Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"

Today I bring you some plants again. I recently found and photographed some lovely flowering specimens of these strange succulents. The species you see here, doesn't really have a common name, but the name 'ghaap' is used for other species in the genus, and this is the easternmost-occurring of them all, thus I think we can use the name 'Eastern Ghaap' for it. The name 'Ghaap' comes from the Khoi-people who used to live in the dry southwestern parts of South Africa, where the majority of the species can be found. The genus name, Hoodia, commemorates a British succulent grower called Mr. Hood. Other than South Africa, the genus Hoodia also occurs in the countries of Namibia, southwestern Angola, central to eastern Botswana, and southern Zimbabwe. It is most diverse in South Africa and Namibia. The species I illustrate here, is Hoodia currorii, named for the navy doctor A. B. Curror, who discovered it when his ship landed on the coast of Angola. From here the species grows into north-western Namibia. But the ones I show here are of the subspecies Hoodia currorii subsp. lugardii, named for discoverer Major Edward Lugard, which ranges from central Botswana eastwards to the far north of South Africa and the far south of Zimbabwe. It is the only Hoodia species that is found here in my province, Limpopo. There's a big gap between its distribution and those of the other members of Hoodia as well as of the main subspecies. This Hoodia reaches a height of about 1 m, the stems a thickness of 8 cm/3.2" and the flowers a diameter of 7.5 cm/3".

Regular readers of my articles will know about the phenomenon called 'convergent evolution' which I mention very often … almost in every second article! That is where animals that are not closely related nevertheless look very similar because of becoming adapted to similar lifestyles, for which a certain kind of body shape is ideal. And this same phenomenon can also be found in the world of plants! Look closely at the photos of the stems of the ghaap. They look very much like those of a cactus! It has the same columnar form as many cacti, it has the same 'build' with ridges going all along the stem, with thorns spaced evenly along each projecting ridge. And yet, a ghaap is not closely related at all to a cactus. It is a member of the Asclepiadaceae, the Milkweed family. As you may gather, that family is indeed mostly composed of weedy plants with soft, twining or scrambling stems. Some of the milkweed family have become adapted to arid climates, such as are found over most of South Africa. What is very useful to plants in arid climates, are storage organs, tissues that can absorb and hold a lot of water. In these dry lands, rain does fall, but at unpredictable times. In any typical year, only a little, or even no rain at all, may fall. But then every so many years there are abundant, heavy showers. If plants can rapidly absorb as much of this water while it is present, and store it, they can stay alive throughout the dry times. The water not absorbed by plants will quickly sink deep into the desert sand, flow away in ephemeral rivers and streams, or fill the large, shallow, temporary lakes called 'pans' where it will eventually evaporate.

One way for plants to store water, is in big subterranean storage organs: tubers, bulbs or corms. These are modified stems, leaves or roots. Several members of the milkweed family have indeed developed underground tubers to store water, as well as food. The food consists of carbohydrates, that is starches and sugars that the plant forms using its green leaves and stems, from water, carbon dioxide, and energy derived from sunlight. So a lot of water as well as food becomes stored underground, and this powers the plant to survive and resprout after having been dormant during dry seasons or entire years.

But plants can also store water and food in their above-ground parts. These plants are called succulents. They can have succulent leaves and/or stems. These parts are noticeably thick and 'juicy'. Many people think that 'succulents' represent a group of plants, a family or order of species that are all related to each other. This is not at all the case. Plants from pretty much across all the different families and orders of flowering plants have independently evolved into succulents, by the process of convergent evolution. Certain families have more succulent members than others, so for instance the cactus family of the Americas, and the Crassula family found almost world-wide, are mostly comprised of succulents. But then you get some families that are typically herbs, woody shrubs or trees of a normal kind, except some of them have become succulent. So it is with the milkweed family. The family includes mainly stem succulents, but it is amazing how many different ways the different succulent Asclepiad species have found of developing and structuring their thickened stems.

In the milkweed or Asclepiad family, the largest group of succulent species are called the Carrion Flowers. We'll get to their flowers later; for now I want to talk about their stems. It is perhaps the case that they all evolved from a species of climbing herb, with long, scrambling and twining stems. These stems became succulent in some kinds, for instance in many species of the genus Ceropegia, which may be the ancestors of all the carrion flowers. Then, the stems became shorter, so that now most carrion flowers are no longer climbing or scrambling plants, but form low tufts or clumps. These plants lost their leaves. At least, the leaves are no longer recognizable as such. They became small points arranged on ridges around the stems. In some, they dry out and become hard, like short thorns. And there you have the ghaap! It is the most drought-adapted group among the carrion flowers. The stems are larger, thicker, and with more ridges than the others, and it has the hardest, sharpest thorn-leaves, so that indeed it looks just like a cactus.

All of these features are adaptations to drought. The thicker the stems, the more water they can store and the more slowly the water will evaporate. This is also why they lost their leaves: in plants, the leaves are mainly how they lose water, since the leaves have flat surfaces as well as little 'breathing holes' called 'stomata' which take in and release air, including water vapour. So in the ghaap, as in cacti, the stems have taken over the function of making food by photosynthesis, which is why the stems are greenish. They have fewer stomata than functional leaves and a smaller surface-to-volume ratio. They're also covered by an outer 'skin' or cuticle, that protects the inner tissues against harsh sunlight, often using pigments that can colour the stems reddish or purplish. The ridges around the stems are such that the stems can absorb water and swell, with the ridges becoming thicker and expanding into the grooves between them when the plant has water to store. The plants have extensive roots close to the soil surface, so that they can quickly absorb water. The leaves no longer function as photosynthesing organs. Instead, they harden and become woody and sharp. Now they function for defense, protecting the succulent stems and the food and water they store, from animals that want to eat them. All of these adaptations have happened in the ghaap, exactly as they have arisen in cacti, to enable the plants to survive as well as possible in dry regions.

But when you look at the flowers, you can see that the ghaap is no close relation to a cactus. Most cacti have quite large and pretty flowers, with lots of petals. They mainly use typical pollinators like butterflies, birds or, in some kinds, bats. But the ghaap has flowers with a large, somewhat concave surface, like a satellite dish, which is made of five petals that have become fused together except for short, thin points on the flower's perimeter. In the very centre there is a small raised ring called an annulus, inside which is a small hollow containing a fascinating structure called a gynostegium. This structure is similar in the ghaap and in all the other carrion flowers. It contains pollen packets that are made to be snagged by the leg or mouthparts of a fly as it inspects the flowers. The flowers smell (mildly in the case of the ghaap) of rotting meat or faeces, so the fly thinks there will be food for its maggots. Many times the fly will even lay eggs on the flower. But the maggots will find no food. If they hatch, they soon die. But the fly, in exploring the flower, hopefully snags one of the pollen packets. When it is done with that flower, it flies, hopefully to another flower on another plant, where the pollen packet will again snag on the gynostegium, this time detaching, even as the fly may pick up a new pollen packet from the new flower. The detached pollen packet will now send a tube down into the gynostegium of the flower on which it has been deposited, from which it will release its pollen to fertilize the ovules in the ovary at the centre of the gynostegium. The fertilized flower will wilt, the petals will fall off, and the ovary will expand and develop into the fruit. The fruit is in the form of a pair of pointed follicles, called 'bokhorinkies' ('goat's horns') in Afrikaans. When ripe, they split open to release the flat seeds, which have white tufts of fluff which allow the wind to carry them far and wide, hopefully to a place they can germinate and grow into new plants.

As conspicuous desert plants, ghaaps are used by humans and animals. The stems can be eaten! If you cut open a stem you'll see it is fresh green and juicy on the inside. The people of the Northern Cape collect the stems, then use knives to cut the thorns off all along the ridges. The stems can be eaten fresh or can be soaked in water before eating. They are bitter in taste but nourishing, and also have an appetite-and-thirst-suppressing, and maybe even stimulating, effect. They're prized by shepherds who gather and eat them while tending their flocks. They will often smoke tobacco after eating the ghaap, because it has a liquorice-like aftertaste which improves the smoke's flavour. Species of Hoodia are now being investigated and experimentally grown all over the world for making an appetite-suppressing supplement from. They're also used to treat indigestion, hypertension, diabetes and stomach ache. A couple of species are used to treat haemorrhoids and pulmonary tuberculosis.

We also found a caterpillar eating one of the plants; I'm sorry that I neglected to photograph it. It's the baby of a beautiful butterfly, the African Monarch, Danaus chrysippus. These caterpillars feed on plants of the milkweed family, to which as I've said the Hoodias and other carrion flowers belong. The caterpillar extracts toxic compounds from the plants to make itself poisonous as protection against predators. While the ghaap is not poisonous, the caterpillar can likely extract and concentrate some of its ingredients to the point where they become toxic.

The eastern ghaap is not currently considered endangered, but it is a rare species, having been recorded only a few times in South Africa. It is a protected species. I actually hope that not too many people learn about it! Otherwise it might get over-harvested. People may take it either for its nourishing or medicinal uses, or for its ornamental value. It is quite an unusual and impressive-looking plant, which people may want to add to their succulent collections and rock gardens, even removing big plants from the veld. Unfortunately it doesn't survive well outside of its natural habitat. It will perish in cold climates, and even a little more moisture than it is used to will cause its roots and stems to rot. I hope to be doing some research on growing this species from wild-harvested seeds, but until we know a way to propagate it and provide nursery-grown specimens, I hope that people will leave it alone.

Willem

28.09.20 Front Page

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