These large shrubs or small trees are up to 6 m tall, usually much-branched from near the base. The young branches are softly woody and have big leaves (to 6 x 8 cm), with 3-5 triangular lobes.
The unisexual flowers are creamy-white with red stripes near the base and produce large green capsules. Oil from the seeds was traditionally used for lighting but nowadays also for manufacturing glycerine, soap and biofuel.
The species is widespread in dry bush and forest in southwestern Madagascar, especially on limestone, and is the only Jatropha that occurs naturally on the island.
As we have seen, the ability to store water is the main characteristic of succulent plants. The most important aspect of the stored water, is how much of it can be withdrawn from the storage tissue and become available to the rest of the plant. This fraction is called the utilizable water and is very different from one succulent to the other.
We usually talk about leaf, stem or root succulents, depending on where most of the water is stored. But because many plants have succulent tissues in more than one type of organ, the distinctions are not always clear.
In many stem succulents, ribs and tubercles help the stems expand and contract as their water content changes. They also assist in directing water from the stems to the roots and in shading different parts of the stem whenever the sun is shining.
Ribs: Stapelia grandiflora (1) and Quaqua pillansii (2)Tubercles: Senecio stapeliiformis (1) and Euph. clandestina (2)
In leaf succulents we sometimes see similar anatomic features, which make changes in volume possible in the storage tissues and thus in the leaves. As the water volume of these leaves decreases, this will lead to folds in them at fixed places, so that they become shorter and flatter.
Curio (Senecio) abbreviatus in the rainy (1) and in the dry season (2). Picture 1 courtesy George HattinghIt has been found that in plants such as this Aloe pearsonii, the leaves may lose up to 60 % of their water and still be able to recover
The globular shape we see in many succulents is optimal for water storage, because it combines maximal volume with minimal surface area.
Euph. globosa
Some plants have an epidermis with so-called idioblasts (extremely enlarged and swollen bladder cells). These may supply up to over half of the total water storing capacity of the leaf.
When the plants start suffering from drought stress, the cells collapse. This obstructs the passage of air to and from the stomata, so that water loss is reduced.
This external storage is most common in the Aizoaceae.
Drosanthemum hispidum (1) and Mesembryanthemum barklyi (2)
Of all the problems succulents face in nature, the main one is the scarcity of water.
In most dry areas the rainfall is unpredictable, with long periods of drought. For at least part of the year, this goes hand in hand with a low humidity (in daytime often lower than 20%), which in combination with high temperatures leads to high evapotranspiration.
Even when there is rain, only part of the water becomes available to plants. The more water comes down in one go, the more runs off or otherwise disappears, especially in rocky and sandy soils.
When water becomes available, succulents must be able to collect it as quickly and thoroughly as possible and they have developed a number of adaptations for that purpose.
Most of them have a shallow rooting system, mainly at a depth between 5 and 15 cm and often extending 10-20 m away from the plant.
A disadvantage of such a root system is that the topsoil may become very hot (70° C or higher) but it allows the plants to absorb even small quantities of rain, dew or mist. The roots often end underneath stones, where they form dense mats. Stones condense dew and mist and collect the water at their base and in crevices; they also protect the roots against drying out.
Shortly before dawn, plants too often reach such low temperatures that a substantial amount of dew accumulates on their surface. (Argyroderma delaetii and Cephalophyllum curtophyllum)
In the dry season many succulents lose their fine roots, but even a little bit of moisture will quickly cause the growth of very fine so-called rain rootlets, which will then absorb nearly all the available moisture.
When enough water becomes available, succulents may take up so much that they literally burst.
A plant such as this Augea capensis may increase its weight several times after rain
This plant of-Fenestraria rhopalophylla is just a few cm across but has roots which may cover up to two m2. Fenestraria occurs in the mist zone on the coast of northern South Africa and southern Namibia, where the sea mist is the main source of water.
Because dew and mist are often more reliable than rainfall, they are important sources of moisture for other plants as well. These plants of Aloe ferox grow not far inland from the south coast in South Africa and regularly receive mist rolling in from the sea.The large leaves of this Aloe microstigma cool down sufficiently at night to collect dew. In many places dew may be the main or even only source of water for months.Mesembryanthemum ( Prenia) sladenianum has spoon-shaped leaves that are perfect for condensing dew and mist
At night, protuberances such as spine tips, hairs, papillae, bladder cells, thorns and spines will become cooler than the rest of the plants and the surrounding air, so that dew condenses at them and is channelled to the roots.
In certain cases, the water can be absorbed directly by these appendages.
Crassula sericea var. velutina uses inflatable epidermis cells
The white scales of this Avonia papyracea are so-called stipules, outgrowths at the base of the leaves. In this case they are much bigger than the leaves themselves and protect these against sun and wind. At the same time these stipules are able to trap water.Crassula barbata
Trichodiadema marlothii Back in 1908, Rudolf Marloth in his famous book Das Kapland, reported the following on plants of this genus: “As soon as one puts a drop of water on a hair at the tip of a shrivelled leaf, the cells suck it up and in a short while the leaf is plump again”.
The term hydathodes is normally used for structures that are in control of guttation (loss of water in the form of drops from the margins of leaves).
The hydathodes that are found in nearly all species of the genus Crassula perform the opposite function: taking up condensed water and atmospheric water vapour. They are arranged in one or two rows along the margin and/or they are distributed over the surface of the leaf. and are often surrounded by trichomes (hair-like growths) which are supposed to assist in trapping water.
Crass. sladeniana
Crass. nudicaulis var. platyphylla
In some plants, only the young leaves have hairs, whereas the older ones are smooth. The first of the two pictures above shows a young plant of Tylecodon paniculatus. In the second picture a mature plant of the same species is accompanied by a plant of T. wallichii (on the left). These species sometimes hybridise in the wild.With their many thin stems, broom-like Euphorbias such as this E. burmannii do not look very well equipped for dry conditions. It seems however that they are rather effective in collecting condensed dew and mist and channelling this moisture to their roots.The same applies to other thin stemmed plants such as this Crassula muscosa v. obtusifolia
