The Phyto-Philes: Cucumber Tree
Created | Updated Jan 4, 2015
The Amazing Cucumber Tree of Socotra
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!"
Again I have for you a very weird plant. This is the Cucumber Tree, Dendrosicyos socotranus. The species is native to the island of Socotra, which lies just off the tip of the 'horn' of Africa. Though closer to Africa, it is politically a part of Yemen, which is on the Arabian Peninsula. The plant's scientific name simply means 'Cucumber Tree of Socotra'. It is the only member of the Cucurbitaceae, the pumpkin, gourd, squash and cucumber family, to reach tree proportions. It has a thick, fat trunk, at the top of which thin, tendril-like branches arise, which bear the leaves. The leaves are somewhat hairy and have finely toothed edges. They grow to about 25 cm/10" in length and width. The tendrils rise for a short distance and then droop down again. Overall, the tree has a small crown for the size of its trunk. It can reach a height of over 3 m/10', and a trunk thickness of about a metre/yard. The flowers are small and yellow-orange, and the fruits are about 3 x 5 cm/1.2" x 2" in size. They turn brick-red when ripe, releasing the seeds. This species can only be grown from seeds, not cuttings.
Pumpkins of the Desert
The pumpkin family is very diverse worldwide, and also in Africa. They are some of the plants that thrive in quite dry regions. Many of them have subterranean tubers. Especially in dry, sandy regions like the Kalahari Desert, there is actually quite a lot of water underground. Rainfall, which is unpredictable but often heavy, once it occurs, rapidly seeps down deep into the sand. Down there it doesn't evaporate as fast as it would have had it remained lying on the surface exposed to the sun. The tuberous desert pumpkins send their roots down deep to get this underground water; the tubers themselves, buried deep under the sand, also don't dry out fast, and act as water reservoirs for the plants. The tubers send up thin, trailing stems to the soil surface. Most wild pumpkin species are able to send up very vigorous and rapidly growing tendrils, which spread around the soil and often clamber over surrounding vegetation, enabling the plant to make good use of the available sunlight.
In South Africa and many other places, some pumpkins have adapted a second growth form. In these cases, the tuber is not completely underground. Especially in hard soil or rocky places, a tuber cannot easily penetrate deep enough or have room to grow big. So these pumpkins develop tubers that are partly or almost completely above-ground. These are called caudices, and plants that have them caudiciforms. The caudiciform pumpkins tend to grow in moister regions than the tuberous pumpkins, because an above-ground tuber dries out faster than one that's buried. But the caudex still acts as a storage organ for nutrients and water. Most wild pumpkins have fairly small caudices, but the plants of the genus Gerrardanthus can have huge, boulder-like caudices growing over a metre/yard tall and wide. In typical caudiciforms, thin tendrils grow out of the top of the caudex, bearing the leaves, flowers and fruit. During the winter or during dry periods, these thin stems die off, but new ones emerge from the caudex when rains come again.
The cucumber tree takes things a final step further. The basal stem is now entirely above ground and becomes tall and stout, so that the plant is indeed a true thick-stemmed tree, or pachycaul. The trunk still acts for storing water. Socotra has a generally dry, semi-desert climate. Cucumber trees have fairly soft, pulpy 'wood' … not true wood … in their trunks. This can hold a lot of moisture, and the thick, corky bark protects them from drying out. It can drop its large leaves during dry times, and take advantage of ample rains by rapid growth of the tip tendrils and production of lots of leaves. While it tolerates the dry climate very well, in cultivation it is very grateful for regular watering.
Socotra has been isolated from the African continent as well as from Arabia for about ten million years. Its flora has links with both. But its plants have also been evolving independently over that time. Today, there are many species found only on the island. Another thing is that these Socotran plants tend to look very weird! The cucumber tree with its great, thick, bottle-like trunk, supporting the small crown of tendrils and big, hairy leaves, has a strange, primeval look. Indeed, it looks somewhat like a person … a stout character with a wild mop of green hair! On the island it is joined by other weird succulent plants like the Socotra Dragon Tree, Dracaena cinnabarii (a relative of the one growing on the Canary Islands), the huge bottle-like Desert Roses, Adenium socotranum (relatives of the Impala Lilies), the weird, succulent, open-fruit "figs", Dorstenia gigas, the strange succulent grape, Cissus subaphylla, a few unique species of the Carrion Flower family, and many more.
People started colonizing Socotra a few thousand years ago, and have caused much harm to the original vegetation. Before, Socotra had no large grazing or browsing animals. Once goats, sheep and camels started browsing the vegetation, much of the island was denuded. The climate became dryer as well. Plant growth in general in a region has a favourable effect on the climate. Plants retain rainfall better than bare soil does. The water is held in the plants rather than sinking or running away. Underneath the plants, the leaf litter and humus layer grows; the soil becomes more fertile and able to retain more water. Plants slowly release moisture into the atmosphere from their leaves, creating moister air over the soil. Life creates favourable conditions for more life. Socotra, as it was before humans and their livestock reached it, must have been much lusher than the largely barren place it is today. But even today, there are still areas where substantial natural vegetation remains. If it were up to me, I'd have the humans of Socotra working hard to re-establish the endemic plants wherever possible.
The cucumber tree is quite vulnerable to overgrazing. Young plants are rather delicate, and probably quite tasty to goats. For plants to get established, they must be protected until they are big enough to be safe from browsers. Fortunately the mentioned succulent grapes, which are distasteful to browsers, grow into dense thickets, and there are also spiny shrubs of the genus Lycium (relatives of tomatoes). Seedlings of the cucumber tree that germinate within these thickets or beneath the spiny crowns can grow for a long time, safe from animals, and eventually become big adult trees.
As you might guess from its prehistoric appearance, the cucumber tree has an ancient lineage. It is quite distinct from other members of the pumpkin family, showing that it has been evolving separately for a very long time. Indeed, it appears to have been a distinct lineage for twice as long as the island of Socotra has existed! So, before the island split off, there must have been cucumber-tree-like things growing on the African continent. As evolution continued and large browsers, most significantly elephants, evolved, these plants were eradicated from the continent, but persisted and kept evolving on this small island.
I have never grown cucumber trees and can't advise on how it is done. But there are people today who grow them, and it doesn't seem to be particularly difficult. They make fascinating succulent subjects, and also, cultivated individuals contribute to the survival of the species as a whole. I honestly hope that soon we could plant great numbers of them back into the wild, along with Socotra's two-hundred-plus other unique plant species.
