Meet the Blazing Stars

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Midsummer in North America is, among other things, Liatris season. These gorgeous plants are often referred to as blazing stars or gayfeathers, which hints at the impact their flowers have on our psyche. Whether in the garden or in the wild, Liatris are a group of plants worth getting to know a bit better.

Liatris is by and large a North American genus with only a single species occurring in the Bahamas. Though we often think of Liatris as prairie plants, the center of diversity for this group is in the southeastern United States. Taxonomically speaking, Liatris are a bit of a conundrum. Something like 40 different species have been described and, where ranges overlap, many putative hybrids have been named.

 Rocky Mountain blazing star ( Liatris ligulistylis )

Rocky Mountain blazing star (Liatris ligulistylis)

Authorities on this group cite ample confusion when it comes to drawing lines between species. Much of this confusion comes from the fact that numerous variants and intergradations exist between the various species. As mentioned, hybridization is not uncommon in this genus, which complicates matters quite a bit.

 Prairie blazing star ( Liatris pycnostachya )

Prairie blazing star (Liatris pycnostachya)

Liatris as a whole appears to have undergone quite an adaptive radiation in North America, with species adapting to specific soils and habitat types. Take, for instance, the case of cylindrical blazing star (L. cylindracea), marsh blazing star (L. spicata), and rough blazing star (L. aspera). The ranges of these species overlap to quite a degree, however, each prefers to grow in soils of specific texture and moisture. Marsh blazing star, as you may have guessed, prefers wetter soils whereas rough blazing star enjoys drier habitats. Cylindrical blazing star seems to enjoy intermediate soil conditions, especially where soil pH is a bit higher. As such, these three species often occur in completely different habitats. However, in places like the southern shores of Lake Michigan, they find themselves growing in close quarters and as a result, a fair amount of hybridization has occurred.

 Rough blazing star ( Liatris aspera )

Rough blazing star (Liatris aspera)

Another example of confusion comes from a species commonly known as the savanna blazing star (Liatris scariosa nieuwlandii). Many different ecotypes of this plant exist and some experts don't quite know how to deal with them all. Sometimes savanna blazing star is treated as a variant of another species called the northern blazing star (Liatris scariosa var. nieuwlandii) and sometimes it is treated as its own distinct species (Liatris nieuwlandii). Until proper genetic work can be done, it is impossible to say which, if any, are correct. 

 Glandular blazing star ( Liatris glandulosa )

Glandular blazing star (Liatris glandulosa)

Taxonomic confusion aside, the various Liatris species and variants are important components of the ecology wherever they occur. Numerous insects feed upon and raise their young on the foliage and few could argue against their flowers as pollinator magnets. All Liatris produce pink to purple flowers in splendid Asteraceae fashion. Every once in a while, an aberrant form is produced that sports white flowers. Though horticulturists have capitalized on this for the garden, at least one authority claims that these white forms are much weaker than their pink flowering parents. At least one species, the pinkscale blazing star (L. elegans), produces large, filamentous white bracts that very much resemble flowers.

 Check out the bracts on the pinkscale blazing star ( L. elegans )!

Check out the bracts on the pinkscale blazing star (L. elegans)!

Liatris are just as interesting below as they are above. The roots, foliage, and flowers all emerge from a swollen underground stem called a corm. The formation of these corms is one reason why some Liatris species have become so popular in our gardens. It makes them extremely hardy during the dormant season. In the spring, the corm starts forming roots. At the same time, tiny preformed buds at the top of the corm begin to grow this years crop of leaves and flowers. By the end of the growing season, the corm has reached its maximum size for that year and the plant draws down the rest of its reserves to wait out the winter.

 Cylindrical blazing star ( Liatris cylindracea )

Cylindrical blazing star (Liatris cylindracea)

During this time, some species form a layer of tissue along the edge of the corm that is much darker in coloration than what was laid down earlier in the season. This has led some to suggest that aging individual Liatris is possible. Experts believe that specimens can readily reach 30 to 40 years of age or more, however, the degree to which these dark bands indicate annual growth is up for a lot of debate. Others have found no correlation with plant age. Regardless, it is safe to say that many Liatris species can live for decades if left undisturbed.

 Scrub blazing star ( Liatris ohlingerae )

Scrub blazing star (Liatris ohlingerae)

All in all, Liatris is a very special, albeit slightly confusing, group of plants. It offers a little something for everyone. What's more, their beauty is only part of the story. These are ecologically important plants that support many great insect species. As summer wears on, make sure to get out there and enjoy the Liatris in your neck of the woods. You will be happy you did!

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

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

 

 

 

What Are Plants Made Of?

Have you ever thought about what plants are made of? I mean, really thought about it. Strip away all the splendor and glory of all the different plant species on this planet and really take a close look at how plants grow and make more plants. It is a fascinating realm and it all has to do with photosynthesis. To go from photons given off by our nearest star to a full grown plant is quite the journey and, at the end of that journey, you may be surprised to learn what plants are all about.

It starts with photons. Leaving the sun they travel out into the universe. Some eventually collide with Earth and make their way to the surface. Plants position their leaves to absorb these photons. Energy from the photons is used to split water molecules inside the chloroplasts. In the process of splitting water, oxygen is released as a byproduct (thanks plants!). Splitting water also releases electrons and hydrogen ions.

These electrons and hydrogen ions are used to make energy in the form of ATP. Along with some electrons, ATP is then used in another cycle known as the Calvin cycle. The point of the Calvin cycle is to take in CO2 and use the energy created prior to reduce carbon molecules into chains of organic molecules. Most of the carbon in a plant comes from the intake of CO2. Through a series of steps (I will spare you the details) plants piece together carbon atoms into long chains. Some of these chains form glucose and some of that glucose gets linked together into cellulose.

Cellulose is the main structural component of plant cells. From the smallest plants in the world (genus Wolffia) all the way up to the largest and tallest redwoods and sequoias (incidentally some of the largest organisms to have ever existed on this planet) , all of them are built out of cellulose. So, in essence, all the plant life you see out there is literally built from the ground up by carbon originating from CO2 gas. Pretty incredible stuff, wouldn't you agree?

Big Things Come In Small Packages

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Meet Blossfeldia liliputana, the smallest species of cactus in the world. With a maximum diameter of only 12 mm, this wonderful succulent would be hard to spot tucked in among the nooks and crannies of rock outcrops. Its species name "liliputana" is a reference to the fictional island of Liliput (Gulliver's Travels) whose inhabitants were said to be rather small. If its size alone wasn't interesting in and of itself, the biology of B. liliputana is also downright bizarre.

B. liliputana is native to arid regions between southern Bolivia and northern Argentina. It appears to prefer growing wedged between cracks in rock as these are usually the spots where just enough soil builds up to put down its roots. Root formation, however, does not happen for quite some time. Most often new individuals bud off from the parent plant. They emerge not from the base, but rather from apical tissues, yet another unique feature of this cactus. What's more, this cactus produces no spines. Instead, its numerous areoles are covered in a dense layer of trichomes that are rather felt-like to the touch.

As you can clearly see, this species is small. It only ever becomes conspicuous when it comes time to flower. Imagine a bunch of tiny white to pink cactus flowers poking out of a crevice. It must be a remarkable sight to see in person. Despite their showy appearance, its is believed that most are self-fertilized.

As mentioned, the size of this cactus isn't the only interesting thing about its biology. B. liliputana is categorized as a poikilohydric organism, meaning it doesn't have the ability to regulate its internal water content. Researchers have found that individual plants can lose up to 80% of their weight in water and can maintain that state for as long as two years without any negative effects. As such, colonies of these tiny cacti often appear shrunken or squished. Once the rains arrive, however, it springs back to its original rounded shape with seemingly no issues. Amazingly, a significant amount of water uptake happens via the fuzzy areoles that cover its surface, hence it does not harm the plant to hold off growing roots for quite some time. 

Speaking of water regulation, B. liliputana holds another record for having the lowest density of stomata of any terrestrial autotrophic vascular plant. Stomata are the pores in which plants regulate water and gas exchange so having so few may have something to do with why this species loses and gains water to such a degree that would kill most other vascular plant species.

Another peculiar quality of this cactus are its seeds. Unlike all other cacti whose seeds are hard and relatively smooth, the seeds of B. liliputana are hairy. Attached to each seed is a small fleshy structure called an aril, which aids in seed dispersal. As it turns out, B. liliputana relies on ants as its main seed dispersers. Ants attracted to the fleshy aril drag the seeds back to their nests, remove and eat the aril, and then discard the seed. This is often good news for the cactus because its seeds end up in nutrient-rich ant middens protected from both the elements and seed predators, often in much more suitable conditions for germination.

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Needless to say, B. liliputana is a bit of an oddball as far as cacti are concerned. Its highly derived features coupled with its bizarre biology has made it difficult for taxonomists to elucidate its relationship to the rest of the cactus family. It certainly deserves its own genus, to which it is the only member, however, it has been added to and removed form a handful of cactus subfamilies over the years. The most recent genetic analyses suggests that it is unique enough to warrant its own tribe within Cactaceae - Blossfeldieae.

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

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

How Aroids Turn Up the Heat

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A subset of plants have evolved the ability to produce heat, a fact that may come as a surprise to many reading this. The undisputed champions of botanical thermogenesis are the aroids (Araceae). Exactly why they do so is still the subject of scientific debate but the means by which heat is produced is absolutely fascinating.

The heat producing organ of an aroid is called the spadix. Technically speaking, a spadix is a spike of minute flowers closely arranged around a fleshy axis. All aroid inflorescences have one and they come in a wide variety of shapes, colors, and textures. To produce heat, the spadix is hooked up to a massive underground energy reserve largely in the form of carbohydrates or sugars. The process of turning these sugars into heat is rather complex and surprisingly animal-like.

 Cross section of a typical aroid inflorescence with half of the protective spathe removed. The spadix is situated in the middle with a rings of protective hairs (top), male flowers (middle), and female flowers (bottom).

Cross section of a typical aroid inflorescence with half of the protective spathe removed. The spadix is situated in the middle with a rings of protective hairs (top), male flowers (middle), and female flowers (bottom).

It all starts with a compound we are rather familiar with - salicylic acid - as it is the main ingredient in Aspirin. In aroids, however, salicylic acid acts as a hormone whose job it is to initiate both the heating process as well as the production of floral scents. It signals the mitochondria packed inside a ring of sterile flowers located at the base of the spadix to change their metabolic pathway.

In lieu of their normal metabolic pathway, which ends in the production of ATP, the mitochondria switch over to a pathway called the "Alternative Oxidase Metabolic Pathway." When this happens, the mitochondria start burning sugars using oxygen as a fuel source. This form of respiration produces heat.

 Thermal imaging of the inflorescence of  Arum maculatum .

Thermal imaging of the inflorescence of Arum maculatum.

As you can imagine, this can be a costly process for plants to undergo. A lot of energy is consumed as the inflorescence heats up. Nonetheless, some aroids can maintain this costly level of respiration intermittently for weeks on end. Take the charismatic skunk cabbage (Symplocarpus foetidus) for example. Its spadix can reach temperatures of upwards of 45 °F (7 °C) on and and off for as long as two weeks. Even more incredible, the plant is able to do this despite freezing ambient temperatures, literally melting its way through layers of snow.

For some aroids, however, carbohydrates just don't cut it. Species like the Brazilian Philodendron bipinnatifidum produce a staggering amount of floral heat and to do so requires a different fuel source - fat. Fats are not a common component of plant metabolisms. Plants simply have less energy requirements than most animals. Still, this wonderful aroid has converged on a fat-burning metabolic pathway that puts many animals to shame. 

 The inflorescence of  Philodendron bipinnatifidum  can reach temps as high as 115 °F (46 °C)

The inflorescence of Philodendron bipinnatifidum can reach temps as high as 115 °F (46 °C)

P. bipinnatifidum stores lots of fat in sterile male flowers that are situated between the fertile male and female flowers near the base of the spadix. As soon as the protective spathe opens, the spadix bursts into metabolic action. As the sun starts to set and P. bipinnatifidum's scarab beetle pollinators begin to wake up, heat production starts to hit a crescendo. For about 20 to 40 minutes, the inflorescence of P. bipinnatifidum reaches temperatures as high as 95 °F (35 °C) with one record breaker maxing out at 115 °F (46 °C)! Amazingly, this process is repeated again the following night.

It goes without saying that burning fat at a rate fast enough to reach such temperatures requires a lot of oxygen. Amazingly, for the two nights it is in bloom, the P. bipinnatifidum inflorescence consumes oxygen at a rate comparable to that of a flying hummingbird, which are some of the most metabolically active animals on Earth.

 The world's largest inflorescence belongs to the titan arum ( Amorphophallus titanum ) and it too produces heat.

The world's largest inflorescence belongs to the titan arum (Amorphophallus titanum) and it too produces heat.

Again, why these plants go through the effort of heating their reproductive structures is still a bit of a mystery. For most, heat likely plays a role in helping to volatilize floral scents. Anyone that has spent time around blooming aroids knows that this plant family produces a wide range of odors from sweet and spicy to downright offensive. By warming these compounds, the plant may be helping to lure in pollinators from a greater distance away. It is also thought that the heat may be an attractant in and of itself. This is especially true for temperate species like the aforementioned skunk cabbage, which frequently bloom during colder months of the year. Likely both play a role to one degree or another throughout the aroid family.

What we can say is that the process of plant thermogenesis is absolutely fascinating and well worth deeper investigation. We still have much to learn about this charismatic group of plants.

LEARN MORE ABOUT AROID POLLINATION HERE

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

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

 

Of Bluebells and Fungi

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Whether in your garden or in the woods, common bluebells (Hyacinthoides non-scripta) are a delightful respite from the dreary months of winter. It should come as no surprise that these spring geophytes are a staple in temperate gardens the world over. And, as amazing as they are in the garden, bluebells are downright fascinating in the wild.

Bluebells can be found growing naturally from the northwestern corner of Spain north into the British Isles. They are largely a woodland species, though finding them in meadows isn't uncommon. They are especially common in sites that have not experienced much soil disturbance. In fact, large bluebell populations are used as indicators of ancient wood lots.

Being geophytes, bluebells cram growth and reproduction into a few short weeks in spring. We tend to think of plants like this as denizens of shade, however, most geophytes get going long before the canopy trees have leafed out. As such, these plants are more accurately sun bathers. On warm days, various bees can be seen visiting the pendulous flowers, with the champion pollinator being the humble bumble bees.

The above ground beauty of bluebells tends to distract us from learning much about their ecology. That hasn't stopped determined scientists though. Plenty of work has been done looking at how bluebells make their living and get on with their botanical neighbors. In fact, research is turning up some incredible data regarding bluebells and mycorrhizal fungi.

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Bluebell seeds tend not to travel very far, most often germinating near the base of the parent. Germination occurs in the fall when temperatures begin to drop and the rains pick up. Interestingly, bluebell seeds actually germinate within the leaf litter and begin putting down their initial root before the first frosts. Often this root is contractile, pulling the tiny seedling down into the soil where it is less likely to freeze. During their first year, phosphorus levels are high. Not only does the nutrient-rich endosperm supply the seedling with much of its initial needs, abundant phosphorus near the soil surface supplies more than enough for young plants. This changes as the plants age and change their position within the soil.

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Over the next 4 to 5 years, the bluebell's contractile roots pull it deeper down into the soil, taking it out of the reach of predators and frost. This also takes them farther away from the nutrient-rich surface layers. What's more, the roots of older bluebells are rather simple structures. They do not branch much, if at all, and they certainly do not have enough surface area for proper nutrient uptake. This is where mycorrhizae come in.

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Bluebells partner with a group of fungi called arbuscular mycorrhiza, which penetrate the root cells, thus greatly expanding the effective rooting zone of the plant. Plants pay these fungi in carbohydrates produced during photosynthesis and in return, the fungi provide the plants with access to far more nutrients than they would be able to get without them. One of the main nutrients plants gain from these symbiotic fungi is phosphorus.

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For bluebells, with age comes new habitat, and with new habitat comes an increased need for nutrients. This is why bluebells become more dependent on arbuscular mycorrhiza as they age. In fact, plants grown without these fungi do not come close to breaking even on the nutrients needed for growth and maintenance and thus live a shortened life of diminishing returns. This is an opposite pattern from what we tend to expect out of mycorrhizal-dependent plants. Normally its the seedlings that cannot live without mycorrhizal symbionts. It just goes to show you that even familiar species like the bluebell can offer us novel insights into the myriad ways in which plants eke out a living.

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

Further Reading: [1] [2]

 

One Mustard, Many Flavors

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What do kale, broccoli, cauliflower, Brussel sprouts, and cabbage have in common? They are all different cultivars of the same species!

Wild cabbage (Brassica oleracea) is native to coastal parts of southern and western Europe. In its native habitat, wild cabbage is very tolerant of salty, limey soils but not so tolerant of competition. Because of this, it tends to grow mainly on limestone sea cliffs where few other plants can dig their roots in.

Despite their popularity as delicious, healthy vegetables, as well as their long history of cultivation, there is scant record of this plant before Greek and Roman times. Some feel that this is one of the oldest plants in cultivation. Along with the countless number of edible cultivars, the wild form of Brassica oleracea can be found growing throughout the world, no doubt thanks to its popularity among humans.

I am always amazed by how little we know about crop wild relatives. Despite the popularity of its many agricultural cultivars, relatively little attention has been paid to B. oleracea in the wild. What we do know is that at least two subspecies have been identified - B. oleracea ssp. bourgeaui and B. oleracea L. ssp. oleracea. As far as anyone can tell, subspecies 'oleracea' is the most wide spread in its distribution whereas subspecies 'bourgeaui'  is only known from the Canary Islands. 

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B. oleracea's long history with humans confuses matters quite a bit. Because it has been cultivated for thousands of years, identifying which populations represent wild individuals and which represent ancient introductions is exceedingly difficult. Such investigations are made all the more difficult by a lack of funding for the kind of research that would be needed to elucidate some of these mysteries. We know so little about wild B. oleracea that the IUCN considers is a species to be "data deficient."

It seems to appreciate cool, moist areas and will sometimes escape from cultivation if conditions are right, thus leading to the confusion mentioned above. It is amazing to look at this plant and ponder all the ways in which humans have selectively bred it into the myriad shapes, sizes, and flavors we know and love (or hate) today! However, we must pay more attention to the wild progenitors of our favorite crops. They harbor much needed genetic diversity as well as clues to how these plants are going to fare as our climates continue to change.

Photo Credit: [1] [2]

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

North America's Pachysandra

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In the interest of full disclosure, I have never been a fan of garden variety Pachysandra. Long before I had any interest in plants or gardening, there was something about this groundcover that simply did not appeal to me. Fast forward more than a decade and my views on the use of Asian Pachysandra in the garden have not changed much. You can imagine my surprise then when I learned that North America has its own representative of this genus - the Allegheny spurge (Pachysandra procumbens).

My introduction to P. procumbens happened during a tour of the Highlands Botanical Garden in Highlands, North Carolina. I recognized its shape and my initial reaction was alarm that a garden specializing in native plants would showcase a non-native species. My worry was quickly put to rest as the sign informed me that this lovely groundcover was in fact indigenous to this region. Indeed, P. procumbens can be found growing in shady forest soils from North Carolina down to Florida and Texas.

This species is yet another representative of a curious disjunction in major plant lineages between North America and eastern Asia. Whereas North America has this single species of Pachysandra, eastern Asia boasts two, P. axillaris and P. terminalis. Such a large gap in the distribution of this genus (as well as many others) seems a bit strange until one considered the biogeographic history of the two continents.

Many thousands of years ago, sea levels were much lower than they are today. This exposed land bridges between continents which today are hundreds of feet under water. During favorable climatic periods, Asia and North America likely shared a considerable amount of their respective floras, a fact we still find evidence of today. The Pachysandra are but one example of a once connected distribution that has been fragmented by subsequent sea level rise. Fossil records of Pachysandra have been found in regions of British Columbia, Washington, Oregon, Wyoming, and North and South Dakota and provide further confirmation of this.

As a species, P. procumbens is considered a subshrub. It is slow growing but given time, populations can grow to impressive sizes. In spring, numerous fragrant, white flower spikes emerge that are slowly eclipsed by the flush of spring leaf growth. The flowers themselves are intriguing structures worthy of close inspection. Their robust form is what gives this genus its name. "Pachys" is Greek for thick and "andros" is Greek for male, which refers to the thickened filaments that support the anthers.

It is hard to say for sure why this species is not as popular in horticulture as its Asian cousins. It tolerates a wide variety of soil types and does well in shade. What's more, it is mostly ignored by all but the hungriest of deer. And, at the end of the day, it took this species to change my mind about Pachysandra. After all, each and every species has a story to tell.

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

Further Reading: [1] [2]

Apocynaceae Ant House

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The dogbane family, Apocynaceae, comes in many shapes, sizes, and lifestyles. From the open-field milkweeds we are most familiar with here in North America to the cactus-like Stapeliads of South Africa, it would seem that there is no end to the adaptive abilities of this family. Being an avid gardener both indoors and out, the diversity of Apocynaceae means that I can be surrounded by these plants year round. My endless quest to grow new and interesting houseplants was how I first came to know a genus within the family that I find quite fascinating. Today I would like to briefly introduce you to the Dischidia vines.

 Bullate leaves help the vine clasp to the tree as well as house ant colonies.

Bullate leaves help the vine clasp to the tree as well as house ant colonies.

The genus Dischidia is native to tropical regions of China. Like its sister genus Hoya, these plants grow as epiphytic vines throughout the canopy of warm, humid forests. Though they are known quite well among those who enjoy collecting horticultural curiosities, Dischidia as a whole is relatively understudied. These odd vines do not attach themselves to trees via spines, adhesive pads, or tendrils. Instead, they utilize their imbricated leaves to grasp the bark of the trunks and branches they live upon.

 The odd, bulb-like leaves of the urn vine ( Dischidia rafflesiana )

The odd, bulb-like leaves of the urn vine (Dischidia rafflesiana)

One thing we do know about this genus is that most species specialize in growing out of arboreal ant nests. Ant gardens, as they are referred to, offer a nutrient rich substrate for a variety of epiphytic plants around the world. What's more, the ants will visciously defend their nests and thus any plants growing within.

 The flowers of   Dischidia ovata

The flowers of Dischidia ovata

Some species of Dischidia take this relationship with ants to another level. A handful of species including D. rafflesiana, D. complex, D. major, and D. vidalii produce what are called "bullate leaves." These leaves start out like any other leaf but after a while the edges stop growing. This causes the middle of the leaf to swell up like a blister. The edges then curl over and form a hollow chamber with a small entrance hole.

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These leaves are ant domatia and ant colonies quickly set up shop within the chambers. This provides ample defense for the plant but the relationship goes a little deeper. The plants produce a series of roots that crisscross the inside of the leaf chamber. As ant detritus builds up inside, the roots begin to extract nutrients. This is highly beneficial for an epiphytic plant as nutrients are often in short supply up in the canopy. In effect, the ants are paying rent in return for a place to live.

Growing these plants can take some time but the payoff is worth. They are fascinating to observe and certainly offer quite a conversation piece as guests marvel at their strange form.

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

Further Reading: [1]

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 Air Plants Drink

   Tillandsia tectorum

 Tillandsia tectorum

Air plants (genus Tillandsia) are remarkable organisms. All it takes is seeing one in person to understand why they have achieved rock start status in the horticulture trade. Unlike what we think of as a "traditional" plant lifestyle, most species of air plants live a life free of soil. Instead, they attach themselves to the limbs and trunks of trees as well as a plethora of other surfaces. 

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Living this way imposes some serious challenges. The biggest of these is the acquisition of water. Although air plants are fully capable of developing roots, these organs don't live very long and they are largely incapable of absorbing anything from the surrounding environment. The sole purpose of air plant roots is to anchor them to whatever they are growing on. How then do these plants function? How do they obtain water and nutrients? The answer to this lies in tiny structures called trichomes. 

Trichomes are what gives most air plants their silvery sheen. To fully appreciate how these marvelous structures work, one needs some serious magnification. A close inspection would reveal hollow, nail-shaped structures attached to the plant by a stem. Instead of absorbing water directly through the leaf tissues, these trichomes mediate the process and, in doing so, prevent the plant from losing more water than it gains. 

The trichomes themselves start off as living tissue. During development, however, they undergo programmed cell death, leaving them hollow. When any amount of moisture comes into contact with these trichomes, they immediately absorb that water, swelling up in the process. As they swell, they are stretched out flat along the surface of the leaf. This creates a tiny film of water between the trichomes and the rest of the leaf, which only facilitates the absorption of more water. 

 Trichomes up close.  

Trichomes up close.  

Because the trichomes form a sort of conduit to the inside of the leaf, water and any nutrients dissolved within are free to move into the plant until the reach the spongy mesophyll cells inside. In this way, air plants get all of their water needs from precipitation and fog. Not all air plants have the same amount of trichomes either. In fact, trichome density can tell you a lot about the kind of environment a particular air plant calls home. 

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The fuzzier the plant looks, the drier the habitat it can tolerate. Take, for instance, one of the fuzziest air plants - Tillandsia tectorum. This species hails from extremely arid environments in the high elevation regions of Ecuador and Peru. This species mainly relies on passing clouds and fog for its moisture needs and thus requires lots of surface area to collect said water. Now contrast that with a species like Tillandsia bulbosa, which appears to have almost no trichome cover. This smoother looking species is native to humid low-land habitats where high humidity and frequent rain provide plenty of opportunities for a drink. 

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Absorbing water in this way would appear to have opened up a plethora of habitats for the genus Tillandsia. Air plants are tenacious plants and worthy of our admiration. One could learn a lot from their water savvy ways. 

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

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

Ferns Afloat

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My introduction to the genus Salvinia was as an oddball aquarium plant floating in a display tank at the local pet store. I knew nothing about plants at the time but I found it to be rather charming nonetheless. Every time the green raft of leaves floated under the filter outlet, water droplets would bead off them like water off of a ducks back. Even more attractive were the upside down forest of "roots" which were actively sheltering a bunch of baby guppies. 

I grew some Salvinia for a few years before my interest in maintaining aquariums faded. I had forgotten about them for quite some time. Much later as I was diving into the wild world of botany, I started revisiting some of the plants that I had grown in various aquariums to learn more about them. It wasn't long before the memory of Salvinia returned. A quick search revealed something quite astonishing. Salvinia are not flowering plants. They are ferns! 

The genus Salvinia is quite wide spread. They can be found growing naturally throughout North, Central, and South America, the West Indies, Europe, Africa, and Madagascar. Sadly, because of their popularity as aquarium and pond plants, a few species have become extremely aggressive invaders in many water ways. More on that in a bit. 

Salvinia is comprised of roughly 12 different species. Of these, at least 4 are suspected to be naturally occurring hybrids. As you have probably already gathered, these ferns live out their entire lives as floating aquatic plants. Their most obvious feature are the pairs of fuzzy green leaves borne on tiny branching stems. These leaves are covered in trichomes that repel water, thus keeping them dry despite their aquatic habit. 

 These are not roots!

These are not roots!

Less obvious are the other types of leaves these ferns produce. What looks like roots dangling below the water's surface are actually highly specialized, finely dissected leaves! I was quite shocked to learn this and to be honest, it makes me appreciate these odd little ferns even more. Its on these underwater leaves that the spores are produced. Specialized structures called sporocarps form like tiny nodules on the tips of the leaf hairs.

Sporocarps come in two sizes, each producing its own kind of spore. Large sporocarps produce megaspores while the smaller sporocarps produce microspores. This reproductive strategy is called heterospory. Microspores germinate into gametophytes containing male sex organs or "antheridia" whereas the megaspores develop into gametophytes containing female sex organs or "archegonia." 

As I mentioned above, some species of Salvinia have become aggressive invaders, especially in tropical and sub-tropical water ways. Original introductions were likely via plants released from aquariums and ponds but their small spores and vegetative growth habit means new introductions occur all too easily. Left unchecked, invasive Salvinia can form impenetrable mats that completely cover entire bodies of water and can be upwards of 2 feet thick!

 Sporocarps galore! 

Sporocarps galore! 

Lots of work has been done to find a cost effective way to control invasive Salvinia populations. A tiny weevil known scientifically as Cyrtobagous singularis has been used with great success in places like Australia. Still, the best way to fight invasive species is to prevent them from spreading into new areas. Check your boots, check your boats, and never ever dump your aquarium or pond plants into local water ways. Provided you pay attention, Salvinia are rather fascinating plants that really break the mold as far as most ferns are concerned. 

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

 

How Trees Fight Disease

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Plants do not have immune systems like animals. Instead, they have evolved an entirely different way of dealing with infections. In trees, this process is known as the "compartmentalization of decay in trees" or "CODIT." CODIT is a fascinating process and many of us will recognize its physical manifestations.

In order to understand CODIT, one must know a little something about how trees grow. Trees have an amazing ability to generate new cells. However, they do not have the ability to repair damage. Instead, trees respond to disease and injury  by walling it off from their living tissues. This involves three distinct processes. The first of these has to do with minimizing the spread of damage. Trees accomplish this by strengthening the walls between cells. Essentially this begins the process of isolating whatever may be harming the living tissues.

This is done via chemical means. In the living sapwood, it is the result of changes in chemical environment within each cell. In heartwood, enzymatic changes work on the structure of the already deceased cells. Though the process is still poorly understood, these chemical changes are surprisingly similar to the process of tanning leather. Compounds like tannic and gallic acids are created, which protect tissues from further decay. They also result in a discoloration of the surrounding wood. 

The second step in the CODIT process involves the construction of new walls around the damaged area. This is where the real compartmentalization process begins. The cambium layer changes the types of cells it produces around the area so that it blocks that compartment off from the surrounding vascular tissues. These new cells also exhibit highly altered metabolisms so that they begin to produce even more compounds that help resist and hopefully stave off the spread of whatever microbes may be causing the injury. Many of the defects we see in wood products are the result of these changes.

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The third response the tree undergoes is to keep growing. New tissues grow around the infected compartment and, if the tree is healthy enough, will outpace further infection. You see, whether its bacteria, fungi, or a virus, microbes need living tissues to survive. By walling off the affected area and pumping it full of compounds that kill living tissues, the tree essentially cuts off the food supply to the disease-causing organism. Only if the tree is weakened will the infection outpace its ability to cope.

Of course, CODIT is not 100% effective. Many a tree falls victim to disease. If a tree is not killed outright, it can face years or even decades of repeated infection. This is why we see wounds on trees like perennial cankers. Even if the tree is able to successfully fight these repeat infections over a series of years, the buildup of scar tissues can effectively girdle the tree if they are severe enough.

CODIT is a well appreciated phenomenon. It has set the foundation for better tree management, especially as it relates to pruning. It is even helping us develop better controls against deadly invasive pathogens. Still, many of the underlying processes involved in this response are poorly understood. This is an area begging for deeper understanding.

Photo Credits: kaydubsthehikingscientist & Alex Shigo

Further Reading: [1]

The Strangest Wood Sorrel

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For me, wood sorrels are a group of plants I usually have to look down to find. This is certainly not the case for Oxalis gigantea. Native to the coastal mountains of northern Chile, this bizarre Oxalis has forgone the traditional herbaceous habit of its cousins in exchange for a woody shrub-like growth form.

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When I first laid eyes on O. gigantea, I thought I was looking at some strange form of Ocotillo. In front of me was a shrubby plant consisting of multiple upright branches that were covered in a dense layer of shiny green leaves occasionally interrupted by yellow flowers. You would think that at this point in my life, aberrant taxa would not longer surprise me. Think again. 

O. gigantea is one of the largest of the roughly 570 Oxalis species known to science. Its woody branches can grow to a height of 2 meters (6 feet)! The branches themselves are quite interesting to look at. They are covered in woody spurs from which clusters of traditional Oxalis-style leaves emerge. Each stem is capable of producing copious amounts of flowers all throughout the winter months. The flowers are said to be pollinated by hummingbirds but I was not able to find any data on this. 

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This shrub is but one part of the Atacama Desert flora. This region of Chile is quite arid,  experiencing a 6 to 10 month dry season every year. What rain does come is often sparse. Any plant living there must be able to cope. And cope O. gigantea does! This oddball shrub is deciduous, dropping its leaves during the dryer months. During that time, these shrubs look pretty ragged. You would never guess just how lush it will become once the rains return. Also, it has a highly developed root system, no doubt for storing water and nutrients to tide them over.  

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O. gigantea has enjoyed popularity as a horticultural oddity over the years. In fact, growing this shrub as a container plant is said to be quite easy. Despite its garden familiarity, O. gigantea is noticeably absent from the scientific literature. In writing this piece, I scoured the internet for any and all research I could find. Sadly, it simply isn't there.

This is all too often the case for unique and interesting plant species like O. gigantea. Like so many other species, it has suffered from the disdain academia has had for organismal research over the last few decades. We humans can and must do better than that. For now, what information does exist has come from horticulturists, gardeners, and avid botanizers from around the world. 

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

Further Reading: [1] [2] 

 

This Isn't Even My Final Form! A Pothos Story

Pothos might be one of the most widely cultivated plants in modern history. These vining aroids are so common that I don't think I can name a single person in my life that hasn't had one in their house at some point or another. Renowned for their hardy disposition and ability to handle extremely low light conditions, they have become famous the world over. They are so common that it is all too easy to forget that they have a wild origin. What's more, few of us ever get to see a mature specimen. The plants living in our homes and offices are mere juveniles, struggling to hang on as they search for a canopy that isn't there.

Trying to find information on the progenitors of these ubiquitous houseplants can be a bit confusing. To do so, one must figure out which species they are talking about. Without a proper scientific name, it is nearly impossible to know which plant to refer to. Common names aside, pothos have also undergone a lot of taxonomic revisions since their introduction to the scientific community. Also, what was thought to be a single species is actually a couple.

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To start with, the plants you have growing in your home are no longer considered Pothos. The genus Pothos seemed to be a dumping ground for a lot of nondescript aroid vines throughout the last century. Many species were placed there until proper materials were thoroughly scrutinized. Today, what we know as a "Pothos" has been moved into the genus Epipremnum. This revision did not put all controversies to rest, however, as the morphological changes these plants go through as they age can make things quite tricky.

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As I mentioned, the plants we keep in our homes are still in their juvenile form. Like all plants, these vines start out small. When they find a solid structure in a decent location, they make their bid for the canopy. Up in a tree in reach of life giving sunlight, these vines really hit their stride. They quickly grow their own version of a canopy that consists of massive leaves nearing 2 feet in length! This is when these plants begin to flower. 

As is typical for the family, the inflorescence consists of a spadix covered by a leafy spathe. The spadix itself is covered in minute flowers and these are the key to properly identifying species. When pothos first made its way into the hands of botanists, all they had to go on were the small, juvenile leaves. This is why their taxonomy had been such a mess for so long. Materials obtained in 1880 were originally named Pothos aureus. It was then moved into the genus Scindapsus in 1908.

Controversy surrounding a proper generic placement continued throughout the 1900's. Then, in the early 1960's, an aroid expert was finally able to get their hands on an inflorescence. By 1964, it was established that these plants did indeed belong in the genus Epipremnum. Sadly, confusion did not end there. The plasticity in forms and colors these vines exhibit left many confusing a handful of species within the group. At various times since the late 1960's, E. aureum and E. pinnatum have been considered two forms of the same species as well as two distinct species. The latest evidence I am aware of is that these two vines are in fact distinct enough to warrant species status. 

The plant we most often encounter is E. aureum. Its long history of following humans wherever they go has led to it becoming an aggressive invader throughout many regions of the world. It is considered a noxious weed in places like Australia, Southeast Asia, India, Pakistan, and Hawai'i (just to name a few). It does so well in these places that it has been a little difficult to figure out where these plants originated. Thanks to some solid detective work, E. aureum is now believed to be native to Mo'orea Island off the west coast of French Polynesia. 

  Epipremnum pinnatum  is similar until you see an adult plant

Epipremnum pinnatum is similar until you see an adult plant

It is unlikely that most folks have what it takes to grow this species to its full potential in their home. They are simply too large and require ample sunlight, nutrients, and humidity to hit their stride. Nonetheless there is something to be said for the familiarity we have with these plants. They have managed to enthrall us just enough to be a fixture in so many homes, offices, and shopping centers. It has also helped them conquer far more than the tiny Pacific island on which they evolved. Becoming an invasive species always seems to have a strong human element and this aroid is the perfect example.

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

Further Reading: [1] [2] [3] 

 

Poinsettias Wild Origins

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Poinsettias are famous the world over for the splash of color they provide indoor spaces during the colder months of the year. The name "poinsettia" is seemingly synonymous with the holiday season. They are so common that it is all too easy to write them off as another disposable houseplant whose only purpose is to dazzle us with a few short weeks of reds and whites. With all of the focus on those colorful bracts, it is easy to lose sight of the fact that these plants have wild origins. What exactly is a poinsettia and where do they come from?

Poinsettia is the common name given to a species of shrub known scientifically as Euphorbia pulcherrima. No one quite knows the exact origin of our cultivated house guests but the species itself is native to the mountains of the Pacific slope of Mexico. It is a scraggly shrub that lives in seasonally dry tropical forests. Mature specimens can grow to be so large and lanky that they almost resemble vines. 

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These shrubs flower throughout winter and into spring. What we think of as large, showy, red and white flower petals are not petals at all. They are actually leafy bracts. Like a vast majority of Euphorbia species, E. pulcherrima produces tiny yellow flowers. They aren't much to look at with the naked eye but take a hand lens to them and you will reveal rather intriguing little structures. The bracts themselves serve similar functions as petals in that their stunning colors are there to attract potential pollinators. 

The bracts also caught the attention of horticulturists as well. Because of their beauty, E. pulcherrima is one of the most widely cultivated plants in human history. As many a poinsettia owner has come to realize, the bracts do not stay colored up all year. In fact, the whole function of these bracts is to save energy on flower production by coloring up leaves that are already in place. The key to the color change lies in the relative amount of daylight. 

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As days grow shorter, the plants begin to mature their flowers. At the same time, changes within the leafy bracts cause them to start producing pigments. When the days become shorter than the nights, the plants go into full reproductive mode. Both red- and white-colored bracts have been found in the wild. As soon as the days start to grow longer than the nights, the plants switch out of reproductive mode and the dazzling color fades. In captivity, this change is mimicked by plunging plants into complete darkness for a minimum of 12 hours per day.

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Another aspect worth considering about this species is its sap. Whereas most plants hailing from Euphorbiacea or spurge family contain toxic sap, the sap of E. pulcherrima is very mild in its toxicity and an absurd amount of plant material would have to be consumed to suffer any serious side effects. Certainly it serves an anti-herbivore purpose in the wild, however, as long as you're not a tiny insect or a gluttonous deer, you have nothing to worry about from this species at least. So there you have it, some food for thought if you feel the urge to purge some spurge in a post-holiday cleanse.

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

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

This Is Not The Bamboo You Are Looking For...

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She has one, he has one, you have one, I have one, the office has one... lets just say "lucky bamboo" has made its way into many a home, office, and waiting room. Popularized by the practice of feng shui and sold for pennies on the dime by New Age stores all over the world, these plants seem to thrive on neglect. It may come as a surprise then that these plants are not a bamboo at all.

These ubiquitous home decorations are actually a species of Dracaena, Dracaena braunii to be exact. It isn't even from the same taxonomical order as bamboo. Whereas bamboo are a type of grass, D. braunii is actually more closely related to lilies. Hailing from Africa, D. braunii grows as an understory shrub in rainforests. This may explain why it does so well in the nutrient poor, low light conditions of most homes. 

In the wild, it can grow upwards of 5 feet tall. In captivity, however, it rarely exceeds 3 feet.. While most people grow theirs in a container of water and pebbles, D. braunii can do equally as well, if not better in potting mix.

Photo Credit: Benoit Giroux

Further Reading: [1]

Why Do Rhododendron Leaves Droop and Curl in the Winter?

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Broad leaved, evergreen plants living in the temperate regions of the world face quite a challenge come winter time. Freezing temperatures, lack of water, and often intense sun can exact quite a toll on living tissues. These are likely just some of the reasons why, relatively speaking, broad leaved evergreens are a rare occurrence in temperate zones. By far the most popular group of plants in this category are the Rhododendrons.

Many a Rhodo lover has said that they can tell how cold it is outside by looking at Rhododendron leaves. Indeed, as temperatures drop, the leaves of these evergreen shrubs frequently droop and curl up like green cigars. These leaf movements do seem to be tied to the weather but their triggers and function have been the source of a lot of debate. Certainly not all Rhododendrons are cold hardy but those that are seem to benefit from reorienting their leaves. Why does this happen?

In the past it has been suggested that leaf reorientation may have something to do with reducing snow load. If the leaves were to remain horizontal, this could cause enough snow buildup to break branches. The fact that a considerable amount of ice and snow can accumulate on branches regardless of leaf position, and largely without harm, seems to suggest that this is not the case. Others have suggested that it could be a way to reduce water loss. As the leaves droop and curl, they are hypothetically increasing the humidity around their leaves and thus reducing their chances of desiccation.

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This seems pretty far fetched for a few reasons. For starters, Rhododendron simply do not open their stomata during the colder months. By keeping them closed, there is no net transfer of water into or out of the leaves. Also, their thick, waxy cuticle keeps water within the leaves from evaporating out as well. Finally, leaf drooping and curling happens long before the ground freezes and therefore doesn't seem to be triggered by a lack of water in the environment.

The leading theories on this phenomenon seem to deal more with issues at the cellular level. The first of these has to do with the sensitive photosynthetic machinery inside the chloroplasts. Leaf drooping may actually be a response to increased light. Though we generally don't think about photosynthesis in the winter months, evergreen plants actually experience the highest light intensities of the year during this time period. Throughout the growing season, they are generally shaded by the overstory. Once the canopy leaves fall, hwoever, things change.

Because the plants are, for the most part, dormant, the photosystems within the chloroplasts have no way of dissipating the energy from the incoming sunlight. Photosystem II is especially vulnerable under such scenarios. Experiments have shown that leaves that were forced to stay horizontal during the winter experienced permanent sun damage and photosynthesized considerably less than leaves that were allowed to droop once favorable temperatures returned. The thought is that by positioning the leaves vertically, the plants are reducing the amount of direct light hitting them throughout winter and therefore reducing the potential for light damage.

These experiments also revealed something else about the changes in leaf position when it comes to shape. As it turns out, curling made no difference in protecting the leaves from light damage. It would seem that drooping and curling are responses to two different types of environmental stress. So, why do the leaves curl?

The answer to this question is physical and one that has gained a lot of research attention in the field of cryogenics. When living tissues freeze, ice crystals build up to the point that they can rupture cell membranes. This is only exacerbated if the tissues thaw out quickly. Anyone that has ever tried to freeze and then thaw leafy vegetables knows what I am talking about.

To best preserve tissues via freezing, they must freeze quickly, which reduces the size of the ice crystals that can form, and then thaw out slowly. Researchers found that Rhododendron leaves freeze completely at temperatures below -8 degrees Celsius (17.6 degrees Fahrenheit), temperatures that occur regularly throughout the range of temperate Rhodo species. Again, experiments were able to demonstrate that flat leaves thaw much more rapidly than curled leaves. This is because a curled leaf exposes far less surface area to the warming sun than does a flat leaf. As such, curled leaves don't thaw out as fast and thus are able to avoid much of the damaging effects of daily freeze-thaw cycles.

Though these are all components of the Rhodo leaf puzzle, there is still much work to be done. What we do know is that leaf drooping and leaf curling are two separate behaviors responding to different environmental pressures. Indeed, it appears that these two traits seem to be tied to cold hardiness in the genus Rhododendron. What the exact triggers are and how they produce the changes in shape and orientation, as well as the mechanics of winter survival at the cellular level are topics that are going to require further study. Until then, I think its safe to say that we can appreciate and, to some degree, rely on the spot forecasting abilities of these wonderful shrubs.

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

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

 

The Other Balsaminaceae

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Have you heard of Hydrocera triflora? I hadn't until just recently. To my surprise, Hydocera is one of only two genera that make up the family Balsaminaceae. What's more, it is a monotypic genus, with this lovely species being the single representative. There is no question that H. triflora has been completely overshadowed by its cousins, the Impatiens. In fact, literature on this species is quite scant across the board.

The first question you may be asking is what differentiates Hydrocera from the Impatiens? The differences are rather subtle. I don't know if I would have considered this plant unique enough to warrant its own genus, however, closer botanical observations tell a more nuanced story. The biggest differences between Hydrocera and Impatiens has to do with flower and fruit morphology.

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For starters, the flowers of Hydrocera consist of a full compliment of 5 sepals and 5 petals. The petals themselves are all free from one another. Contrast this with Impatiens, whose flowers mostly consist of 3 sepals and 4 petals that are fused into pairs. The second major difference lies in the fruits. Many of us will be familiar with the explosive capsules of the various Impatiens species, each of which contains many seeds. Hydrocera on the other hand, produces berries that contain 5 seeds. Such vastly different developmental pathways in reproductive structures appear to be enough to warrant the taxonomic separation between the two genera.

The next question one might asking is why are Impatiens so diverse while Hydrocera contains only a single species? This is anyone's guess, really, but there has been at least a few hypotheses put forward that sound plausible. One has to do with habitat preference. Impatiens are largely plants of upland forests and montane environments. Such habitats may offer more potential for diversification due to high heterogeneity in resources and lots of potential for isolation of various populations. Contrast this with the habitat of H. triflora. Though it occurs throughout a wide swath of lowland Asia and India, it is semi-aquatic and these types of habitats may be more restrictive for diversification.

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Another possibility has to do with seed dispersal. As mentioned above, Impatiens produce lots of seeds per capsule and, with their explosive habit, can disperse them over relatively large distances. Contrast this with Hydrocera. When the berries mature, they fall into the water and sink. They remain submerged until rot or various aquatic organisms eat away at the fleshy coating. Once the seeds have been freed, air sacs cause them to float on the currents until seasonal drying brings them back into contact with the mud. Though this is certainly an effective method for dispersal, the lower seed production rate coupled with being at the mercy of the currents means that Hydrocera is probably considerably less likely to find itself in new habitats.

Again, this is largely speculation at this point. We simply don't know enough about this oddball of the balsam world to make any serious conclusions. Luckily H. triflora is not a species under immediate threat. It seems to do quite well throughout its range, frequently occurring in flooded ditches and rice patties. Still, such stories underlie the importance of fostering and funding organism-focused research.

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

Further Reading: [1] [2]