The Genus Ceropegia Recently Got a Whole Lot Bigger

Ceropegia_haygarthii-three-flowers.jpg

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]

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]

Daffodil Insights

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

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

Narcissus poeticus

Narcissus poeticus

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

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

Illustration_Narcissus_poeticus0.jpg

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

Narcissus tazetta

Narcissus tazetta

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

Narcissus cyclamineus

Narcissus cyclamineus

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

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

Narcissus triandrus

Narcissus triandrus

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

Narcissus viridiflorus  - a green, fall-blooming daffodil

Narcissus viridiflorus - a green, fall-blooming daffodil

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

The endangered  Narcissus alcaracensis .

The endangered Narcissus alcaracensis.

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

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

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

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

 

Meet The Powder Gun Moss

I get very excited when I am able to identify a new moss. This is mainly due to the fact that moss ID is one of my weakest points. I was sitting down on a rock the other day taking a break from vegetation surveys when I looked to my right and saw something peculiar. The area was pretty sloped and there was some exposed soil in the vicinity. Covering some of that soil was what looked like green fuzz. Embedded in that fuzz were these strange green urns.

I busted out my hand lens and got a closer look. This was definitely a moss but one I had never seen before. The urns turned out to be capsules. Later, a bit of searching revealed this to be a species of moss in the genus Diphyscium. This genus is the largest within the family Diphysciaceae and here in North America, we have two representatives - D. foliosum and D. mucronifolium.

These peculiar mosses have earned themselves the common name 'powder gun moss.' The reason for this lies in those strange sessile capsules. Unlike other mosses that send their capsules up on long, hair-like seta in order to disperse their spores on the faintest of breezes, the Diphyscium capsules remain close to the ground. In lieu of wind, a powder gun moss uses rain. In much the same way puffball mushrooms harness the pounding of raindrops, so too do the capsules of the powder gun moss. Each raindrop that hits a capsule releases a cloud of spores that are ejected into an already humid environment full of germination potential.

Luckily for moss lovers like myself, the two species of Diphyscium here in North America tend to enjoy very different habitats. This makes a positive ID much more likely. D. foliosum prefers to grow on bare soils whereas D. mucronifolium prefers humid rock surfaces. Because of this distinction, I am quite certain the species I encountered is D. foliosum. And what a pleasant encounter it was. Like I said, it isn't often I accurately ID a moss so this genus now holds a special place in my mind.

Further Reading: [1] [2]

 

The Fuzziest of Flowers

Describing plants can be quite a task for taxonomists. When a new species is discovered, the honor of naming it often goes to the discoverer. At the very least, they have some input. Some folks go for the more traditional rout and give the plant a descriptive name rooted in either Latin or Greek. Others decide to name the plant in honor of a botanist of the past or perhaps a loved one. Still others take a stranger approach in order to immortalize a famous celebrity. However, in doing so they risk taking something away from the species in question.

Instead of a descriptive name that clues you in on specific features of the plant, instead you hit an etymological dead end in which you are stuck with nothing more than a last name. This became quite apparent to University of Alabama botanist John Clark when it was time to name a newly discovered plant species from South America. 

Had things been slightly different, the recently discovered Kohleria hypertrichosa would have been named after Chewbacca. One look at the flowers of this species and you can understand why. The long tubular petals of this gesneriad are covered in dense, fuzzy hair. This is unlike any other plant known to science. The appearance of these odd fuzz balls may seem puzzling at first but considering where this plant was found growing, it quickly becomes apparent that these flowers are a marvelous adaptation in response to climate. 

Kohleria hypertrichosa is only known to grow in a very narrow swath of mountainous cloud forest in the Ecuadorian Andes. At home between elevations of 3,600 and 6,600 feet above sea level, this wonderful gesneriad experiences some pretty low temperatures for a tropical region. It is likely that the thick layer of hairs keeps the flowers a bit warmer than the surrounding air, offering a welcoming microclimate for pollinators. This could potentially make them much more likely to be pollinated in a habitat where pollinators may be in short supply. 

At the end of the day, Clark decided to stick with a more traditional name for this new species. Its scientific name is no less interesting as a result. The specific epithet 'hypertrichosa' is derived from a condition in humans known as hypertrichosis, or werewolf syndrome, in which a person grows excessive amounts of body hair. 

Photo Credit: Andreas Kay [1]

Further Reading: [1]

How North America Lost Its Asters

It's that time of year in northern North America where many of the most famous and easily recognized species come into flower, the asters. Some of my favorite plants once resided in this genus, but did you know that referring to our North American representatives as "asters" is no longer taxonomically accurate?

Since the time of Linnaeus, plants and animals have been categorized based on morphological similarities. With recent advances made in the understanding and sequencing of DNA, a new and more refined method of classifying the relationships of living organisms has been added to the mix. Much of what has been taken for granted for the last few decades is being changed. One group that has been drastically overhauled are the North American asters. At one time there were roughly 180 species of North American flowering plants that found themselves in the genus Aster. Today, there is only one, Aster alpinus, which enjoys a circumboreal distribution. 

Because the concept of "Aster" was developed using an Old World species (Aster amellus), New World asters were not granted that distinction. The New World species have shown to have their own unique evolutionary history and thus new genera were either assigned or created. By far, the largest New World genus that came out of this revisions is Symphyotrichum. This houses many of our most familiar species including the New England aster (Symphyotrichum novae-angliae). Some of the other genera that absorbed New World aster include Baccharis, Archibaccharis, Ericameria, Solidago, and Machaeranthera, just to name a few.

Taxonomy is often a difficult concept to wrap your head around. It is constantly changing as we come up with better ways of defining organisms. Even the concept of a species is something biologists have a hard time agreeing on. Surely, genetic analyses offer some of the best methods we have to date, a fact that the Angiosperm Phylogeny Group is constantly refining.

For some, this is all a bunch of silly name changes but for others this is the most important and dynamic form of natural science on the planet. Having a standard for naming organisms is a crucial component of understanding biodiversity. With a name, you can take the next step in getting to know and understand a beloved species. One thing to consider is that, as species are split and regrouped, often times what was thought to be one species turns out to be many. In the case of organisms which are threatened or endangered, a split like that can unveil a disastrous elevation into a far more dismal ranking.

Further Reading: [1] [2]