The survival of the fattest: Saving water, part 1

To stay alive, living organisms must be able to maintain a healthy water balance. In other words, over a certain period at least as much water must enter them as leave them.
Adaptations for collecting and storing water obviously are not very useful without suitable means to conserve the water as well.
As Gordon Rowley in his “Illustrated Encyclopedia of Succulents” (1978) put it so nicely: ”A storeroom for water is of no use if it lacks a door to prevent the contents from escaping”.
Although some water is lost through the surfaces  of leaves and stems, most of the loss is caused by transpiration through the breathing pores, so the key to the water balance is reduction of transpiration.
Transpiration rates are also influenced by factors such as temperature, humidity, presence and intensity of sunlight, precipitation, wind, land slope etc.
On hot days, the soil surface may reach a temperature of 75 ° C, whereas a few centimeters higher up it may be up to 40 ° C  cooler. In between, there is a layer of still air.
Similar conditions exist above plant surfaces and evidently they have a strong influence on transpiration. The layer is easily thinned or even destroyed by wind.
Many succulents have their leaves or stems arranged in such a way that pockets of still air are formed.
Often, these outgrowths also provide a certain shade and reflect or scatter sunlight hitting the plants.
The stems or leaves may also be covered in a mantle of spines, hairs etc. This coat produces a layer of more or less still air and thereby reduces transpiration; it also gives a certain amount of shade, which helps to diminish exposure to strong radiation – especially when the cover is light in colour.
Spines serve the same purpose as hairs. Usually they are modified branches, leaves or parts of leaves (but in the case of Euphorbia they are often hardened flower stalks and in Monsonia/Sarcocaulon they are hardened leaf stalks).

HAIRS

Crassula mesembryanthemoides

Pelargonium barklyi


In Huernia pillansii the hairs are in fact very elongated tubercles

SPINES

Monsonia crassicaulis


Euphorbia eustacei

STIPULES
 
The name of this plant (Avonia papyracea) refers to the white papery scales which hide the tiny green leaves almost completely. The scales are actually stipules (outgrowths at the bases of leafstalks);  in this case they are much bigger than the actual leaves and protect these against sun and wind. At the same time the stipules are able to trap water.


In Pelargonium hystrix the stipules look rather different   

 



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The survival of the fattest: Collecting water

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

For other posts in this series click here.