The Ceropegias Welcome a New Member

Photos by David Styles

The genus Ceropegia is home to some of my favorite plants. Not only are they distant cousins of the milkweeds (Asclepias spp.), they sport some of the most interesting floral morphologies whose beauty is only exceeded by their fascinating pollination syndromes. Recently, Ceropegia expert and friend of the podcast Dr. Annemarie Heiduk brought to my attention the recent description of a species named in her honor.

Ceropegia heidukiae hails from KwaZulu-Natal, South Africa, and, at current, is believed to be endemic to a habitat type called the Northern Zululand Mistbelt Grassland. Morphologically, it has been described as an erect perennial herb. Unlike many of its cousins, C. heidukiae does not vine. Instead, it grows a slender stem with opposite, ovate leaves that just barely reaches above the surrounding grasses. By far the most striking feature of this plant are its flowers.

Photos by David Styles.

Ceropegia heidukiae produces elaborate trap flowers at the tips of its slender stems during the month of December (summer in the Southern Hemisphere). Each flower is comprised a greenish-gold, striped tube made of fused petals and topped with a purple, star-like structure with fine hairs. These flowers were the key indication that this species was previously unknown to science. Additionally, a sweet, acidic scent was detected during the relatively short blooming period.

Their beauty aside, the anatomy and scent of these flowers hints at what may very well be a complex and specific pollination syndrome. Indeed, scientists like Dr. Heiduk are revealing amazing chemical trickery within the flowers of this incredible genus, including one species that mimics the smell of dying bees. Who knows what kinds of relationships this new species has evolved in its unique habitat. Only plenty of observation and experimentation will tell and I anxiously await future studies.

A view of the Northern Zululand Mistbelt Grassland where Ceropegia heidukiae was found.

Sadly, C. heidukiae lives in one of South Africa’s most threatened habitat types. South Africa’s Biodiversity Act currently classifies the Northern Zululand Mistbelt Grassland as endangered due to factors like timber plantations and unsustainable grazing. Hopefully with the recognition of unique species like C. heidukiae, more attention can be given to sustainable use of the Northern Zululand Mistbelt Grassland such that both the people and the species that rely on it can continue to do so for generations to come.

Photo Credits: David Styles

Further Reading: [1] [2]

A Rare Succulent Member of the Milkweed Family

Photo by: Gennaro Re

Across nearly every ecosystem on Earth, biodiversity tends to follow a pattern in which there are a small handful of very common species and many, many more rare species. It would seem our knowledge of plants follows a similar pattern; we know a lot about a small group of species and very little to nothing about most others. Take, for example, a succulent relative of the milkweeds known to science as Whitesloanea crassa. Despite its occurrence in specialist succulent plant collections, we know next to nothing about the natural history of this species or if it even still exists in the wild at all.

Without flowers, one would be hard pressed to place this odd succulent within a family. Even when in bloom, proper analysis of its taxonomic affinity requires a close inspection of the floral morphology. What W. crassa exhibits is a highly derived morphology well-adapted to its xeric environment. Native to Somalia, it was said to grow on bare ground and its appearance supposedly matches the rocks that dominate its desert habitat. Never producing leaves or branches, the main body of W. crassa consists of a succulent, quadrangular stem that slowly grows upwards as it ages.

Flowers are produced in a dense inflorescence, which is most often situated near the base of the plant. Each flower is very showy at maturity, consisting of a fleshy, fused, 5-lobed corolla decorated in shades of pink and red. As far as I can tell, this is not one of stinkier members of the family. Though I have found pictures of flowers crawling with maggots, most growers fail to comment on any strong odors. In fact, aside from limited care instructions, detailed descriptions of the plant represent the bulk of the scientific information available on this odd species.

Maggots crawling around inside the flowers indicates this species mimics carrion as its pollination mechanism. Photo by: Flavio Agrosi

As I mentioned, it is hard to say whether this species still exists in the wild or not. The original mention of this plant in the literature dates back to 1914. A small population of W. crassa was found in northern Somalia and a few individuals were shipped overseas where they didn’t really make much of an impact on botanists or growers at that time. It would be another 21 years before this plant would receive any additional scientific attention. Attempts to relocate that original population failed but thanks to a handful of cultivated specimens that had finally flowered, W. crassa was given a proper description in 1935. After that time, W. crassa once again slipped back into the world of horticultural obscurity.

A few decades later, two additional trips were made to try and locate additional W. crassa populations. Botanical expeditions to Somalia in 1957 and again in 1986 did manage to locate a few populations of this succulent and it is likely that most of the plants growing in cultivation today are descended from collections made during those periods. However, trying to find any current information on the status of this plant ends there. Some say it has gone extinct, yet another species lost to over-collection and agriculture. Others claim that populations still exist but their whereabouts are kept as a closely guarded secret by locals. Though such claims are largely unsubstantiated, I certainly hope the latter is true and the former is not.

Photo by: Flavio Agrosi

Our knowledge of W. crassa is thus restricted to what we can garner from cultivated specimens. It is interesting to think of how much about this species will remain a mystery simply because we have been unable to observe it in the wild. Despite these limitations, cultivation has nonetheless provided brief windows into it’s evolutionary history. Because of its rock-like appearance, it was assumed that W. crassa was related to the similar-looking members of the genus Pseudolithos. However, genetic analysis indicates that it is not all that closely related to this genus. Instead, W. crassa shares a much closer relationship to Huernia and Duvalia.

This is where the story ends unfortunately. Occasionally one can find cultivated individuals for sale and when you do, they are usually attached to a decent price tag. Those lucky enough to grow this species successfully seem to hold it in high esteem. If you are lucky enough to own one of these plants or to have at least laid eyes on one in person, cherish the experience. Also, consider sharing said experiences on the web. The more information we have on mysterious species like W. crassa, the better the future will be for species like this. With any luck, populations of this plant still exist in the wild, their locations known only to those who live nearby, and maybe one day a lucky scientist will finally get the chance to study its ecology a little bit better.

Photo Credits: [1] & Flavio Agrosi [2] [3] [4]

Further Reading: [1] [2]

A Tree That Makes Poisonous Rats

For many organisms, poisons are an effective means to keep from being eaten. However, making poisons can be costly. Depending on the compounds involved, poison synthesis can require a lot of nutrients that could be directed elsewhere. This is why some animals acquire poisons through their diet. Take, for instance, the monarch butterfly. As its caterpillars feed on milkweed, they sequester the milkweed toxins in their tissues, which makes them unpalatable into adulthood. Cases like this abound in the invertebrate world, but recently scientists have confirmed that at least one mammal has evolved a similar strategy.

Meet the African crested rat (Lophiomys imhausi). Its large size and bold color patterns make it look like the result of a passionate encounter between a porcupine and a skunk. However, it is 100% rat and it has a fascinating defense strategy that begins with a tree native throughout parts of eastern Africa aptly referred to as the poison arrow tree (Acokanthera schimperi).

An African crested rat displaying its crest of toxic hairs and aposematic color pattern. [SOURCE]

The poison arrow tree is a member of the milkweed family (Apocynaceae), and like many of its relatives, this species produces potent toxins that can cause heart failure. The toxic nature of this tree has not been lost on humans. In fact, the particular strain of toxin it produces is referred to as ouabaïne or “arrow poison” as indigenous peoples have been coating their arrows with its sap for millennia. It turns out that humans aren’t the only mammals to find use for this sap either. The African crested rat uses it too.

The African crested rat grows highly specialized crest of hairs along its back. These hairs are thick and porous and when the rat feels threatened, it erects the crest and shows off its stark black and white coloring. It has been noted in the past that predators such as dogs that try to eat the rat run the risk of collapsing into convulsions and dying so the idea was put forth that that crest of hairs was toxic. Only recently has this been confirmed.

By studying a group of these rodents, scientists observed an interesting behavior. Many of the rats in their study would chew and lick twigs and branches of the poison arrow tree and then chew and lick their crest. What this behavior does is transfer the plant toxins onto those specialized hairs. The high surface area of each hair means they can soak up a lot of the toxins. Surprisingly, the rats appear to be resistant to the sap’s toxic effects. Perhaps they possess modified sodium pumps in their heart muscles that counter the effects of the toxin. Or, they may possess a highly specialized gut flora that breaks down the toxins. Either way, the rats do not show any signs of poisoning from this behavior.

A close-up view of the African crested rat’s poison anointed hairs. Photo by Sara B. Weinstein

The rats don’t have to do this very often to remain poisonous. By talking with locals that still use the poison arrow tree sap on their arrows, researchers learned that the compounds are extremely stable. Once coated, arrows will remain toxic for years. As such, the African crested rat likely doesn’t need constant application for this defense mechanism to remain effective.

As far as we know, this is the first example of a mammal sequestering plant toxins as a form of defense. It is amazing to think that a defense strategy evolved by a plant to avoid being eaten can be co-opted by a rat so that it too can avoid being eaten. Sadly, it is feared that this unique relationship between rat and tree is starting to disappear. Though more research is needed to accurately assess their numbers, there is growing evidence that African crested rats are on the decline. Hopefully with a bit more attention, these trends can be properly assessed and conservation measures can be put into place. In the meantime, please avoid putting any and all rats in your mouth.

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

Further Reading: [1]

Learn to Love Bluevine

I will admit that it took me a bit to figure this plant out. At first I thought I was looking at a significant bindweed infestation. These heart shaped leaves were twinging all over our fence. Then it flowered and I realized that this was no bindweed. This mysterious vine was none other than bluevine (also commonly called honeyvine, Cynanchum laeve)

Believe it or not, this is a species of milkweed. Though not in the genus Asclepias, it nonetheless belongs within the same family (Apocynaceae) and is close enough in relation to function as a host for species such as the charismatic monarch butterfly.

Mention this in many of the native plant groups and you are bound to be met with resistance. Because this species can be weedy, many people seem to want to overlook its value as a food source for monarch caterpillars. There is even scientific evidence to suggest that there are no significant differences in fitness and survival among caterpillars raised on either common milkweed or bluevine. The authors of one study even make the conclusion that,

“Given the abundance of honeyvine milkweed in the east-central United States, this species may be a more important host plant for the monarch than has been generally recognized.”

The biggest problem people seem to have with bluevine is that it can be very aggressive in disturbed soils. In many places it is considered a serious agricultural pest. Like its milkweed cousins, its seeds erupt from pods and are born on light, feathery filaments. Because of this they can travel great distances on the slightest breeze. They germinate readily and, once established, the plant can regrow from rootstock.

Regardless of where you stand on bluevine, there is no denying that it is an interesting species. Its flowers are packed into clusters and smell heavily of honey. They are primarily visited by small solitary bees. As is typical of the family, bluevine produces some serious chemical defenses. As such, it is generally ignored by mammalian herbivores but is readily consumed by many of the other native milkweed specialists in North America. So, I urge you to consider giving bluevine a chance. You may grow to love its hardy disposition and its great ecological value.

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