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.
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.
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
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. nudicaulis var. platyphylla
