The Succulent Passionflowers

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Succulent passionflowers?! It took me a minute to get my head wrapped around the idea. It wasn’t until I saw one in flower that I truly understood. The genus Adenia is found throughout east and west Africa, Southeast Asia, and hits its peak diversity in Madagascar. It comprises approximately 100 species and, as a whole, is poorly understood. Today I would like to introduce you to this bizarre genus within Passifloraceae.

Adenia glauca

Adenia glauca

Adenia is, to date, the second largest genus within the Passionflower family and yet delineating species has been something of a nightmare for botanists over the years. At least some of this confusion lies within the diversity of this odd group. It has been said that few angiosperm lineages surpass Adenia in the diversity of growth forms they exhibit. Though all could be considered succulent to some degree, Adenia runs the gamut from trees to vines, and even tuberous herbs.

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Even within individual species, the overall form of these plants can vary widely depending on the conditions under which they have been growing. Their succulent nature and that fact that many species can reach rather large proportions means that herbarium records for this group are scant at best. Many are only known from a single, incomplete collection of a few bits and pieces of plant. Also, juvenile plants often look very different from their adult forms, making timing of the collection crucial for proper analysis.

To complicate matters more, all Adenia are dioecious, meaning that individual plants are either male or female. Male and female flowers of individual species look pretty distinct and differ a bit from what we have come to expect out of the passionflower family. Often collections were made on only a single sex. This is further complicated by the fact that these plants often exhibit very short flowering seasons. Most come into bloom right before the onset of the rainy season and are entirely leafless at that point in time. Because of this, it has been extremely difficult to accurately match flowering collections to vegetative collections. As such, nearly 1/4 of all Adenia species are missing descriptions of either male or female flowers and their fruits.

Female flower of  Adenia reticulata

Female flower of Adenia reticulata

Male flowers of  Adenia digitata

Male flowers of Adenia digitata

Flowers of  Adenia firingalavensis

Flowers of Adenia firingalavensis

Fruits of  Adenia hondala

Fruits of Adenia hondala

Even genetic work has failed to clear up much of the mysteries that surround this group. Some studies suggest that Adenia is sister to all other genera within Passifloraceae whereas others have even suggested it to be nestled neatly within the genus Passiflora. The most recent work hints at a placement among the tribe Passifloreae. If this confuses you, you are certainly not alone. Until a more complete sampling effort is done on Adenia, I think it is safe to say that this genus will be holding onto its taxonomic mysteries for the foreseeable future.

Adenia globosa

Adenia globosa

All Adenia are perennial plants but how they manage this differs from species to species. Some put all of their energy into underground tubers, producing annual stems and leaves that die back each year. Others don’t produce any tubers and instead store all of their water and nutrients within thick stems. This has made at least a handful of species a hit with succulent growers around the world. It is always an interesting sight to see a giant caudiciform trunk or base with bunches of spindly stems spraying out from the top.

Leaves and fruit of  Adenia cissampeloides

Leaves and fruit of Adenia cissampeloides

Juvenile  Adenia glauca

Juvenile Adenia glauca

Adenia are also extremely toxic plants. The conditions under which these plants evolved are tough and it appears that this group doesn’t want to take any chances on losing any biomass to herbivores. The main class of compounds they produce are called lectins. These proteins cause myriad issues within animal bodies including rapid cell death, blood clotting, inhibition of protein synthesis, and a disruption of ribosome and DNA function. Needless to say, its in any critters best interest to avoid nibbling on any species of Adenia. Even handling and pruning of these plants merits caution.

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Whether you’re a botanist, taxonomist, gardener, or just curious about plant diversity, Adenia is a wonderful example of just how many unknowns are still out there. Regardless of their taxonomic status, these are fascinating species, each with a wonderful ecology and intriguing evolutionary history. These plants are hardy survivors and a great example of the lengths a genus can go to when presented with new opportunities. Undoubtedly many more species await description but the plants we currently know of are fascinating to say the least.

Adenia pechuelii

Adenia pechuelii

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Further Reading: [1] [2]

The Grafted Cactus Origin Story

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Many of you have undoubtedly met this interesting cactus before. Some  of you probably own one. Commonly referred to as 'Hibotan' or "moon  cactus," these are not a single species cactus but rather two different  cacti that have been grafted together.

The colorful top part is known scientifically as  Gymnocalycium mihanovichii. It is endemic to Paraguay and some provinces  of Argentina. In the wild it is not nearly this colorful. The specimens  sold in garden shops all over the world are actually mutant varieties that do not produce chlorophyll, thus revealing other pigments that are normally masked by green. The color of these mutants can range from  yellows to reds and even deep purples. Without chlorophyll, these mutants would normally die as seedlings.

The wild version of  Gymnocalycium mihanovichii  is a lot less coloreful.

The wild version of Gymnocalycium mihanovichii is a lot less coloreful.

Provided their host cactus is kept happy, mutant  Gymnocalycium mihanovichii  will flower.

Provided their host cactus is kept happy, mutant Gymnocalycium mihanovichii will flower.

At some point in time, someone got it in their head that they could graft these colorful mutants onto other species of cacti and perhaps they would survive. This is exactly what has happened. Interestingly enough, the bottom host cactus isn't even in the same genus as the moon cactus. Grafting is most often done on a species of Hylocereus (the same genus responsible for dragon fruit). How and why this host was chosen I do not know. Either way, armed with this knowledge, I hope you have gained a new found appreciation for these seemingly ubiquitous house plants.

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

Further Reading: [1]

The Genus Ceropegia Recently Got a Whole Lot Bigger

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The succulent and climbing members of the milkweed family (Apocynaceae) have been gaining a lot of popularity among houseplant growers and for good reason. These wonderful plants produce some of the most elaborate flowers most of us will ever encounter and many of them smell quite strongly. Whereas houseplant enthusiasts recognize multiple genera of these spectacular plants, recent taxonomic work suggests lumping them all into one single genus - Ceropegia.

Such a massive taxonomic move has caused its fair share of drama. Folks seem to get quite ornery when it comes to shifts in nomenclature, especially when it involves this many species. However, when you dive into this group of plants, you really start to see how shaky the ground was that supported the previous classification systems. Evolution, after all, is not a neat and tidy process and we can learn a lot from the succulent and climbing asclepiads regarding the importance of combining morphological and genetic data into our taxonomic decisions.

Ceropegia (Stapelia) hirsuta

Ceropegia (Stapelia) hirsuta

Botanists have been obsessing over this group for decades. Historically speaking, four major groups have been recognized: those with caudiciform stems (genus Brachystelma), the stem succulent stapeliads (which include genera such as Stapelia, Huernia, Orbea, Caralluma, and others), the climbers (genus Ceropegia), and the so-called early divergent group (which include genera such as Anisotoma, Conomitra, Dittoceras, and others). Together these groups total something like 762 species and represent the tribe Ceropegieae.

The taxonomic status of the various members of Ceropegieae have always been up for debate. Early work was based on surprisingly few species and relied heavily on morphological characters such and corolla shape, stem anatomy, and pubescence. Since the 1950’s, many more species have been discovered and that is where a lot of the trouble began. Much of the early characters that were used to draw lines between various groups were suddenly blurred. Genera were created and absorbed by various authors in an attempt to get a handle on how this tribe evolved.

Ceropegia  ( Brachystelma )  tuberosum

Ceropegia (Brachystelma) tuberosum

Things got even more complicated as various stapeliads and Ceropegia attracted the attention of horticulturists. As new species became available, many varieties were haphazardly named and genera such as Stapelia were further split to accommodate some of the peculiar nuances in floral shapes, colors, and sizes. It wasn’t until some genetic work was done that the need for a major overhaul of the Ceropegieae tribe became apparent.

Unfortunately, this early molecular work suffered from low resolution. Very few genera were used and among those, only a handful of gene regions were analyzed. Still, the picture that was developing was that the historical understanding of Ceropegieae was surprisingly misleading. For instance, the genera that made up the stapeliad group appeared to be nested quite firmly within the genus Ceropegia. Though equally as limited in scope, consecutive work in the early 2000’s added further evidence to the idea that the four groups that made up Ceropegieae were so genetically similar that most should be nested somewhere within Ceropegia.

Ceropegia  ( Duvalia )  modesta

Ceropegia (Duvalia) modesta

Though not without controversy, this early molecular work convinced enough taxonomists to take a closer look at each of the four groups. With more resolution and a finer grasp on the diversity in form of these plants, taxonomists started to question the validity of some taxa. Indeed, the closer anyone looked, the more the lines between genera started to blur.

For example, Ceropegia and Brachystelma have long been separated on the basis of floral structure. Ceropegia were considered to adhere to a single corolla structure involving long, tubular flowers whereas Brachystelma were thought to be more variable in form. The discovery of new species clearly demonstrates that there are far too many exceptions to this system for it to be valid.

Fig. 1. Variation in the corolla and corona in the traditional concept of  Ceropegia : A–C,  C. salicifolia , Nepal,  Bruyns 2507  (BM, K); D–E,  C. melanops , Ethiopia,  Gilbert 3050  (K); F—H,  C. meleagris , Nepal,  Bruyns 2496  (K); I–J,  C. loranthiflora , Ethiopia,  Gilbert 2851   (K). [scale-bars or subdivisions indicate mm; A, D, F, I, corolla from  side; B, G, corolla dissected to show location of corona; C, E, H, J,  corona from side].   [SOURCE]

Fig. 1. Variation in the corolla and corona in the traditional concept of Ceropegia: A–C, C. salicifolia, Nepal, Bruyns 2507 (BM, K); D–E, C. melanops, Ethiopia, Gilbert 3050 (K); F—H, C. meleagris, Nepal, Bruyns 2496 (K); I–J, C. loranthiflora, Ethiopia, Gilbert 2851 (K). [scale-bars or subdivisions indicate mm; A, D, F, I, corolla from side; B, G, corolla dissected to show location of corona; C, E, H, J, corona from side]. [SOURCE]

Fig. 2. Variation in the corolla and corona in the traditional concept of  Brachystelma : A–C,  B. brevipedicellatum , South Africa,  Bruyns 2372 ; D–F,  B. mafekingense , Namibia,  Bruyns 1954  (K, WIND); G–J,  B. gymnopodum , South Africa,  Bruyns 2078   (NBG). [scale-bars or subdivisions indicate mm; A, corolla from front,  D, G, corolla from side; B, E, H, corolla dissected to show location of  corona; C, J, corona from front; F, I, corona from side].   [SOURCE]

Fig. 2. Variation in the corolla and corona in the traditional concept of Brachystelma: A–C, B. brevipedicellatum, South Africa, Bruyns 2372; D–F, B. mafekingense, Namibia, Bruyns 1954 (K, WIND); G–J, B. gymnopodum, South Africa, Bruyns 2078 (NBG). [scale-bars or subdivisions indicate mm; A, corolla from front, D, G, corolla from side; B, E, H, corolla dissected to show location of corona; C, J, corona from front; F, I, corona from side]. [SOURCE]

Such is also the case for other anatomical features such as whether plants climb or not. Again, there are plants in both genera that deviate from these patterns, thus making it impossible to nail down any set of characters that maintain the split between these two genera. Also, it would seem that some authors were trying to pull a fast one on readers. Back in 2007, Meve and Liede-Schumann claimed there were “a wide array of morphological features” that separate these two genera but failed to reveal any but those mentioned here. There are multiple species of Ceropegia and Brachystelma that simply do not conform to this historical classification.

Similarly, Ceropegia and the various stapeliads have been separated on the basis of stem and floral anatomy. Historically speaking, the stapeliads were thought to consist of fleshy, succulent stems with tubercules and reduced or absent leaves, whereas Ceropegia were considered to be slender climbers. Again, with more species having been discovered, these distinctions grew more and more blurry.

The succulent stems of  Ceropegia cimiciodora .

The succulent stems of Ceropegia cimiciodora.

It turns out that there are many Ceropegia with fleshy, succulent stems and the only major difference between the two genera is the lack of angles in the stems of some Ceropegia. The structure and presentation of their flowers also stands on shaky ground. There is so much similarity between the flowers of some of the succulent Ceropegia and the early diverging stapeliads that one would be hard pressed to identify any character that clearly separates them.

Between all of the molecular work and the anatomical scrutiny, it was clear that something needed to be done to clean up the taxonomic status of Ceropegieae. Keeping things separate may make sense to some but considering the group as a whole instead of from a collector’s standpoint, trying to find enough distinct characters to preserve the historical treatment would make things way too messy. In 2017 it was suggested that because there are no clear differences between the four groups within this tribe, all members were to be lumped back in to the genus Ceropegia.

Ceropegia  ( Stapelia )  flavopurpurea

Ceropegia (Stapelia) flavopurpurea

Although this most recent treatment still recognizes some morphological differences between these plants (thus multiple subsections are recognized), the lack of genetic differentiation between groups long thought to be distinct really does support this decision. Because of historical precedents, Ceropegia won out as the main generic classification.

Personally I find this work to be extremely exciting. It involved a lot of wonderful detective work and a whole lot of attention to detail. I think the end result paints a far better picture for our understanding of how these plants evolved. I am especially floored that some of the earlier morphological notes turned out to be quite useful in this modern understanding. Even more exciting is the fact that now we know that many of what we thought were “unique” characters amoung the various species actually evolved multiple times throughout the history of this group. This is why I will never get upset by taxonomic changes. They may be working documents but each step we take helps us understand evolution that much more.

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

Further Reading: [1]

Getting to Know Sansevieria

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The houseplant hobby is experiencing something of a renaissance as of late. With their popularity on various social media platforms, easy to grow plant species and their cultivars are experiencing a level of popularity they haven't seen in decades. One genus of particular interest to houseplant hobbyists is Sansevieria.

Despite their popularity, the few Sansevieria species regularly found in cultivation come attached with less than appealing common names. Mother-in-law's tongue, Devil's tongue, and snake plant all carry with them an air of negativity for what are essentially some of the most forgiving houseplants on the market. What few houseplant growers realize is that those dense clumps of upright striped leaves tucked into a dark corner of their home belong to a fascinating genus worthy of our admiration. What follows is a brief introduction to these enigmatic houseplants.

Sansevieria cylindrica

Sansevieria cylindrica

Sansevieria ballyi

Sansevieria ballyi

The Sansevieria we encounter in most nurseries are just the tip of the iceberg. Sansevieria is a genus comprise of about 70 different species. I say 'about' because this group is a taxonomic mess. There are a couple reasons for this. For starters, the vast majority of Sansevieria species are painfully slow growers. It can take decades for an individual to reach maturity. As such, they have never really presented nursery owners with much in the way of economic gain and thus only a few have received any commercial attention.

Another reason has to do with the fiber market during and after World War II. In hopes of discovering new plant-based fibers for rope and netting, the USDA collected many Sansevieria but never formally described most of them. Instead, plants were assigned numbers in hopes that future botanists would take the time needed to parse them out properly.

A third reason has to do with the variety of forms and colors these plants can take. Horticulturists have been fond of giving plants their own special cultivar names. This complicates matters as it is hard to say which names apply to which species. Often the same species can have different names depending on who popularized it and when.

Sansevieria grandis in situ .

Sansevieria grandis in situ.

Regardless of what we call them, all Sansevieria hail from arid regions of Africa, Madagascar and southern Asia. In the wild, many species resemble agave or yucca and, indeed, they occupy similar niches to these New World groups. Like so many other plants of arid regions, Sansevieria evolved CAM photosynthesis as a means of coping with heat and drought. Instead of opening up their stomata during the day when high temperatures would cause them to lose precious water, they open them at night and store CO2 in the form of an organic acid. When the sun rises the next day, the plants close up their stomata and utilize the acid-stored carbon for their photosynthetic needs.

The wonderfully compact  Sansevieria pinguicula .

The wonderfully compact Sansevieria pinguicula.

Often you will encounter clumps of Sansevieria growing under the dappled shade of a larger tree or shrub. Some even make it into forest habitats. Most if not all species are long lived plants, living multiple decades under the right conditions. These are just some of the reasons that they make such hardy houseplants.

The various Sansevieria appear the sort themselves out along a handful of different growth forms. The most familiar to your average houseplant enthusiast is the form typified by Sansevieria trifasciata. These plants produce long, narrow, sword shaped leaves that point directly towards the sky. Many other Sansevieria species, such as S. subspicata and S. ballyi, take on a more rosetted form with leaves that span the gamut from thin to extremely succulent. Still others, like S. grandis and S. forskaalii, produce much larger, flattened leaves that grow in a form reminiscent of a leaky vase. 

Sansevieria trifasciata  with berries .

Sansevieria trifasciata with berries.

Regardless of their growth form, a majority of Sansevieria species undergo radical transformations as they age. Because of this, adults and juveniles can look markedly different from one another, a fact that I suspect lends to some of the taxonomic confusion mentioned earlier. A species that illustrates this nicely is S. fischeri. When young, S. fischeri consists of tight rosettes of thick, mottled leaves. For years these plants continue to grow like this, reaching surprisingly large sizes. Then the plants hit maturity. At that point, the plant switches from its rosette form to producing single leaves that protrude straight out of the ground and can reach heights of several feet! Because the rosettes eventually rot away, there is often no sign of the plants previous form.

A young  Sansevieria fischeri  exhibiting its rosette form.

A young Sansevieria fischeri exhibiting its rosette form.

A mature  Sansevieria fischeri  with its large, upright, cylindrical leaves.

A mature Sansevieria fischeri with its large, upright, cylindrical leaves.

If patient, many of the Sansevieria will reach enormous sizes. Such growth is rarely observed as slow growth rates and poor housing conditions hamper their performance. It's probably okay too, considering the fact that, when fully grown, such specimens would be extremely difficult to manage in a home. If you are lucky, however, your plants may flower. And flower they do!

Though there is variation among the various species, Sansevieria all form flowers on either a simple or branched raceme. Flowers range in color from greenish white to nearly brown and all produce a copious amount of nectar. I have even noticed sickeningly sweet odors emanating from the flowers of some captive specimens. After pollination, flowers give way to brightly colored berries, hinting at their place in the family Asparagaceae.

A flowering  Sansevieria hallii .

A flowering Sansevieria hallii.

As a whole, Sansevieria can be seen as exceptional tolerators, eking out an existence wherever the right microclimate presents itself in an otherwise harsh landscape. Their extreme water efficiency, tolerance of shade, and long lived habit has lent to the global popularity of only a few species. For the majority of the 70 or so species in this genus, their painfully slow growth rates means that they have never made quite a splash in the horticulture trade.

Nonetheless, Sansevieria is one genus that even the non-botanically minded among us can pick out of a lineup. Their popularity as houseplants may wax and wane but plants like S. trifasciata are here to stay. My hope is that all of these folks collecting houseplants right now will want to learn more about the plants they bring into their homes. They are more than just fancy decorations, they are living things, each with their own story to tell. 

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

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

Aloe or Agave?

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Convergent evolution is the process by which unrelated organisms evolve similar traits in response to similar environmental constraints. One amazing example of convergent evolution has occurred among the Aloe and Agave. These two distinct lineages are separated both in space and time and yet they often look so similar that it can be hard for the average person to tell them apart. With that in mind, lets consider the similarities and differences between these two lineages.

To start, Aloe and Agave hail from two completely different spots on the botanical family tree. Each also has its own unique geographic origin. Agave is a New World genus with species ranging in their distribution from tropical South America north into arid portions of North America. Genetic analysis places the genus Agave in the family Asparagaceae.

Agave americana  in bloom

Agave americana in bloom

Aloe, on the other hand, enjoys an Old World distribution, from Africa and Madagascar to the Arabian Peninsula as well as many islands scattered throughout the Indian Ocean. Taxonomically speaking, Aloe has undergone more than a few revisions through time, however, recent genetic work suggests that the Aloe belong to the family Asphodelaceae.

Experts believe that the lineages that gave rise to these two distinct genera branched off from a common ancestor some 93 million years ago. Despite all of that intervening time and space, the rigors of their arid habitats have managed to shape these plants in strikingly similar ways. Morphologically speaking, there is a lot of superficial similarity between Aloe and Agave.

Aloe hereroensis in situ

Aloe hereroensis in situ

Both groups exhibit water-storing, succulent leaves arranged in rosettes. These leaves are often adorned with spines or other protrusions aimed at deterring herbivores. Both groups also utilize CAM photosynthesis for their energy needs. When it comes time to flower, both groups frequently produce brightly colored, tubular flowers arranged at the tip of long stalks.

It is worth noting that the harsh environments that have shaped these two plant lineages also seems to have induced a backup plan for reproduction. Both Aloe and Agave produce tiny offshoots called "pups." These pups gain nourishment from the parent plant until they are large enough to fend for themselves. All pups are clones but if the parent plant had what it takes to survive in that spot, there is a good chance that its cloned offspring will as well. That way, even if sexual reproduction fails, these cloned progeny will get another shot.

Despite all of this convergence, these two lineages nonetheless exhibit vastly different developmental pathways and thus there are plenty of differences separating the two. For starters, slice into the leaves of each type and you will quickly find one major morphological difference. As many already know, Aloe leaves are largely filled with a gooey pulp and not much else. Aloe leaves function as water storage organs. Agave also store plenty of liquid in their leaves, however, they also produce numerous long strands of fiber that provide much more structural integrity.

Cross section of an Aloe leaf showing gelatinous pulp.

Cross section of an Aloe leaf showing gelatinous pulp.

Agave leaf showing fibrous interior.

Agave leaf showing fibrous interior.

Aloe and Agave each have evolved their own reproductive strategies as well. Aloe are perennial bloomers. Under the right conditions, many Aloe species will produce a profusion of flower stalks year after year. The stalks emerge from between the leaves and are largely pollinated by birds and insects in their native habitats. Agave, on the other hand, are monocarpic meaning they invest all of their energy into one single bloom. The Agave flowering stalk emerges from the center of the rosette and are pollinated by myriad insects, birds, and even bats. After flowering is complete, the main Agave plant dies.

Aloe flowers

Aloe flowers

Agave flowers

Agave flowers

Convergent evolution will never cease to amaze me. Despite millions of years and hundreds of miles separating these two lineages, Aloe and Agave have nonetheless been shaped in similar ways by similar environmental conditions.

Photo Credits: Wikimedia Commons

Further Reading: [1]

The Mystery of the Ghost Plant

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As houseplants enjoy a resurgence in our culture, untold numbers of novice and expert growers alike will have undoubtedly tried their luck at a succulent or two. Succulent, of course, is not a taxonomic division, but rather a way of describing the anatomy of myriad plants adapted to harsh, dry environments around the world. One of the most common succulents in the trade is the ghost plant (Graptopetalum paraguayense).

I would bet that, if you are reading this and you grow houseplants, you have probably grown a ghost plant at one point or another. They are easy to grow and will propagate a whole new plant from only a single leaf. Despite its worldwide popularity, the ghost plant is shrouded in mystery and confusion. To date, we know next to nothing about its ecology. Much of this stems from poor record keeping and the fact that we have no idea exactly where this species originated.

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That's right, we do not know the location of its native habitat. Records indicate that the first plants to find their way into human hands were imported into New York in 1904. Apparently, they were growing as "weeds" at the base of some South American cacti. Plants were lucky enough to wind up in the hands of competent botanists and the species has ended up with the name Graptopetalum paraguayense. The specific epithet "paraguayense" was an indication of much confusion to come as it was thought that the ghost plant originated in Paraguay.

Time has barely improved our knowledge. Considering many of its relatives hail from Mexico, it gradually became more apparent that South America could not claim this species as its own. Luck changed only relatively recently with the discovery of a population of a unique color variant of the ghost plant on a single mountain in northeastern Mexico. A thorough search of the area did not reveal any plants that resemble the plant so many of us know and love. It has been suggested that the original population from which the type species was described is probably growing atop an isolated mountain peak somewhere nearby in the Chihuahuan Desert.

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Despite all of the mystery surrounding this species, we can nonetheless elucidate some aspects about its biology by observing plants in cultivation. It goes without saying that the ghost plant is a species of dry, nutrient-poor habitats. Its succulence and tolerance of a wide array of soil conditions is a testament to its hardy disposition. Also, if plants are grown in full sun, they develop a bluish, waxy coating on their leaves. This is likely a form of sunscreen that the plant produces to protect it from sun scorch. As such, one can assume that its native habitat is quite sunny, though its ability to tolerate shade suggests it likely shares its habitat with shrubby vegetation as well. Given enough time and proper care, ghost plants will produce sprays of erect, 5 pointed flowers. It is not known who might pollinate them in the wild.

It is always interesting to me that a plant can be so well known to growers while at the same time being a complete mystery in every other way. A search of the literature shows that most of the scientific attention given to the ghost plant centers on potentially useful compounds that can be extracted from its tissues. Such is the case for far too many plant species, both known and unknown alike. Perhaps, in the not too distant future, some intrepid botanist will at last scramble up the right mountain and rediscover the original habitat of this wonderful plant. Until then, I hope this small introduction provides you with a new found appreciation for this wonderfully adaptable houseplant.

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

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

 

The Mighty Saguaro Cactus

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Where does one begin with a plant like the saguaro cactus (Carnegiea gigantea)? It is recognized the world over for its iconic appearance yet its native range is disproportionately small compared to its popularity. It is easily one of the most spectacular plants I have ever encountered and I will never forget the sound the wind makes as it blows over its spiny pleated trunk. It would be impossible to sum up our collective knowledge of this species in one article, however, I feel that some form of an introduction is necessary. Today I want to honor this icon of the Sonoran Desert.

The saguaro is the only member of the genus Carnegiea, which is part of a subtribe of cacti characterized by their columnar appearance. Despite its unique taxonomic affinity, the evolutionary origins of this cactus remains a bit of a mystery. Though it is undoubtedly related to other columnar cacti of the Americas, a proper family tree seems to be just out of our reach. Due to lots of convergent and parallel evolution as well as conflicts between genealogies and species histories, we still aren't sure of its evolutionary origins. What we do know about this species on a genetic level is nonetheless quite interesting. For instance the saguaro has one of the smallest chloroplast genomes of any non-parasitic plant and we aren’t exactly sure why this is the case.

Saguaro are long lived cacti. Estimating age of a cactus can be rather tricky considering that they don’t produce annual growth rings. This is where long term monitoring projects have come in handy. By observing hundreds of saguaro throughout the Sonoran Desert, experts believe that saguaro can regularly reach ages of 150 to 170 years and some individuals may be able to live for more than 200 years. Amazingly, it is thought that saguaro will not begin to grow their characteristic arms until they reach somewhere around 50 to 100 years of age. That being said, some saguaro never bother growing arms. It all depends on where the conditions they experience throughout their lifetime.

Growth for a saguaro depends on where they are rooted. Under favorable conditions, a saguaro can grow to heights of 50 feet or more, with the world record holder clocking in at a whopping 78 feet in height. Such growth becomes all the more impressive when you realize just how agonizingly slow the process can be. Studies have shown that juvenile saguaro only put on about 1.5 inches of growth in their first eight years of life.

Despite preconceived notions about the hardy nature of most cacti, saguaro have proven to be rather specific in their needs. They are limited in their growth and distribution by the availability of water and warm temperatures. Saguaro, especially young individuals, cannot tolerate periods of prolonged frost. Additionally, germination and seedling survival occur most frequently only during the wettest years. In fact, one study showed that successful years for reproduction in these beloved cacti were tied to volcanic eruptions that cooled the climate just enough to allow the young saguaro to become established.

Outside of volcanic eruptions, saguaro appear to have friends in the surrounding vegetation. Studies have shown that saguaro seedlings seem to do best when growing under the shade of trees like the palo verde (Parkinsonia florida), ironwood (Olneya tesota), and mesquite (Prosopis velutina). The microclimates produced by these trees are much more favorable for saguaro growth than are open desert conditions. In essence, these trees serve as nurseries for young saguaro until they are large enough to handle more exposed conditions. Their nursery habits are not mutually beneficial however as research suggests that saguaro eventually compete with the trees that once protected them for precious resources like nutrients and water.

Saguaros outgrowing their palo verde nurse tree. 

Saguaros outgrowing their palo verde nurse tree. 

At roughly 35 years of age, a saguaro will begin to flower. Flowers are small compared to the size of the cactus but they are abundant. Most flowers are produced at the apex of the cactus and it is thought that the growth of saguaro arms is largely a way of increasing the reproductive potential of large individuals. The flowers are cream colored and night scented. They open in the evening but will stay open and continue to produce nectar well into the morning hours.

Though a wide variety of animals will visit these flowers, the main pollinators are bees during the day and lesser long-nosed bats at night. Interestingly, it has been found that certain amino acids within the nectar of the saguaro can actually help female bats sustain lactation while raising their young, making them a valuable food source for these flying mammals. Catering to such a broad spectrum of potential pollinators is thought to have evolved as a means of increasing seed set. Each saguaro ovary contains many ovules and the more pollen that makes it onto the stigma, the more seeds will be produced.

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A lesser long-nosed bat pollinates a saguaro bloom.

A lesser long-nosed bat pollinates a saguaro bloom.

Due to their size and abundance, it is easy to understand why the saguaro is such an ecologically important species in the Sonoran Desert ecosystem. In essence, they function similar to trees in that they serve as vital sources of shelter and food for myriad desert animals. Woodpeckers, especially the gila and the gilded flicker, regularly hollow out and build nests in saguaro trunks. These hollows are subsequently used by many different bird, mammal, and reptile species. The flowers and fruits are important sources of food for wildlife.

Gila woodpecker with its nesting hole.

Gila woodpecker with its nesting hole.

Gila woodpecker holes become homes for other birds like owls. 

Gila woodpecker holes become homes for other birds like owls. 

On rare occasions, woodpecker holes can even become home to other cacti!

On rare occasions, woodpecker holes can even become home to other cacti!

I sincerely hope that this brief introduction does at least some justice to the wonderful organism that is the saguaro cactus. The Sonoran Desert would be a shell of an ecosystem without its presence. What’s more, it has played a significant role in the culture of this region for millennia. Though it appears quite numerous on the landscape, the long-term status of the saguaro is cause for concern. Numerous declines have been reported throughout its range. With its slow growth rates and infrequent recruitment events, the saguaro can be quite sensitive to rapid changes in its environment. Luckily it has received special protection laws throughout its US range.

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


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

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]

A Truly Bizarre Cactus From The Amazon

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When we think of cacti, we tend to think of dry deserts and sandy soils. Few of us would ever jump to the trunk of a tree, nestled in a humid rainforest, and experiencing periodic inundation. Yet, such a habitat is the hallmark of one of the world's strangest species of cactus - Selenicereus witii. In more ways than one, this species is truly aberrant.

Whereas epiphytic cacti aren't novel, the habits of S. witii surely push the limits of what we know about the entire cactus family. Despite having been discovered in 1899, little attention has been paid to this epiphytic cactus. What we do know comes from scant herbarium records and careful observation by a small handful of botanists.

S. witii is endemic to a region of central Amazonia and only grows in Igapó, or seasonally flooded, blackwater forests. It makes its living on the trunks of trees and its entire morphology seems particularly adapted to such a harsh lifestyle. Unlike most cacti, S. witii doesn't seem to bother with water storage. Instead, its stems grow completely appressed to the trunks of trees. Roots emerge from near the spine-bearing areoles and these help to anchor it in place. 

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Because they are often exposed to bright sunlight, the stems produce high amounts of chemical pigments called betalains. These act as sun block, protecting the sensitive photosynthetic machinery from too much solar radiation. These pigments also give the plant a deep red or purple color that really stands out against the trunks of trees. 

Like all members of this genus, S. witii produces absolutely stunning flowers. However, to see them, your best bet is to venture out at night. Flowers usually begin to open just after sundown and will be closed by morning. And my, what flowers they are! Individual blooms can be upwards of 27 cm long and 12.5 cm wide (10 in by 5 in)! They are also said to produce an intense fragrance. Much of their incredible length is a nectar tube that seems to be catered to a specific group of sphinx moths, whose proboscis is long enough to reach the nectar at the bottom.

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The seeds of S. witii are just as aberrant as the rest of the cactus. They are rather large and shaped like a kidney. Cross sections reveal that most of their size is devoted to hollow air chambers. Indeed, the seeds float like tiny pieces of cork when placed in water. This is likely an adaptation resulting from their preferred habitat.

As mentioned above, S. witii has only been found growing in seasonally flooded forests. What's more, plants only occur on the trunks of large trees right at the high water line. In fact, the highly appressed nature of its stems seems to suggest that this species can withstand periodic submergence in fast flowing water. The seeds must also cope with flooding and it is likely that their buoyant nature aids in seed dispersal during these periods. 

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All in all, this is one weird cactus. Although it isn't alone in its tropical epiphytic habit, it certainly takes the cake for being one of the most derived. Aside from a few publications, little attention has been given to this oddball. It would appear that the seasonal flooding of its preferred habitat has simply chased this cactus up into the trees, the environmental demands of which coaxed out strange but ingenious adaptations from its genome. The good news is that where it does occur, S. witii seems to grow in high numbers.

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

Further Reading: [1]

The Holoparasitic Mistletoes

Flowers of   Tristerix aphyllu s

Flowers of Tristerix aphyllus

The order collectively referred to as mistletoes is incredibly diverse. They range in size from rather large trees down to little more than a couple leaves, barely recognizable on their hosts. Even more unique are the mistletoes that have foregone much of what we would readily recognize as an actual plant. These parasitic plants have adopted an endophytic lifecycle, living their entire lives within the vascular tissues of their host plants, only visible to observers when in flower. 

Tristerix aphyllus is one such species. Its hosts are cacti in the genus Echinopsis (formerly Trichocereus) native to Columbia and Chile. Being an endophyte, the majority of this mistletoe lives as a mycelial-like network of filaments that wrap around the vascular tissues of the host cactus. The only part of the mistletoe that ever emerges are the flowers. They come in both red and yellow forms. What may appear to be lovely cactus covered in red flowers are actually the flowers of Tristerix. Strangely enough, the occasional small leaf is produced on the flowering branches. Though there is chlorophyll in the leaf, researchers believe that they perform little if any photosynthesis.

Fruiting   Tristerix aphyllu s

Fruiting Tristerix aphyllus

This is not a parasitic relationship that is unique to cacti either. Africa has its own endoparasitic mistletoe as well. However, as we have discussed before, Africa does not have any native cacti (http://on.fb.me/1zPbac7). Instead, through convergent evolution, plants in the genus Euphorbia have followed similar adaptive trajectories. As such, at least one species of African mistletoe has followed suit.

Flowers of   Viscum minimum

Flowers of Viscum minimum

A species known scientifically as Viscum minimum finds the cactus-like Euphorbia horrida and E. polygona to its liking. Like Tristerix, Viscum minimum is endoparasitic, living entirely within the tissues of its Euphorbia host until it decides to flower. It too produces brightly colored berries that aid in its dispersal to a new host. 

The main seed dispersers are birds. After consumption, a bird either regurgitates the embryo or passes it out the other end. If that bird happens to be sitting on a host cactus or Euphorbia, the embryo will grow into a seedling that quickly taps into its new host and begins its internal parasitic life. It will not be seen again until it flowers.

Viscum minimum  beginning to set seed.

Viscum minimum beginning to set seed.

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


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