Leafy Cacti?

Pereskia aculeata

Pereskia aculeata

At first glance, there is little about a Pereskia that would suggest a relation to what we know as cacti. Even a second, third, and forth glance probably wouldn't do much to persuade the casual observer that these plants have a place on cacti family tree. All preconceptions aside, Pereskia are in fact members of the family Cactaceae and quite interesting ones at that.

Most people readily recognize the leafless, spiny green stems of a cactus. Indeed, this would appear to be a unifying character of the family. Pereskia is proof that this is not the case. Though other cacti occasionally produce either tiny, vestigial leaves or stubby succulent leaves, Pereskia really break the mold by producing broad, flattened leaves with only a hint of succulence.

Pereskia spines are produced from areoles in typical cactus fashion.

Pereskia spines are produced from areoles in typical cactus fashion.

What's more, instead of clusters of Opuntia-like pads or large, columnar trunks, Pereskia are mainly shrubby plants with a handful of scrambling climbers mixed in. Similar to their more succulent cousins, the trunks of Pereskia are usually adorned with clusters of long spines for protection. Additionally, each species produces the large, showy, cup-like blooms we have come to expect from cacti.

They are certainly as odd as they are beautiful. As it stands right now, taxonomists recognize two clades of Pereskia - Clade A, which are native to a region comprising the Gulf of Mexico and Caribbean Sea (this group is currently listed under the name Leuenbergeria) and Clade B, which are native to regions just south of the Amazon Basin. This may seem superficial to most of us but the distinction between these groups has a lot to teach us about the evolution of what we know of as cacti. 

Pereskia grandifolia

Pereskia grandifolia

Genetically speaking, the genus Pereskia sorts out at the base of the cactus family tree. Pereskia are in fact sister to all other cacti. This is where the distinction between the two Pereskia clades gets interesting. Clade A appears to be the older of the two and all members of this group form bark early on in their development and their stems lack a feature present in all other cacti - stomata. Stomata are microscopic pours that allow the exchange of gases like CO2 and oxygen. Clabe B, on the other hand, delay bark formation until later in life and all of them produce stomata on their stems.

The reason this distinction is important is because all other cacti produce stomata on their stems as well. As such, their base at the bottom of the cactus tree not only shows us what the ancestral from of cactus must have looked like, it also paints a relatively detailed picture of the evolutionary trajectory of subsequent cacti lineages. It would appear that the ancestor of all cacti started out as leafy shrubs that lacked the ability to perform stem photosynthesis. Subsequent evolution saw a delay in bark formation, the presence of stomata on the stem, and the start of stem photosynthesis, which is a defining feature of all other cacti.

Pereskia aculeata

Pereskia aculeata

If you are as excited about Pereskia as I am, then you , my friend, are in luck. A handful of Pereskia species have found their way into the horticulture trade. With a little luck attention to detail, you too can share you home with one of these wonderful plants. Just be warned, they get tall and their spines, which are often hidden by the leaves, are a force to be reckoned with. Tread lightly with these wonderfully odd cacti. Celebrate their as the evolutionary wonders that they are!

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2]

 

 

The Desert Mistletoe: Evolution In Action

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There are a multitude of mistletoes on this planet (for example: 1, 2, 3) and all of them are parasites to one degree or another. I find parasitic plants absolutely fascinating because there are as many variations on this lifestyle as there are hosts to parasitize. On a recent botanical adventure in the Sonoran Desert, I met yet another representative of this group - the desert mistletoe (Phoradendron californicum). Once I knew what I was looking at, I could not wait to do some research. As it turns out, this species has garnered quite a bit of attention over the years and it is teaching us some interesting tidbits on how parasites may evolve.

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The desert mistletoe is not hard to spot, especially during the driest parts of the year when most of its host trees have shed their leaves. It looks like a leafless, tangled mass of pendulous stems sitting among the branches of larger shrubs and trees. It can be found growing throughout both the Mojave and Sonoran deserts and appears to prefer leguminous trees including palo verde (Parkinsonia florida), mesquite (Prosopis spp.), and Acacia.

The desert mistletoe is a type of hemiparasite, which means it is capable of performing photosynthesis but nonetheless relies on its host tree for water and other nutrients. Lacking leaves, the desert mistletoe meets all of its photosynthetic needs via its green stems. Its leafless habit also makes its flowers and fruit all the more conspicuous. Despite their small size, its flowers are really worth closer inspection. When in bloom, a desert mistletoe comes alive with the hum of various insects looking for energy-rich nectar and pollen. Even before you spot them, you can easily tell if there is a blooming mistletoe nearby as the flowers give off a wonderfully sweet aroma. It appears that the desert mistletoe takes no chances when it comes to reproduction in such an arid climate.

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As I mentioned above, the desert mistletoe has been the subject of inquiry over the last few decades. Researchers interested in how parasitic plants evolve have illuminated some intriguing aspects of the biology of this species, especially as its relates to host preference. It would appear that our interest in this species seems to be situated at an important time in its evolutionary history. Not all populations of desert mistletoe "behave" in the same way. In fact, each seems to be heading towards more intense specialization based on its preferred host.

By performing seed transplant experiments, researchers have demonstrated that various populations of desert mistletoe seem to be specializing on specific tree species. For instance, when seeds collected from mistletoe growing on Acacia were placed on paleo verde or mesquite, they experienced significantly less germination than if they were placed on another Acacia. Though the exact mechanisms aren't clear at this point in time, evidence suggests that the success of desert mistletoe may be influenced by various hormone levels within the host tree, with isolated populations becoming more specialized on the chemistry of their specific host in that region.

Speaking of isolation, there is also evidence to show that populations of desert mistletoe growing on different host trees are reproductively isolated as well. Populations growing on mesquite trees flower significantly later than populations growing on Acacia or palo verde. Essentially this means that their genes never have the chance to mix, thus increasing the differences between these populations. Again, it is not entirely certain how the host tree may be influencing mistletoe flowering time, however, hormones and water availability are thought to play a role.

Another intriguing idea, and one that has yet to be tested, are the roles that seed dispersers may play in this evolutionary drama. After pollination, the desert mistletoe produces copious amounts of bright red berries that birds find irresistible. Two birds in particular, the northern mockingbird and the Phainopepla, aggressively defend fruiting mistletoe shrubs within their territories. It could be possible that these birds may be influencing which trees the seeds of the desert mistletoe end up on. Again, this is just a hypothesis but one that certainly deserves more attention.

A Phainopepla on the lookout for mistletoe berries.

A Phainopepla on the lookout for mistletoe berries.

Love them or hate them, there is something worth admiring about mistletoes. At the very least, they are important components of their native ecology. What's more, species like the desert mistletoe have a lot to teach us about the way in which species interact and what that means for biodiversity.

Photo Credit: [1]

Further Reading: [1] [2] [3] [4]

Daffodil Insights

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Daffodils seem to be everywhere. Their horticultural popularity means that, for many of us, these plants are among the first flowers we see each spring. Daffodils are so commonplace that it's as if they evolved to live in our gardens and nowhere else. Indeed, daffodils have had a long, long history with human civilization, so much so that it is hard to say when our species first started to cohabitate. Our familiarity with these plants belies an intriguing natural history. What follows is a brief overview of the world of daffodils. 

If you are like me, then you may have gone through most of your life not noticing much difference between garden variety daffodils. Though many of us will be familiar with only a handful of daffodil species and cultivars, these introductions barely scratch the surface. One may be surprised to learn that as of 2008, more than 28,000 daffodil varieties have been named and that number continues to grow each and every year. Even outside of the garden, there is some serious debate over the number of daffodil species, much of this having to do with what constitutes a species in this group.

Narcissus poeticus

Narcissus poeticus

As I write this, all daffodils fall under the genus Narcissus. Estimates as to the number of species within Narcissus range from as few as 50 to as many as 80. The genus itself sits within the family Amaryllidaceae and is believed to have originated somewhere between the late Oligocene and early Miocene, some 18 to 30 million years ago. Despite its current global distribution, Narcissus are largely Mediterranean plants, with peak diversity occurring on the Iberian Peninsula. However, thanks to the aforementioned long and complicated history in cultivation, it has become quite difficult to understand the full range of diversity in form and habitat of many species. To understand this, we first need to understand a bit about their reproductive habits.

Much of the evolution of Narcissus seems to center around floral morphology and geographic isolation. More specifically, the length of the floral tube or "corona" and the position of the sexual organs within, dictates just who can effectively pollinate these plants. The corona itself is not made up of petals or sepals but instead, its tube-like appearance is due to a fusion of the stamens into the famous trumpet-like tube we know and love.

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Variation in corona shape and size has led to the evolution of three major pollination strategies within this genus. The first form is the daffodil form, whose stigma is situated at the mouth of the corolla, well beyond the 6 anthers. This form is largely pollinated by larger bees. The second form is the paperwhite form, whose stigma is situated more closely to or completely below the anthers at the mouth of the corona. This form is largely pollinated by various Lepidoptera as well as long tongued bees and flies. The third form is the triandrus form, which exhibits three distinct variations on stigma and anther length, all of which are situated deep within the long, narrow corona. The pendant presentation of the flowers in this group is thought to restrict various butterflies and moths from entering the flower in favor of bees.

Narcissus tazetta

Narcissus tazetta

The variations on these themes has led to much reproductive isolation among various Narcissus populations. Plants that enable one type of pollinator usually do so at the exclusion of others. Reproductive isolation plus geographic isolation brought on by differences in soil types, habitat types, and altitudinal preferences is thought to have led to a rapid radiation of these plants across the Mediterranean. All of this has gotten extremely complicated ever since humans first took a fancy to these bulbs.

Narcissus cyclamineus

Narcissus cyclamineus

Reproductive isolation is not perfect in these plants and natural hybrid zones do exist where the ranges of two species overlap. However, hybridization is made much easier with the helping hand of humans. Whether via landscape disturbance or direct intervention, human activity has caused an uptick in Narcissus hybridization. For centuries, we have been mixing these plants and moving them around with little to no record as to where they originated. What's more, populations frequently thought of as native are actually nothing more than naturalized individuals from ancient, long-forgotten introductions. For instance, Narcissus populations in places like China, Japan, and even Great Britain originated in this manner.

All of this mixing, matching, and hybridizing lends to some serious difficulty in delineating species boundaries. It would totally be within the bounds of reason to ask if some of the what we think of as species represent true species or simply geographic varieties on the path to further speciation. This, however, is largely speculative and will require much deeper dives into Narcissus phylogenetics.

Narcissus triandrus

Narcissus triandrus

Despite all of the confusion surrounding accurate Narcissus taxonomy, there are in fact plenty of true species worth getting to know. These range in form and habit far more than one would expect from horticulture. There are large Narcissus and small Narcissus. There are Narcissus with yellow flowers and Narcissus with white flowers. Some species produce upright flowers and some produce pendant flowers. There are even a handful of fall-blooming Narcissus. The variety of this genus is staggering if you are not prepared for it.

Narcissus viridiflorus  - a green, fall-blooming daffodil

Narcissus viridiflorus - a green, fall-blooming daffodil

After pollination, the various Narcissus employ a seed dispersal strategy that doesn't get talked about enough in reference to this group. Attached to each hard, black seed are fatty structures known as eliasomes. Eliasomes attract ants. Like many spring flowering plant species around the globe, Narcissus utilize ants as seed dispersers. Ants pick up the seeds and bring them back to their nests. They go about removing the eliasomes and then discard the seed. The seed, safely tucked away in a nutrient-rich ant midden, has a much higher chance of germination and survival than if things were left up to simple chance. It remains to be seen whether or not Narcissus obtain similar seed dispersal benefits from ants outside of their native range. Certainly Narcissus populations persist and naturalize readily, however, I am not aware if ants have any part in the matter.

The endangered  Narcissus alcaracensis .

The endangered Narcissus alcaracensis.

Despite their popularity in the garden, many Narcissus are having a hard go of it in the wild. Habitat destruction, climate change, and rampant collecting of wild bulbs are having serious impacts on Narcissus numbers. The IUCN considered at least 5 species to be endangered and a handful of some of the smaller species already thought to be extinct in the wild. In response to some of these issues, protected areas have been established that encompass at least some of the healthy populations that remain for some of these species.

If you are anything like me, you have ignored Narcissus for far too long. Sure, they aren't native to the continent on which I live, and sure, they are one of the most commonly used plants in a garden setting, but every species has a story to tell. I hope that, armed with this new knowledge, you at least take a second look at the Narcissus popping up around your neighborhood. More importantly, I hope this introduction makes you appreciate their wild origins and the fact that we still have much to learn about these plants. I have barely scratched the surface of this genus and there is more more information out there worth perusing. Finally, I hope we can do better for the wild progenitors of our favorite garden plants. They need all the help they can get and unless we start speaking up and working to preserve wild spaces, all that will remain are what we have in our gardens and that is not a future I want to be a part of.

Photo Credits: [1] [2] [3] [4] [5] [6] [7]

Further Reading: [1] [2] [3] [4] [5] [6] [7] [8] [9]

 

The Wild World of the Creosote Bush

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Apart from the cacti, the real rockstar of my Sonoran experience was the creosote bush (Larrea tridentata). Despite having been quite familiar with creosote as an ingredient, I admit to complete ignorance of the plant from which it originates. Having no familiarity with the Sonoran Desert ecosystem, I was walking into completely new territory in regard to the flora. It didn’t take long to notice creosote though. Once we hit the outskirts of town, it seemed to be everywhere.

If you are in the Mojave, Sonoran, and Chihuahuan Deserts of western North America, you are never far from a creosote bush. They are medium sized, slow growing shrubs with sprays of compact green leaves, tiny yellow flowers, and fuzzy seeds. Apparently what is thought of as one single species is actually made up of three different genetic populations. The differences between these has everything to do with chromosome counts. Populations in the Mojave Desert have 78 chromosomes, Sonoran populations have 52 chromosomes, and Chihuahuan have 26. This may have to do with the way in which these populations have adapted to the relative amounts of rainfall each of these deserts receive throughout the year, however, it is hard to say for sure.

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Regardless, creosote is supremely adapted to these xeric ecosystems. For starters, their branching architecture coupled with their tiny leaves are arranged so as to make the most out of favorable conditions. If you stare at these shrubs long enough, you may notice that their branches largely orient towards the southeast. Also, their leaves tend to be highly clustered along the branches. It is thought that this branching architecture allows the creosote to minimize water loss while maximizing photosynthesis.

Deserts aren’t hot 24 hours per day. Night and mornings are actually quite cool. By taking advantage of the morning sun as it rises in the east, creosote are able to open their stomata and commence photosynthesis during those few hours when evapotranspiration would be at its lowest. In doing so, they are able to minimize water loss to a large degree. Although their southeast orientation causes them to miss out on afternoon and evening sun to a large degree, the benefits of saving precious water far outweigh the loss to photosynthesis. The clustering of the leaves along the branches may also reduce overheating by providing their own shade. Coupled with their small size, this further reduces heat stress and water loss during the hottest parts of the day.

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Creosote also secrets lots of waxy, resinous compounds. These coat the leaves and to some extent the stems, making them appear lacquered. It is thought that this also helps save water by reducing water loss through the leaf cuticle. However, the sheer diversity of compounds extracted from these shrubs suggests other functions as well. It is likely that at least some of these compounds are used in defense. One study showed that when desert woodrats eat creosote leaves, the compounds within caused the rats to lose more water through their urine and feces. They also caused a reduction in the amount of energy the rats were able to absorb from food. In other words, any mammal that regularly feeds on creosote runs the risk of both dehydration and starvation. This isn’t the only interesting interaction that creosote as with rodents either. Before we get to that, however, we first need to discuss roots.

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Creosote shrubs have deep root systems that are capable of accessing soil water that more shallowly rooted plants cannot. This brings them into competition with neighboring plants in intriguing ways. When rainfall is limited, shallowly rooted species like Opuntia gain access to water before it has a chance to reach deeper creosote roots. Surprisingly this happens more often than you would think. The branching architecture of creosote is such that it tends to accumulate debris as winds blow dust around the desert landscape. As a result, the soils directly beneath creosote often contain elevated nutrients. This coupled with the added shade of the creosote canopy means that seedlings that find themselves under creosote bushes tend to do better than seedlings that germinated elsewhere. As such, creosote are considered nurse plants that actually facilitate the growth and survival of surrounding vegetation. So, if recruitment and resulting competition from vegetation can become such an issue for long term creosote survival, why then do we still so much creosote on the landscape?

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The answer may lie in rodents and other burrowing animals in these desert ecosystems. Take a look at the base of a large creosote and you will often find the ground littered with burrows. Indeed, many a mammal finds the rooting zone of the creosote shrub to be a good place to dig a den. When these animals burrow under shallowly rooted desert plants, many of them nibble on and disturb the rooting zones. Over the long-term, this can be extremely detrimental for the survival of shallow rooted species. This is not the case for creosote. Its roots run so deep that most burrowing animals cannot reach them. As such, they avoid most of the damage that other plants experience. This lends to a slight survival advantage for creosote at the expense of neighboring vegetation. In this way, rodents and other burrowing animals may actually help reduce competition for the creosote.

Barring major disturbances, creosote can live a long, long time. In fact, one particular patch of creosote growing in the Mojave Desert is thought to be one of the oldest living organisms on Earth. As creosote shrubs grow, they eventually get to a point in which their main stems break and split. From there, they begin producing new stems that radiate out in a circle from the original plant. These clones can go on growing for centuries. By calculating the average growth rate of these shrubs, experts have estimated that the Mojave specimen, affectionately referred to as the “King Clone,” is somewhere around 11,700 years old!

The ring of creosote that is King Clone.

The ring of creosote that is King Clone.

For creosote, its slow and steady wins the race. They are a backbone of North American desert ecosystems. Their structure offers shelter, their seeds offer food, and their flowers support myriad pollinators. Creosote is one shrub worthy of our respect and admiration.

Photo Credit: [1] [2]

Further Reading: [1] [2] [3] [4]

The Giant Genomes of Geophytes

Canopy plant ( Paris japonica )

Canopy plant (Paris japonica)

A geophyte is any plant with a short, seasonal lifestyle and some form of underground storage organ ( bulb, tuber, thick rhizome, etc.). Plants hailing from a variety of families fall into this category. However, they share more than just a similar life history. A disproportionate amount of geophytic plants also possess massive genomes. 

As we have discussed in previous posts, life isn't easy for geophytes. Cold temperatures, a short growing season, and plenty of hungry herbivores represent countless hurdles that must be overcome. That is why many geophytes opt for rapid growth as soon as conditions are right. However, they don't do this via rapid cell division. 

Dutchman's breeches ( Dicentra cucullaria ) emerging with preformed buds.

Dutchman's breeches (Dicentra cucullaria) emerging with preformed buds.

Instead, geophytes spend the "dormant" months pre-growing all of their organs. What's more, the cells that make up their leaves and flowers are generally much larger than cells found in non-geophytes. This is where that large genome comes into plant. If they had to wait until the first few weeks of spring to start their development, a large genome would only get in the way. Their dormant season growth means that these plants don't have to worry about streamlining the process of cellular division. They can take their time. 

As such, an accumulation of genetic material isn't detrimental. Instead, it may actually be quite beneficial for geophytes. Associated with large genomes are things like larger stomata, which helps these plants better regulate their water needs. The large genomes may very well be the reason that many geophytic plants are so good at taking advantage of such ephemeral growing conditions. 

When the right conditions present themselves, geophytes don't waste time. Pre-formed organs like leaves and flowers simply have to fill with water instead of having to wait for tissues to divide and differentiate. Water is plentiful during the spring so geophytes can rely on turgor pressure within their large cells for stability rather than investing in thick cell walls. That is why so many spring blooming plants feel so fleshy to the touch. 

Taken together, we can see how large genomes and a unique growth strategy have allowed these plants to exploit seasonally available habitats. It is worth noting, however, that this is far from the complete picture. With such a wide variety of plant species adopting a geophytic lifestyle, we still have a lot to learn about the secret lives of these plants.

Photo Credits: [1] [2]

Further Reading: [1]

An Ancient Hawaiian Moss

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The cloud forests of Kohala Mountain on the island of Hawai'i are home to a unique  botanical community. One plant in particular is quite special as it may be one of the most ancient clonal organisms in existence. Look down at your feet and you may find yourself surrounded by a species of moss known as Sphagnum palustre. Although this species enjoys a broad distribution throughout the northern hemisphere, its presence on this remote volcanic island is worth closer inspection. 

Hawai'i is rather depauperate in Sphagnum representatives and those that have managed to get to this archipelago are often restricted to growing in narrow habitable zones between 900 to 1,900 meters in elevation as these are the only spots that are cool and wet enough to support Sphagnum growth. Needless to say, successful colonization of the Hawaiian Islands by Sphagnum has been a rare event.  The fact that Sphagnum palustre was one of the few that did should not come as any surprise. What should surprise you, however, is how this particular species has managed to persist. 

Mounds of  S. palustre  in its native habitat. 

Mounds of S. palustre in its native habitat. 

Hawaiian moss aficionados have long noted that the entire population of Kohala's S. palustre mats never seem to produce a single female individual. Indeed, this moss is dioicous, meaning individuals are either male or female. As such, many have suspected that the mats of S. palustre growing on Kohala represented a single male individual that has been growing vegetatively ever since it arrived as a spore on the island. The question then becomes, how long has this S. palustre individual been on Kohala?

To answer that, researchers decided to take a look at its DNA. What they discovered was surprising in many ways. For starters, all plants were in fact males of a single individual. A rare genetic trait was found in the DNA of every population they sampled. This trait is so rare that the odds of it turning up in any number by sheer chance is infinitesimally small. What this means is that every S. palustre population found on Kohala is a clone of a single spore that landed on the mountain at some point in the distant past. Exactly how distant was the next question the team wanted to answer. 

A lush cloud forest on the slopes of Kohala.

A lush cloud forest on the slopes of Kohala.

The first clue to this mystery came from peat deposits found on the slopes of the mountain. Researchers found remains of S. palustre in peat deposits that were dated to somewhere around 24,000 years old. So, it would appear that S. palustre has been growing on Kohala since at least the late Pleistocene. But how long before that time did this moss arrive?

Again, DNA was the key to unlocking this mystery. By studying the rate at which mutations arise and fix themselves within the genetic code of this plant, they were able to estimate the average rate of mutation through time. By sampling different moss populations on Kohala, they could then use those estimates to figure out just how long each mat has been growing. Their estimates suggest that the ancestral male sport arrived on Hawai'i somewhere between 49,000 and 50,000 years ago and it has been cloning itself ever since. 

A large mat of  S. palustre

A large mat of S. palustre

As if that wasn't remarkable in and of itself, their thorough analysis of the genetic diversity within S. palustre revealed a remarkable amount of genetic diversity for a clonal organism. Though not all genetic mutations are beneficial, enough of them have managed to fix themselves into the DNA of the moss clones over thousands of years. The DNA of S. palustre is challenging long-held assumptions about genetic diversity of asexual organisms.

Of course, no conversation about Hawaiian botany would be complete without mention of invasive species. As one can expect at this point, Kohala's S. palustre populations are being crowded out by more aggressive vegetation introduced from elsewhere in the world. Unlike a lot of Hawaiian plants, however, the clonal habit of S. palustre puts a more nuanced twist to this story. 

Because Sphagnum is spongy yet durable, it has often been used as packing material. Packages stuffed with S. palustre from Kohala have been sent all over the island and because of this, S. palustre is now showing up en masse on other islands in the archipelago. Sadly, when it starts to grow in habitats that have never experienced the ecosystem engineering traits of a Sphagnum  moss, S. palustre gets pretty out of hand. It's not just packages that spread it either. All it takes is one sprig of the moss stuck on someone's boot to start a new colony elsewhere. The unique flora elsewhere in the Hawaiian archipelago have not evolved to compete with S. palustre and as a result, escaped populations are rapidly changing the ecology to the detriment of other endemic Hawaiian plants. 

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] 

Are Algae Plants?

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I was nibbling on some nori the other day when a thought suddenly hit me. I don't know squat about algae. I know it comes in many shapes, sizes, and colors. I know it is that stuff that we used to throw at each other on the beach. I know that it photosynthesizes. That's about it. What are algae? Are they even plants?

The shortest answer I can give you is "it depends." The term algae is a bit nebulous in and of itself. In Latin, the word "alga" simply means "seaweed." Algae are paraphyletic, meaning they do not share a recent common ancestor with one another. In fact, without specification, algae may refer to entirely different kingdoms of life including Plantae (which is often divided in the broad sense, Archaeplastida and the narrow sense, Viridiplantae), Chromista, Protista, or Bacteria.

Caulerpa racemosa , a beautiful green algae.

Caulerpa racemosa, a beautiful green algae.

Taxonomy being what it is, these groupings may differ depending on who you ask. The point I am trying to make here is that algae are quite diverse from an evolutionary standpoint. Even calling them seaweed is a bit misleading as many different species of algae can be found in fresh water as well as growing on land.

Cyanobacteria are photosynthetic bacteria, not plants.

Cyanobacteria are photosynthetic bacteria, not plants.

Take for instance what is referred to as cyanobacteria. Known commonly as blue-green algae, colonies of these photosynthetic bacteria represent some of the earliest evidence of life in the fossil record. Remains of colonial blue-green algae have been found in rocks dating back more than 4 billion years. As a whole, these types of fossils represent nearly 7/8th of the history of life on this planet! However, they are considered bacteria, not plants.

Diatoms (Chromista)

Diatoms (Chromista)

Diatoms (Chromista) are another enormously important group. The single celled, photosynthetic organisms are encased in beautiful glass shells that make up entire layers of geologic strata. They comprise a majority of the phytoplankton in the world's oceans and are important indicators of climate. However, they belong to their own kingdom of life - Chromista or the brown algae.

To bring it back to what constitutes true plants, there is one group of algae that really started it all. It is widely believed that land plants share a close evolutionary history with a branch of green algae known as the stoneworts (order Charales). These aquatic, multicellular algae superficially resemble plants with their stalked appearance and radial leaflets.

A nice example of a stonewort ( Chara braunii ).

A nice example of a stonewort (Chara braunii).

It is likely that land plants evolved from a Chara-like ancestor that may have resembling modern day hornworts that lived in shallow freshwater inlets. Estimates of when this happen go back as far as 500 million years before present. Unfortunately, fossil evidence is sparse for this sort of thing and mostly comes in the form of fossilized spores and molecular clock calculations.

Porphyra umbilicalis   - One of the many species of red algae frequently referred to as nori.

Porphyra umbilicalis  - One of the many species of red algae frequently referred to as nori.

Now, to bring it back to what started me down this road in the first place. Nori is made from algae in the genus Porphyra, which is a type of Rhodophyta or red algae. Together with Chlorophyta (the green algae), they make up some of the most familiar groups of algae. They have also been the source of a lot of taxonomic debate. Recent phylogenetic analyses place the red algae as a sister group to all other plants starting with green algae. However, some authors prefer to take a broader look at the tree and thus lump red algae in a member of the plant kingdom. So, depending on the particular paper I am reading, the nori I am currently digesting may or may not be considered a plant in the strictest sense of the word. That being said, the lines are a bit blurry and frankly I don't really care as long as it tastes good.

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1] [2] [3] [4]

 

How a Giant Parasitic Orchid Makes a Living

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Imagine a giant vine with no leaves and no chlorophyll scrambling over decaying wood and branches of a warm tropical forest. As remarkable as that may seem, that is exactly what Erythrorchis altissima is. With stems that can grow to upwards of 10 meters in length, this bizarre orchid from tropical Asia is the largest mycoheterotrophic plant known to science.

Mycoheterotrophs are plants that obtain all of their energy needs by parasitizing fungi. As you can probably imagine, this is an extremely indirect way for a plant to make a living. In most instances, this means the parasitic plants are stealing nutrients from the fungi that were obtained via a partnership with photosynthetic plants in the area. In other words, mycoheterotrophic plants are indirectly stealing from photosynthetic plants.

In the case of E. altissima, this begs the question of where does all of the carbon needed to build a surprising amount of plant come from? Is it parasitizing the mycorrhizal network associated with its photosynthetic neighbors or is it up to something else? These are exactly the sorts of questions a team from Saga University in Japan wanted to answer.

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All orchids require fungal partners for germination and survival. That is one of the main reasons why orchids can be so finicky about where they will grow. Without the fungi, especially in the early years of growth, you simply don't have orchids. The first step in figuring out how this massive parasitic orchid makes its living was to identify what types of fungi it partners with. To do this, the team took root samples and isolated the fungi living within.

By looking at their DNA, the team was able to identify 37 unique fungal taxa associated with this species. Most surprising was that a majority of those fungi were not considered mycorrhizal (though at least one mycorrhizal species was identified). Instead, the vast majority of the fungi associated with with this orchid are involved in wood decay.

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

To ensure that these wood decay fungi weren't simply partnering with adult plants, the team decided to test whether or not the wood decay fungi were able to induce germination of E. altissima seeds. In vitro germination trials revealed that not only do these fungi induce seed germination in this orchid, they also fuel the early growth stages of the plant. Further tests also revealed that all of the carbon and nitrogen needs of E. altissima are met by these wood decay fungi.

These results are amazing. It shows that the largest mycoheterotrophic plant we know of lives entirely off of a generalized group of fungi responsible for the breakdown of wood. By parasitizing these fungi, the orchid has gained access to one of the largest pools of carbon (and other nutrients) without having to give anything back in return. It is no wonder then that this orchid is able to reach such epic proportions without having to do any photosynthesizing of its own. What an incredible world we live in!

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Photo Credits: [1] [2]

Further Reading: [1]

Rein In Those Seeds

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Plants living on islands face a bit of a conundrum. In order to get to said islands, the ancestors of those plants had to exhibit extreme seed or spore dispersal strategies. However, if plants are to persist after arriving to an island, long-distance dispersal becomes rather risky. In the case of oceanic islands, seeds or spores that travel too far end up in the water. As such, we often observe an evolutionary reduction in dispersal ability for island residents. 

Islands, however, are not always surrounded by water. You can have "islands" on land as well. The easiest example for most to picture would be the alpine zone of a mountain. Species adapted to these high-elevation habitats find it hard to compete with species native to low-elevation habitats and are therefore stuck on these "islands in the sky." Less obvious are islands created by a specific soil type. 

Take, for instance, gypseous soils. Such soils are the result of large amounts of gypsum deposits at or near the soil surface. Gypseous soils are found in large quantities throughout parts of western North America, North and South Africa, western Asia, Australia, and eastern Spain. They are largely the result of a massive climatic shift that occurred during the Eocene, some 50 million years ago. 

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Massive mountain building events during that time were causing large reductions in atmospheric CO2 concentrations. The removal of this greenhouse gas via chemical weathering caused a gradual decline in average temperatures around the world. Earth was also becoming a much drier place and throughout the areas mentioned above, hyper-saline lakes began to dry up. As they did, copious amount of minerals, including gypsum, were left behind. 

These mineral-rich soils differ from the surrounding soils in that they contain a lot of salts. Salt makes life incredibly difficult for most terrestrial plants. Life finds a way, however, and a handful of plant species inevitably adapted to these mineral-rich soils, becoming specialists in the process. They are so specialized on these types of soils that they simply cannot compete with other plant species when growing in more "normal" soils. 

Essentially, these gypseous soils function like soil or edaphic islands. Plants specialized in growing there really don't have the option to disperse far and wide. They have to rein it in or risk extirpation. For a group of plants growing in gypseous soils in western North America, this equates to changes in seed morphology. 

Mentzelia is a genus of flowering plants in the family Loasaceae. There are somewhere around 60 to 70 different species, ranging from annuals to perennials, and forbs to shrubs (they are often referred to as blazing stars but since that would lead to too much confusion with Liatris, I will continue to refer to them as Mentzelia).

For most species in this genus, seed dispersal is accomplished by wind. Plants growing on "normal" soils produce seeds with a distinct wing surrounding the seed. A decent breeze will dislodge them from their capsule, causing them blow around. With any luck some of those seeds will land in a suitable spot for germination, far from their parents. Such is not the case for all Mentzelia though. When researchers took a closer look at species that have specialized on gypseous soils, they found something intriguing. 

Mentzelia  phylogeny showing reduction in seed wings.

Mentzelia phylogeny showing reduction in seed wings.

The wings surrounding the seeds of gypseous Mentzelia were either extremely reduced in size or had disappeared altogether. Just as it makes no sense for a plant living on an oceanic island to disperse its seeds far out into the ocean, it too makes no sense for gypseous Mentzelia to disperse their seeds into soils in which they cannot compete. It is thought that limited dispersal may help reinforce the types of habitat specialization that we see in species like these Mentzelia. The next question that must be answered is whether or not such specialization and limited dispersal comes at the cost of genetic diversity. More work will be needed to understand such dynamics. 

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2]

 

Hydatellaceae: The Other Basal Angiosperms

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Though rather obscure to most of the world, the genus Trithuria has enjoyed somewhat of a celebrity status in recent years. A paper published in 2007 lifted this tiny group of minuscule aquatic plants out of their spot in Poales and granted them a place among the basal angiosperm lineage Nymphaeales. This was a huge move for such little plants. 

The genus Trithuria contains 12 species, the majority of which reside in Australia, however, two species, T. inconspicua and T. konkanensis, are native to New Zealand and India. They are all aquatic herbs and their diminutive size and inconspicuous appearance make them easy to miss. For quite some time these odd plants were considered to be a group of highly reduced monocots. Their original placement was in the family Centrolepidaceae. All of that changed in 2007.

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Close inspection of Trithuria DNA told a much different story. These were not highly reduced monocots after all. Instead, multiple analyses revealed that Trithuria were actually members of the basal angiosperm lineage Nymphaeales. Together with the water lilies (Nymphaeaceae) and the fanworts (Cabombaceae), these plants are living representatives of some of the early days in flowering plant evolution. 

Of course, DNA analysis cannot stand on its own. The results of the new phylogeny had to be corroborated with anatomical evidence. Indeed, closer inspection of the anatomy of Trithuria revealed that these plants are truly distinct from members of Poales based on a series of features including furrowed pollen grains, inverted ovules, and abundant starchy seed storage tissues. Taken together, all of these lines of evidence warranted the construction of a new family - Hydatellaceae.

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The 12 species of Trithuria are rather similar in their habits. Many live a largely submerged aquatic lifestyle in shallow estuarine habitats. As you may have guessed, individual plants look like tiny grass-like rosettes. Their small flower size has lent to some of their taxonomic confusion over the years. What was once thought of as individual flowers were revealed to be clusters or heads of highly reduced individual flowers. 

Reproduction for these plants seems like a tricky affair. Some have speculated that water plays a role but close inspections of at least one species revealed that very little pollen transfer takes place in this way. Wind is probably the most common way in which pollen from one plant finds its way to another, however, the reduced size of these flowers and their annual nature means there isn't much time and pollen to go around. It is likely that most of the 12 species of Trithuria are self-pollinated. This is probably quite useful considering the unpredictable nature of their aquatic habitats. It doesn't take much for these tiny aquatic herbs to establish new populations. In total, Trithuria stands as living proof that big things often come in small packages. 

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

 

Meet The Ghostworts

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I love parasitic plants and I love liverworts. Imagine my excitement then when I learned that there are at least two species of parasitic liverworts! These bizarre little plants are currently the only parasitic non-vascular plants known to science. 

The first description of a ghostwort dates back to 1919. Although no description of habitat was given, the account describes a set of liverwort thalli containing no chlorophyll and whose cells were full of mycorrhizal fungi. They were assigned to the genus Aneura and that was that. Further descriptions of this plant would not be made for more than a decade.

A ghostwort gametophyte with spike-like sporophytes.

A ghostwort gametophyte with spike-like sporophytes.

Proper attention was not given to this group until the 1930's. More plants started turning up among the humus and mosses of forests and wetlands throughout Finland, Sweden, and Scotland. A more thorough workover of specimens was made and the plants were moved into their own genus, Cryptothallus, which accurately captured their subterranean habit. They were given the name Cryptothallus mirabilis.

Another species of Cryptothallus was discovered in Costa Rica in 1977. It was named Cryptothallus hirsutus. Only one other collection of these species was made and it remains the lesser known of the two species. It is interesting to note the disparity between their ranges, with C. mirabilis inhabiting northern portions of Europe, and C. hirsutus only known from those two collections in Central America. Regardless, these odd liverworts have received a bit more attention in recent years.

It seems that the ghostworts manage to capture the attention of anyone who looks hard enough. For instance, a handful of attempts have been made to cultivate ghostworts in a controlled lab setting. Originally, plants were grown exposed to varying levels of light but try as the may, researchers were never able to coax the plants into producing chlorophyll. It would appear that these tiny liverworts are in fact some sort of parasite.

Spike-like sporophytes with a branching gametophyte. 

Spike-like sporophytes with a branching gametophyte. 

Proper evidence of their parasitic lifestyle was finally demonstrated 2003. Researchers were able to grow C. mirabilis in specialized observation chambers in order to understand what is going on under the soil. As it turns out, those numerous mycorrhizal connections mentioned in the original description are the key to survival for the ghostworts. The team showed that the ghostwort tricks fungi in the genus Tulasnella into forming mycorrhizal connections with its cells. These fungi also happen to be hooked up to a vast network of pine and birch tree roots.

By tricking the fungi, into an association, the ghostworts are able to steal carbohydrates that the fungi gain from the surrounding trees. Like all mycoheterotrophs, the ghostworts are essentially indirect parasites of photosynthetic plants. Their small size and relative rarity on the landscape likely helps these plants go unnoticed by the fungi but much more work needs to be done to better understand such dynamics.

Ghostworts look more like fungi than plants.

Ghostworts look more like fungi than plants.

In 2008, phylogenetic attention was paid to the ghostworts in order to better understand where they fit on the liverwort branch of the tree. As it turns out, Cryptothallus appears to be nestled quite comfortably within the genus Aneura. Because of this, the authors suggest disposing of the genus Cryptothallus altogether. Outside of simply placing this species back in its originally described genus, it affiliation with Aneura is quite interesting from an evolutionary standpoint.

Other liverworts in the genus Aneura are also known to form mycorrhizal relationships with Tulasnella. Unlike the ghostworts, however, these liverworts are fully capable of photosynthesis. Because these intimate fungal relationships were already in place before the ghostworts began evolving towards a fully parasitic lifestyle, it suggests that the saprophytic nature of Tulasnella fungi may have actually facilitated this jump. 

The cryptic nature of the ghostworts has left many a botanist wanting. Their subterranean habit makes them incredibly hard to find. Who knows what secrets this group still holds. Future discoveries could very well add more species to the mix or, at the very least, greatly expand the known range of the other two.

Photo Credits: Brian Eversham [1] [2] [3] [4]

Further Reading: [1] [2] [3]

 

On the Ecology of Krameria

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There is something satisfying about saying "Krameria." Whereas so many scientific names act as tongue twisters, Krameria rolls of the tongue with a satisfying confidence. What's more, the 18 or so species within this genus are fascinating plants whose lifestyles are as exciting as their overall appearance. Today I would like to give you an overview of these unique parasitic plants.

Commonly known as rhatany, these plants belong to the family Krameriaceae. This is a monotypic clade, containing only the genus Krameria. Historically there has been a bit of confusion as to where these plants fit on the tree of life. Throughout the years, Krameria has been placed in families like Fabaceae and Polygalaceae, however, more recent genetic work suggests it to be unique enough to warrant a family status of its own. 

Regardless of its taxonomic affiliation, Krameria is a wonderfully specialized genus of plants with plenty of offer the biologically curious among us. All 18 species are shrubby, though at least a couple species can sometimes barely qualify as such. They are a New World taxon with species growing native as far south as Paraguay and Chile and as far north as Kansas and Colorado. They generally inhabit dry habitats.

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As I briefly mentioned above, most if not all of the 18 species are parasitic in nature. They are what we call "hemiparasites" in that despite stealing from their hosts, they are nonetheless fully capable of photosynthesis. It is interesting to note that no one has yet been able to raise these plants in captivity without a host. It would seem that despite being able to photosynthesize, these plants are rather specialized parasites. 

That is not to say that they have evolved to live off of a specific host. Far from it actually. A wide array of potential hosts, ranging from annuals to perennials, have been identified. What I find most remarkable about their parasitic lifestyle is the undeniable advantage it gives these shrubs in hot, dry environments. Research has found that despite getting a slow start on growing in spring, the various Krameria species are capable of performing photosynthesis during extremely stressful periods and for a much longer duration than the surrounding vegetation. 

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The reason for this has everything to do with their parasitic lifestyle. Instead of producing a long taproot to reach water reserves deep in the soil, these shrubs invest in a dense layer of lateral roots that spread out in the uppermost layers of soil seeking unsuspecting hosts. When these roots find a plant worth parasitizing, they grow around its roots and begin taking up water and nutrients from them. By doing this, Krameria are no longer limited by what water or other resources their roots can find. Instead, they have managed to tap into large reserves that would otherwise be locked away inside the tissues of their neighbors. As such, the Krameria do not have to worry about water stress in the same way that non-parasitic plants do. 

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By far the most stunning feature of the genus Krameria are the flowers. Looking at them it is no wonder why they have been associated with legumes and milkworts. They are beautiful and complex structures with a rather specific pollination syndrome. Krameria flowers produce no nectar to speak of. Instead, they have evolved alongside a group of oil-collecting bees in the genus Centris.

One distinguishing feature of Krameria flowers are a pair of waxy glands situated on each side of the ovary. These glands produce oils that female Centris bees require for reproduction. Though Centris bees are not specialized on Krameria flowers, they nonetheless visit them in high numbers. Females alight on the lip and begin scraping off oils from the glands. As they do this, they inevitably come into contact with the stamens and pistil. The female bees don't feed on these oils. Instead, they combine it with pollen and nectar from other plant species into nutrient-rich food packets that they feed to their developing larvae.  

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Following fertilization, seeds mature inside of spiny capsules. These capsules vary quite a bit in form and are quite useful in species identification. Each spine is usually tipped in backward-facing barbs, making them excellent hitchhikers on the fur and feathers of any animal that comes into contact with them.  

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]

A Surprising Realization About Leaf Windows

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I will never forget the first time I laid eyes on a Lithops. These odd little succulents are truly marvels of evolution. The so-called "living stones" really do earn their name as most are exquisitely camouflaged to match the gravelly soils in which they grow. If bizarre color patterns weren't enough, Lithops, as well as many other succulents, live their lives almost completely buried under the soil. All one ever really sees is the very tip of their succulent leaves and the occasional flower.

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It is the tips of those leaves that make people swoon. Lithops belong to a hodgepodge mix of succulent genera and families that produce windowed leaves. Aside from their striking patterns, the tips of their leaves are made up of layers of translucent cells, which allow light to penetrate into the interior of the leaf where the actual photosynthetic machinery is housed. Their semi-translucent leaves, coupled with their nearly subterranean habit, have led to the assumption that the leaf windows allow the plants to continue photosynthesis all the while being mostly buried. Despite the popularity of this assumption, few tests had been performed to see whether or not the windows function as we think. All of that changed back in the year 2000.

As hinted at above, a variety of succulent plants have converged on a similar leaf morphology. This is where things get a bit strange. Not all plants that exhibit the leaf window trait find themselves buried in the soil. Others, such as Peperomia graveolens for example, produce the photosynthetic tissues well above the soil. Examples like this led at least some researchers to second guess the common assumption of windows increasing photosynthesis and the resulting investigations were surprising to say the least. 

Peperomia graveolens

Peperomia graveolens

A duo of researchers decided to test the assumption that leaf windows increase photosynthesis by channeling light directly to the photosynthetic machinery inside. The researchers used tape to cover the leaf windows of a variety of succulent plant species. When they compared photosynthetic rates between the two groups, not a single difference was detected. Plants who had their leaves covered photosynthesized the same amount as plants with uncovered leaves. These data were quite shocking. Because they tested this assumption across a variety of plant species, the results suggested that the function of windowed leaves isn't as straight forward as we thought. These findings raised more questions than they solved.

Subsequent experiments only served to reinforce the original findings. What's more, some even showed that plants with covered windows actually photosynthesized more than plants with uncovered windows. It seems that windowed leaves function in a completely opposite manner than the popular assumption. The key to this patterns may lie in heat exchange. When the researchers took the temperature of the interior of the leaves in each group, they found that internal leaf temperatures were significantly higher in the uncovered group and this has important implications for photosynthesis for these species.

Fenestraria rhopalophylla

Fenestraria rhopalophylla

High leaf temperatures can be extremely damaging to photosynthetic proteins. If too much light filters through, leaf temperatures can actually hit damaging levels. This is one reason that many of these plant species have adopted this bizarre semi-subterranean habit. Plants that experienced such high temperatures throughout the course of a day had permanent damage done to their photosystems. This led to a reduction of fitness over time. Such lethal temperature spikes did not happen to leaves that had been covered.

Haworthia truncata

Haworthia truncata

If you're anything like me, at this point you must be questioning the role of the leaf windows entirely. Why would they be there if they may actually hurt the plants in the long run? Well, this is where knowing something about the habitat of each species comes into play. Not all leaf windows are created equal. The patterns of their windows vary quite a bit depending on where the plants evolved. In 2012, a paper was published that looked at the patterns of Lithops leaf windows in relation to their place of origin. Not all Lithops grow in the same conditions and various species hail from regions with vastly different climates.

What the paper was able to demonstrate was that Lithops native to regions that experience more acerage annual rainfall have much larger window areas on their leaves than Lithops native to drier regions. Again, the underpinnings of this discovery nonetheless have to do with light availability. Wetter areas experience more cloud cover than drier areas so Lithops growing where its cloudy have to cope with a lot less sun than their more xeric-growing cousins. As such, having a larger window allows more diffuse light into the leaf for photosynthesis without having to worry about the damaging temperatures.

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The reverse is true for Lithops from drier climates. They have smaller leaf windows because they experience more days with direct sun. These species tended to have much smaller windows, which reduced the amount of sunlight entering the leaf. This serves to keep internal leaf temperatures within a much safer range, thus protecting the delicate proteins inside. As it turns out, leaf windows seem to represent a trade-off between photosynthesis and overheating. What's more, some window-leaved species seem to be evolving away from the light transmitting function of their cousins living in shadier conditions. If anything, this serves as a reminder that simply because something seems obvious, that doesn't mean its always true. Stay curious, my friends!

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1] [2] [3] [4] [5] [6]

The Extraordinary Catasetum Orchids

Male  Catasetum osculatum

Male Catasetum osculatum

Orchids, in general, have perfect flowers in that they contain both male and female organs. However, in a family this large, exceptions to the rules are always around the corner. Take, for instance, orchids in the genus Catasetum. With something like 166 described species, this genus is interesting in that individual plants produce either male or female flowers. What's more, the floral morphology of the individual sexes are so distinctly different from one another that some were originally described as distinct species. 

Female  Catasetum osculatum

Female Catasetum osculatum

In fact, it was Charles Darwin himself that first worked out that plants of the different sexes were indeed the same species. The genus Catasetum enthralled Darwin and he was able to procure many specimens from his friends for study. Resolving the distinct floral morphology wasn't his only contribution to our understanding of these orchids, he also described their unique pollination mechanism. The details of this process are so bizarre that Darwin was actually ridiculed by some scientists of the time. Yet again, Darwin was right. 

Catasetum longifolium

Catasetum longifolium

If having individual male and female plants wasn't strange enough for these orchids, the mechanism by which pollination is achieved is quite explosive... literally. 

Catasetum orchids are pollinated by large Euglossine bees. Attracted to the male flowers by their alluring scent, the bees land on the lip and begin to probe the flower. Above the lip sits two hair-like structures. When a bee contacts these hairs, a structure containing sacs of pollen called a pollinia is launched downwards towards the bee. A sticky pad at the base ensures that once it hits the bee, it sticks tight. 

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Bees soon learn that the male flowers are rather unpleasant places to visit so they set off in search of a meal that doesn't pummel them. This is quite possibly why the flowers of the individual sexes look so different from one another. As the bees visit the female flowers, the pollen sacs on their back slip into a perfect groove and thus pollination is achieved. 

Eulaema polychroma  visiting  Catasetum integerrimum

Eulaema polychroma visiting Catasetum integerrimum

The uniqueness of this reproductive strategy has earned the Catasetum orchids a place in the spotlight among botanists and horticulturists alike. It begs the question, how is sex determined in these orchids? Is it genetic or are there certain environmental factors that push the plant in either direction? As it turns out, light availability may be one of the most important cues for sex determination in Catasetum

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A paper published back in 1991 found that there were interesting patterns of sex ratios for at least one species of Catasetum. Female plants were found more often in younger forests whereas the ratios approached an even 1:1 in older forests. What the researchers found was that plants are more likely to produce female flowers under open canopies and male flowers under closed canopies. In this instance, younger forests are more open than older, more mature forests, which may explain the patterns they found in the wild. It is possible that, because seed production is such a costly endeavor for plants, individuals with access to more light are better suited for female status. 

Catasetum macrocarpum

Catasetum macrocarpum

Aside from their odd reproductive habits, the ecology of these plants is also quite fascinating. Found throughout the New World tropics, Catasetum orchids live as epiphytes on the limbs and trunks of trees. Living in the canopy like this can be stressful and these orchids have evolved accordingly. For starters, they are deciduous. Most of the habitats in which they occur experience a dry season. As the rains fade, the plants will drop their leaves, leaving behind a dense cluster of green pseudobulbs. These bulbous structures serve as energy and water stores that will fuel growth as soon as the rains return. 

Catasetum silvestre in situ

Catasetum silvestre in situ

The canopy can also be low in vital nutrients like nitrogen and phosphorus. As is true for all orchids, Catasetum rely on an intimate partnership with special mychorrizal fungi to supplement these ingredients. Such partnerships are vital for germination and growth. However, the fungi that they partner with feed on dead wood, which is low in nitrogen. This has led to yet another intricate and highly specialized relationship for at least some members of this orchid genus. 

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Mature Catasetum are often found growing right out of arboreal ant nests. Those that aren't will often house entire ant colonies inside their hollowed out pseudobulbs. This will sometimes even happen in a greenhouse setting, much to the chagrin of many orchid growers. The partnership with ants is twofold. In setting up shop within the orchid or around its roots, the ants provide the plant with a vital source of nitrogen in the form of feces and other waste products. At the same time, the ants will viciously attack anything that may threaten their nest. In doing so, they keep many potential herbivores at bay.  

Female  Catasetum planiceps

Female Catasetum planiceps

To look upon a flowering Catasetum is quite remarkable. They truly are marvels of evolution and living proof that there seems to be no end to what orchids have done in the name of survival. Luckily for most of us, one doesn't have to travel to the jungles and scale a tree just to see one of these orchids up close. Their success in the horticultural trade means that most botanical gardens house at least a species or two. If and when you do encounter a Catasetum, do yourself a favor and take time to admire it in all of its glory. You will be happy that you did. 

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8] [9] 

Further Reading: [1] [2] [3] [4] [5]

The Incredible Feat of a Resurrection Plant

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It is understandable why one would look at the crispy brown bundle of a Selaginella lepidophylla and think that it was dead. No wonder then why this hardy spikemoss has become such a novelty item for those looking for a unique gift. Indeed, even the common name of "resurrection plant" suggests that this species miraculously returns from the dead with the simple addition of water. A dormant resurrection plant is far from dead, however. It is in a state of dormancy that we are still struggling to understand.

Selaginella lepidophylla is native to the Chihuahuan desert, spanning the border between the US and Mexico. This is a harsh habitat for most plants, let alone a Lycophyte. However, this lineage has not survived hundreds of millions of years by being overly sensitive to environmental change and S. lepidophylla is a wonderful reminder of that.

As you can probably imagine, tolerating near-complete desiccation can be pretty beneficial when your habitat receives an average of only 235 mm (9.3 in) of rain each year. A plant can either store water for those lean times or go dormant until the rains return. The latter is exactly what S. lepidophylla does. As its water supply dwindles, the whole body of the plant curls up into a tight ball and waits. No roots anchor it to the ground. It is at the mercy of the winds as it blows around like a tiny tumbleweed until it winds up wedged into a crack or crevice.

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When the rains return, S. lepidophylla needs to be ready. Wet this crispy bundle and watch as over the course of about a day, the dormant ball unfurls to reveal the stunning body of a photosynthetic spikemoss ready to take advantage of moist conditions. Such conditions are short lived, of course, so after a few days drying out, the plant shrivels up and returns to its dormant, ball-like state. How does the plant manage to do this? Why doesn't it simply die? The answer to these questions has been the subject of quite a bit of debate and investigation. 

What we do know is that part of its success has to do with curling up into a ball. Without water in its tissues, its sensitive photosynthetic machinery would easily become damaged by punishing UV rays. By curling up, the plant essentially shelters these tissues from the sun. Indeed, plants that were kept from curling up experienced irreversible damage to their photo systems and were not as healthy as plants that did curl up. To this, the plant owes its success to rather flexible cell walls. Unlike other plants that snap when folded, the cells of S. lepidophylla are able to fold and unfold without any major structural damage.

As far as metabolism and chemistry is concerned, however, we are still trying to figure out how S. lepidophylla survives such drastic shifts. For a while it was thought that, similar to other organisms that undergo such dramatic desiccation, the plant relies on a special sugar called trehalose. Trehalose is known to bind to important proteins and membranes in other desiccation-tolerant organisms, thus protecting them from damage and allowing them to quickly return to their normal function as soon as water returns.

An analysis of non-desiccating Selaginella species, however, showed that S. lepidophylla doesn't produce a lot of trehalose. Though it is certainly present in its tissues, more wet-loving species of Selaginella contain much higher amounts of this sugar. Instead, it has been found that other sugars may actually be playing a bigger role in protecting the inner workings of this plant. Sorbitol and xylitol are found in much higher concentrations within the tissues of S. lepidophylla, suggesting that they may be playing a bigger role than we ever realized. More work is needed to say for sure.

Finally, it would appear that S. lepidophylla is able to maintain enzyme activities within its cells at much higher levels during desiccation periods than was initially thought possible. When dried, some enzymes were found to be working at upwards of 75% efficiency of those found in hydrated tissues. This is really important for a plant that needs to respond quickly to take advantage of fleeting conditions. Along with quick production of new enzymes, this "idling" of enzymatic activity during dormancy is thought to not only protect the plant from too much respiration, but also allows it to hit the ground running as soon as favorable conditions return. 

Despite our lack of understanding of the process, it is amazing to watch this resurrection plant in action. To see something go from a death-like state to a living, photosynthetic organism over the course of a day is truly a marvel worth enjoying.

Photo Credits: [1] [2]

Further Reading: [1]

Resin Midges, Basal Angiosperms, and a Strange Pollination Syndrome

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When we try to talk about clades that are "basal" or "sister" to large taxonomic groups, your average listener either consciously or unconsciously thinks "primitive." Primitive has connotations of something that under-developed or unfinished. This is simply not the case. Take, for instance, a family of basal angiosperms called Schisandraceae.

This family is nestled within the order Austrobaileyales, which, along with a small handful of other families, represent the earliest branches of the angiosperm family tree still alive today.  To call them primitive, however, would be a serious misnomer. Because they diverged so early on, these lineages represent serious success stories in flowering plant evolution, having survived for hundreds of millions of years. Instead, we must think of them as fruitful early experiments in angiosperm evolution.

Floral morphology of and interaction between midge and their larvae (white arrows) in Illicium dunnianum

Still, the proverbial proof is in the pudding and if there was any sort of physical evidence one could put forth to remove our hierarchical prejudices about the taxonomic position of these plants, it would have to be their bizarrely specific pollination syndromes.  Members of the family Schisandraceae have entered into intense relationships with a group of flies known as midges and their interactions are anything but primitive. 

We will start with two species of plant native throughout parts of Asia. Meet as Illicium dunnianum and Illicium tsangii. More will be familiar with this genus than they may realize as Illicium gives us the dreaded star anise flavor our grandparents liked to sneak into our cookies as kids (but I digress). These particular species, however, have more to offer the world than flavoring. They are also very important plants for a group of gall midges in the genus Clinodiplosis.

The midges cannot reproduce without I. dunnianum or I. tsangii. You see, these midges lay their eggs within the flowers of these plants and, in doing so, end up pollinating them in the process. At first glance it may seem like a very one-sided relationship. Female midges deposit their eggs all along the carpels packed away inside large, fleshy whorl of tepals. As the midges crawl all over the reproductive organs looking for a suitable place to lay, they inevitably pick up and deposit pollen. 

Floral morphology and interaction between midge larvae (white arrows) in  Illicium tsangii

This is not the end of this relationship though. After eggs have been deposited, something strange happens to the Illicium flowers. For starters, they develop nursery chambers around the midge larvae. Additionally, their tepals begin producing heat. Enough heat is produced to keep the nursery chamber temperature significantly warmer than the ambient air temperature. What's more flower heating intensifies throughout the duration of fruit development. It was originally hypothesized that this heating had something to do with floral odor volatilization and seed incubation, however, experiments have shown that at least seed development in these two shrubs is not influenced by floral heat in any major way. The same cannot be said for the midge larvae. 

As the flowers mature and give way to developing seeds, the midge larvae are hard at work feeding on tiny bits of the flowers themselves. When researchers looked at midge larvae development on these Illicium species, they found that they were completely dependent upon the floral heat for survival. Any significant drop in temperature caused them to die. Essentially, the plants appear to be producing heat more for the midges than for themselves. It may seem odd that these two plants would invest so much energy to heat their flowers so that midge larvae feeding on their tissues can survive but such face-value opinions rarely stand in ecology.

One must not forget that those larvae grow up to be adult midges that will go on to pollinate the Illicium flowers the following season. Although the plants are taking a bit of a hit by allowing the larvae to develop within their tissues, they are nonetheless ensuring that enough pollinators will be around to repeat the process again. If that wasn't cool enough, the relationship between each of these plants and their pollinators are rather specific and the authors of at least one paper believe that the midges that pollinate each species are new to science. 

Now, if I haven't managed to convince you that this angiosperm sister lineage is anything but primitive, then let's take a look at another genus within the family Schisandraceae that have taken this midge pollination syndrome to the next level. This story also takes place in Asia but instead involves a genus of woody vines known as Kadsura

Like the Illicium we mentioned earlier, a handful of Kadsura species rely on midges for pollination. The way in which they go about maintaining this relationship is a bit more involved. The midges that are attracted by Kadsura flowers are known as resin midges and their larvae live off of plant resins. The flowers of Kadsura are another story entirely. They are as odd as they are beautiful. 

Flowers, pollinators ,and their larvae (white arrows) in  Kadsura heteroclita .

Flowers, pollinators ,and their larvae (white arrows) in Kadsura heteroclita.

In male flowers, stamens are arranged in dense, cone-like structures called androecia whereas the female flowers contain a compact shield-like structure with the uppermost part of the stigma barely emerging. This is called a gynoecium. Even weirder, the male flowers of one particularly strange species, Kadsura coccinea, produce large, swollen inner tepals. 

Once Kadsura flowers begin to open, visiting midges are not far behind. Male flowers seem to attract more midges than female flowers and it is thought that this has to do with varying amounts of special attractant chemicals produced by the flowers themselves. Regardless, midges set to work exploring the blooms with males looking for mates and females looking for a place to lay their eggs. 

When a suitable spot has been found, females will deposit their eggs into the floral tissues with their ovipositor. The wounded plant tissues immediately begin producing resin, not unlike a wounded pine tree. In the case of K. coccinea, it would appear that the oddly swollen tepals are specifically targeted by female midges for egg laying. They too produce resin upon having eggs laid within. 

The oddball flowers of Kadsura coccinea showing swollen tepals.

The function of plant resins in many cases are to fight off pathogens. From beetles to fungi, resin helps plug up and seal off wounds. This does not seem to be the case in the Kadsura-midge relationship though. The so-called "brood chambers" within the floral tissues go on producing resin for upwards of 6 days after the midge eggs were laid. Eventually the floral parts whither and drop off but the midge larvae seem to be quite happy in their resin-filled homes. 

As it turns out, the resin midge larvae feed on the viscous resin as their sole food source. Instead of trying to ward off these pesky little insects, the plants seem to be encouraging them to raise their offspring within! Just as we saw in the Asian Illicium, these Kadsura vines seem to be providing brood sites for their pollinators. Also, just as the Illicium-midge relationship thought to be species specific, each species of Kadsura appears to have its own specific species of resin midge pollinator! K. coccinea even goes as far as to produce tepals specifically geared towards raising midge larvae, thus keeping them away from their more valuable reproductive organs. In return for the nursery service, Kadsura have their pollinators all to themselves.

Pollination mutualisms in which plants trade raising larvae for pollen transfer are extremely derived and some of the most specialize plant/animal interactions on the planet. To find such relationships in these basal or sister lineages is living proof that these plants are anything but primitive. In the energy-reproductive investment trade-off, it appears that ensuring ample pollinator opportunities far outweighs the cost of providing them with nursery chambers. It is remarkable to think just how intertwined the relationships between these plants and there pollinators truly are. Take that, plant taxonomic prejudices! 

Photo Credits: [1] [2]

Further Reading: [1] [2] 

 

The Nitrogen-Fixing Abilities of Cycads

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Long before the first legumes came onto the scene, the early ancestors of Cycads were hard at work fixing atmospheric nitrogen. However, they don't do this on their own. Despite being plentiful in Earth's atmosphere, gaseous nitrogen is not readily available to most forms of life. Only a special subset of organisms are capable of turning gaseous nitrogen into forms usable for life. Some of the first organisms to do this were the cyanobacteria, which has led them down the path towards symbioses with various plants on many occasions. 

Cycads are but one branch of the gymnosperm tree. Their lineage arose at some point between the Carboniferous and Permian eras. Throughout their history it would seem that Cycads have done quite well in poor soils. They owe this success to a partnership they struck up with cyanobacteria. Although it is impossible to say when exactly this happened, all extant cycads we know of today maintain this symbiotic relationship with these tiny prokaryotic organisms. 

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

The relationship takes place in Cycad roots. Cycads don't germinate with cyanobacteria in tow. They must acquire them from their immediate environment. To do so, they begin forming specialized structures called precoralloid roots. Unlike other roots that generally grow downwards, these roots grow upwards. They must situate themselves in the upper layer of soil where enough light penetrates for cyanobacteria to photosynthesize.

The cyanobacteria enter into the precoralloid roots through tiny cracks and take up residence. This causes a change in root development. The Cycad then initiates their development into true coralloid roots, which will house the cyanobacteria from that point on. Cycads appear to be in full control of the relationship, dolling out carbohydrates in return for nitrogen depending on the demands of their environment. Coralloid roots can shed and reform throughout the lifetime of the plant. It is quite remarkable to think about how nitrogen-fixing symbiotic relationships between plants and microbes have evolved independently throughout the history of life on this planet.

Photo Credits: [1] [2]

Further Reading: [1] [2]

 

The Hidden Anatomy of Grass Flowers

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Grass flowers have their own unique beauty. Examine them with a hand lens and a whole new world of angiosperm diversity suddenly opens up. Unlike other flowering plants, their charm lies not in showy sepals or petals, but in an intricacy centered around the utilization of wind for pollination. However, such floral organs are not lacking. Grass flowers do in fact produce a perianth, the function of which has been highly modified.

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To see what I am referring to, you need to do some dissection under a scope. Pull off a flower and peel away the sheaths (the palea and lemma) that cover it. Inside you will see an ovary complete with feathery stigmas as well as the anthers. At the base of the ovary sits a pair of scales called lodicules. These lodicules are thought to be the rudimentary remains of the perianth. They certainly don't resemble sepals or petals but that is because the function of these structures is not to attract pollinators. They assist in pollination in another way.

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When grass flowers are ready for reproduction, the lodicules begin to swell. This swelling serves to push apart the rigid palea and lemma that protected the flowering parts as they developed. Once apart, the anthers and stigma are free to emerge and let wind do the dirty work for them. Lodicules differ quite a bit from species to species in their size, shape, and overall appearance. Much of this is likely tied to the overall structure in grass flowers.

Photo Credits: [1] [2]

Further Reading: [1]

 

Evidence Of Carnivory In Teasel

As far as carnivorous plants are concerned, the common teasel (Dipsacus fullonum) seems like a strange fit. Observe this plant up close, however, and you might notice something interesting. Its leaves are perfoliate and form a cup-like depression where they attach to the main stem. Not only does this cup regularly fill with water, it also frequently traps small insects.

Many have speculated over the function of this anatomical trap. Much of this speculation has centered around the idea that it may serve as a form of protection for the flowers located above. Insect herbivores climbing up the stem in search of food instead find a moat of water. Some inevitably fall in and drown in the process. Other hypotheses have been put forward as well including the possibility of something approaching carnivory. 

The idea that common teasel could be, to some degree, carnivorous never really went away. For most of this time it has remained entirely theoretical. There simply was no empirical evidence available to say otherwise. All of that changed with a 2011 study published in PLOS. A research duo finally put this theory to the test in the first ever experiment to see if teasel gains any sort of nutrient benefit from its insect victims.

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By systematically supplying teasel plants with insect prey, the team was able to look at how plants responded to the addition of a potential meal. They added various levels of insect larvae to some plants and removed them from others. For their study, evidence would come in the form of some sort of physiological response to the feeding treatments. If teasel really is obtaining nutrients from its insect victims, it stands to reason that those nutrients would be allocated to either growth or reproduction.

The resulting data offers the first evidence that teasel may in fact be benefiting from the insect carcasses. Although the team found no evidence that plants supplemented with insects were increasing in overall biomass, they did see a positive effect on not only the number of seeds produced but also their size. In other words, when fed a diet of insects, the plants weren't growing any larger but they were producing larger amounts of heavier seeds. This is a real boon for a plant with a biennial life cycle like teasel. The more healthy seeds they can produce, the better.

As exciting as these finds are, one must temper their expectations. As the authors themselves state in their paper, these findings must be replicated in order to say for certain that the effects they measured were due to the addition of insect prey. Second, no chemical analyses were made to determine if the plants are actively digesting these insects or even how available nutrients may be absorbed. Simply put, more work is needed. Perhaps teasel is a species that, evolutionary speaking, is on its way to becoming a true carnivore. We still can't say for sure. Nonetheless, they have given us the first evidence in support of a theory that went more than a century without testing. It is interesting to think that there is a strong possibility that if someone wants to see a carnivorous plant, they need go no further than a fallow field.

Photo Credits: [1] [2]

Further Reading: [1]

Red or White?

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Who doesn't love a nice oak tree? One cannot overstate their importance both ecologically and culturally. Although picking an oak tree out of a lineup is something many of us are capable of doing, identifying oaks to species can be a bit more challenging. This is further complicated by the fact that oaks often hybridize. Still, it is likely you have come across some useful tips and tricks for narrowing down your oak choices. One such trick is distinguishing between the red oaks and the white oaks. If you're anything like me, this is something you took for granted for a while. Is there anything biologically or ecologically meaningful to such a split?

In short, yes. However, a true appreciation of these groups requires a deeper look. To start with, oaks are members of the genus Quercus, which belongs in the family Fagaceae. Globally there are approximately 400 species of oak and each falls into one of three categories - the red oaks (section Lobatae), the white oaks (section Quercus), and the so-called "intermediate" oaks (section Protoblanus). For the sake of this article, I will only be focusing on the red and white groups as that is where most of the oak species reside. The intermediate oak group is made up of 5 species, all of which are native to the southwestern United States and northwestern Mexico.

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As is common with oak identification, reliable techniques for distinguishing between the two groups can be tricky. Probably the most reliable feature is located on the inner surface of the acorn cap. In white oaks, it is hairless or nearly so, whereas in red oaks, it is covered in tiny hairs. Another useful acorn feature is the length of time it takes them to germinate. White oak acorns mature in one season and germinate in the fall. As such, they contain lower levels of tannins. Red oak acorns (as well as those of the intermediate group) generally take at least two seasons to mature and therefore germinate the following spring. Because of this, red oak acorns have a much higher tannin content. For more information on why this is the case, read this article.

Tyloses in white oak xylem.

Less apparent than acorns is the difference in the wood of red and white oaks. The wood of white oaks contains tiny structures in their xylem tissues called tyloses. These are absent from the wood of red oaks. The function of tyloses are quite interesting. During extreme drought or in the case of some sort of infection, they cut off regions of the xylem to stop the spread of an embolism or whatever may be infecting the tree. As such, white oaks tend to be more rot and drought resistant. Fun fact, tyloses are the main reason why white oak is used for making wine and bourbon barrels as it keeps them from leaking their contents.

More apparent to the casual observer, however, is leaf shape. In general, the white oaks produce leaves that have rounded lobes, whereas the red oaks generally exhibit pointed lobes with a tiny bristle on their tips. At this point you may be asking where an unlobed species like shingle oak (Quercus imbricaria) fits in. Look at the tip of its leaf and you will see a small bristle, which means its a member of the red oak group. Similarly, the buds of these two groups often differ in their overall shape. White oak buds tend to be smaller and often have blunted tips whereas the buds of red oaks are generally larger and often pointed.

Tricky leaves of the shingle oak ( Quercus imbricaria ). Note the bristle tip!

Tricky leaves of the shingle oak (Quercus imbricaria). Note the bristle tip!

Despite this broad generalizations, exceptions abound. This is further complicated by the fact that many species will readily hybridize. Quercus is, after all, a massive genus. Regardless, oaks are wonderful species chock full of ecological and cultural value. Still, oak appreciation is something we all need more of in our lives. I encourage you to try some oak identification of your own. Get outside and see if you can use any of these tricks to help you identify some of the oaks in your neighborhood.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3]