Many families of plants which live in comparatively dry regions have evolved structures to withstand long periods of dryness. Such plants are scientifically known as xerophytes. Many such structures, though not all, can be classed under the heading of succulence – the provision of fleshy, water-retaining tissue, often protected in some way from the scorching effect of the sun.are perhaps the most obvious example of plants exhibiting this form of adaptation but, in fact, they compose only one of the many plant families which have succulent members.
Even the most highly specialised succulent plant requires some water, and no plant can live in completely arid desert. Some do live, nevertheless, in regions where therainfall is as little as 3 or 4 in., and may be restricted to a single month. This, however, is exceptional, and night temperatures, which may fall to almost freezing-point, provoke heavy dew, which plays a considerable part in the life of such plants. In many regions where succulents grow, a fairly heavy rainfall is concentrated into a few weeks, while during the rest of the year little or no rain falls. But it must not be thought that succulent plants inhabit only semi-desert regions. Some of the Cactaceae, for instance, live as epiphytes – that is, perched on trees or rocks – in the tropical rain-forests of South America; and many of the others live in the African scrub and jungle.
The conditions in high mountains are basically similar to those in deserts, due to almost equally restricted rainfall, drying winds and, often, very limited soil. In winter the ground may be frozen and water is withheld from the plants. The hardy succulents, such as sedums and sempervivums, some of which are native to Britain, have been evolved in such conditions.
The grower of succulents does not usually include the hardy kinds; but this is a rather arbitrary outlook, for hardiness is relative, and somewhich live on the high plateaux of Mexico, or in the Andes, may be covered in snow during the winter. That they and the other hardy succulents survive in the cold is due to their being dry at these times.
Some of the South African succulents will withstand a soil surface temperature of 60°C. (40°F.) and a relative airas low as 10 per cent. Those species that live in scrub or forest are obviously accustomed to greater atmospheric humidity.
The structural mechanisms developed to deal with severe drought conditions are varied and ingenious. They must clearly be devoted (1) to absorbing water quickly when it is available, via an extensive-system, and (2) to reducing water loss (transpiration) to the minimum. In this way the maximum water is retained in the tissues — sometimes as much as 95 per cent, of the plant’s volume is water. In forms with , these are almost always protected in some way against the full force of the sun. Often only a thin layer of cells is left tocarryout , bywhich (together with minerals from the soil) the plant’s food supply is produced: the rest of the tissue consists of water-storage cells. The outer skin is usually thick, often coated with wax or with white hairs, which act as an insulating medium.
The rosette arrangement of leaves is common, since it permits an extensive photosynthesising surface in the minimum space.
More’ often the leaves tend to diminish in number and size, and to become cylindrical or globular in shape, which reduces the area of transpiration in relation to volume. They may be closely pressed to the, or in extreme examples the stem is dispensed with and the plant becomes a single, more or less globular body. The leaves may wither and shrink in the dry period, when no growth occurs at all.
Some of these ‘plant-bodies’ grow so that only the round, flattish top is exposed to the sun. In some curious examples a ‘window’ is formed at the top which reduces the power of the sun’s rays but allows sufficient to reach the chlorophyll layer deep inside the plant-body.
This is one extreme: the other is the more or less complete absence of leaves. Most of the cacti are leafless or bear leaves for only a short time, and some other succulents, for example the euphorbias, are similar. Such plants, known as stem succulents, are thick and fleshy, consisting almost entirely of water-storage cells, with a thin photosynthesising layer protected by a thick skin — a function of the leaves taken over by the stem. They are usually globular or columnar, shapes again which minimise the proportion of surface area to volume. Many stem succulents also shrink in the dry season, to plump out again at the first rains.
Of course the minimisation of surface area reduces photosynthesis as well as transpiration. Hence the growth of the most highly drought-adapted forms is slow.
Another group accepted as succulents includes plants with normal leaves but which have a swollen or bulbous stem above ground. Ordinary bulbous and tuberous plants, with their swollen parts below ground, are not regarded as succulents, though indeed they are often adaptations to at least a seasonal period of drought.
Many people only like cacti if they flower readily and at an early age. Here are somegenera which can be recommended, though to obtain they must be repotted regularly, watered properly and, in most cases, given full sun. Good nurserymen will advise on the best flowering species:
Chamaecereus sylvestrii, Echinopsis eyriesii, most species of Gymnoca/ycium,, , Notocactas, Parodia and Rebutia.
Among epiphytic cacti which need a warmer, moister atmosphere, and also do well in rooms, are Aporocactus, Epiphyllwn hybrids, Rhipsalidopsis rosea andspp.
Other succulents which give regular, showy blooms are Crassula falcata, Delosperma spp., many Echeveria spp., Euphorbia bojeri and E. splendens,blossfeldiana and its modern varieties, Lampranthus and other shrubby Mesembryanthemum spp., Oscularia spp., Rochea coccinea, and spp.
It is fairly common to find various garden plants with a flattened or distorted stem. This is usually due to fasciation, a phenomenon in which the growing point of a stem divides and multiplies abnormally. Fasciated succulents are known as cristates.
Among succulents, especially cacti, this is very common, and there are a large number of cristate forms, which are sometimes beautiful and always extraordinary. Some cristates develop in one direction only, resulting in a flat fan-like growth which in time becomes convoluted, not unlike coral; in others a vast number of tiny heads is produced, or sometimes the rib and tubercle formation becomes entirely irregular. Examples of the latter two are often referred to as monstrosities rather than cristates. The Latin word monstrosus or cristatus after the specific name indicates a fasciated variety.
Cristate cacti are reproduced by grafting or, if suitable, from; cristate succulents other than cacti are increased by cuttings. seldom reproduces cristation.