Since water is one of the essentials of plant life, it is not surprising to find that many adaptations are shown by plants in connection with the water supply. According to the nature of these adaptations, plants have been grouped as Xerophytes, Hydrophytes and Mesophytes. Xerophytes are plants which can withstand very dry conditions, hydrophytes are plants which live in moist places or actually in water and mesophytes occur in places which are neither excessively wet nor dry but have a moderate water supply.
These plants are characterized by the possession of certain adaptations known as xeromorphic features, some of which enable them to survive under conditions of drought. The danger of desiccation to which xerophytes are subject may be due to a low rate of absorption of water by the, or to a high rate of evaporation from the aerial parts, or to both these causes. The following are the most important conditions which result in the occurrence of xeromorphic features :—
A region may be physically dry owing to insufficient rainfall, or to the nature of the soil, which allows the rain to drain through. Physical dryness is a characteristic of the deserts of hot countries and the sandy soils and mountain screes in Europe. In such situations theof plants have difficulty in obtaining enough water.
Dryness of the Air
In hot deserts and on mountains and moors which are exposed to intense sunlight and strong winds the air is very dry, so that the rate of transpiration is high. In such places there is danger of desiccation owing to the plant losing water from thefaster than it can absorb it from the soil.
The roots of plants are unable to absorb sufficient water from a cold soil, so that soils which are very wet may nevertheless show a xerophytic vegetation because the water is unavailable for plant use owing to the low temperature of the soil. Such a soil is said to be physiologically dry, and many Alpine soils show this type of dryness. Xerophytic vegetation is also characteristic of moors and salt-marshes, and until recently these areas were considered to show physiological drought. In the case of moors the difficulty of water absorption was supposed to be due to the acidity of the water in peaty soil. In salt-marshes the high osmotic pressure of the salt solution in the soil was said to be the factor which hindered absorption. But it has been shown that absorption docs take place freely in peaty and saline soils, so that the above theories do not explain the xeromorphic features of the plants which grow in such soils. The real cause of the xerophytism exhibited by moorland and salt-marsh plants is not yet properly understood.
Xerophytcs show many structural adaptations by which resistance to drought is achieved. Some xerophytes show increased powers of absorption, while others store up water in special tissue from which it can be drawn when required. Many xerophytcs have peculiarities of structure in the leaves which restrict the rate of transpiration.
Resistance to Drought by Increased Power of Absorption
As a rule the roots of xerophytes are extensively developed in comparison with the shoots, where the depth of the soil permits this. Some desert plants have long and slender roots that reach to great depths or extend over wide areas. The advantage of a largesystem lies in the great area of absorptive surface that is provided, and in the increased chance of tapping distant water supplies. But frequently the soil of deserts is shallow and underlaid by rocks, which prevents the deep penetration by the roots, so that many desert plants, and in particular, have but a small root system.
Resistance to Drought through Possession of Water-storage Tissue
In many xerophytes the leaves orare swollen, and are provided with a special water-storage tissue. Such plants are known as succulents, and are able to survive conditions of extreme drought by making use of their internal water supplies. Whenever the rate of water absorption is so low that the assimilatory tissue suffers from shortage, water passes from the storage tissue to the assimilatory cells, so permitting the continuance of . The houselcek is a succulent which grows between the tiles on the roofs of old houses, and which consists of a short bearing a close rosette of thick fleshy leaves. The biting stonecrop is another succulent with fleshy leaves which grows in stony places and is sometimes found on the tops of walls. Large quantities of water are stored in the swollen of desert , and some species of these plants are capable of losing as much as 50 per cent, of their water content without injury.
Succulence is also a characteristic of certain salt-marsh plants such as saltwort, sea purslane and glasswort, which are all more or less fleshy. But although they show this feature they cannot withstand drought as sucoessfully as the desert xerophytes, and quickly wilt and die if the water supply becomes too scanty. The succulence of salt-marsh plants does not help them in the way that it helps the desert succulents, and it may merely be a reaction of the plant to the presence of salt in the soil. The saltwort is known to lose much of its fleshiness if grown in soils poor in sodium chloride, and many ordinary plants become fleshy if grown in salt solution.
Resistance to Drought by Diminishing the Rate of Transpiration
Xerophytic plants show manyfor reducing the rate of transpiration, of which the following are the most important:— Thick Cuticle and Epidermal Cell Walls
These features prevent the loss of water through the epidermis, e.g. holly, conifers, laurel. A Covering of Hairs on the Leaf
The lower sides of coltsfoot leaves and both sides of the leaves of mullein and the Alpine plant edelweiss have a dense covering of hairs which help to reduce transpiration by maintaining a layer of moist air between the stomata and the outside air. The sinking of Stomata in Pits and Leaf rolling
In Pinus the stomata are sunk in pits, and in the sand-dune grass, Ammophila arenaria, they occur in grooves on the upper surface of the. Here again the effect is to reduce transpiration by placing a space containing moist air between the stomata and the atmosphere. In Ammophila this effect is increased by rolling, I.e. on dry days the edges of the leaf are curved round so as to enclose a chamber into which the stomata open, whereas on wet days the leaf flattens out. Reduction of the Leaf Surface
Many xerophytes, such as the heaths, have very small leaves. A pine tree has long narrow leaves, and the cypress has flat scale-like leaves which present a small transpiring surface to the air. In the broom the small leaves soon drop off and the long green stems perform the assimilatory functions. In this case, further protection from the heating effect of the sun is afforded by the verticalof the stems. Many cacti have no leaves, the functions of which are taken over by the swollen stems. Dwarfness of Habit
The close arrangement of the leaves and the low growing nature of many xeromorphic plants gives them protection from the drying wind.
The Value of Xeromorphic Features
Until recently it was thought that the xeromorphic features concerned with water storage and reduction of transpiration were the most important of the structural adaptations of xerophytes that enable them to survive in dry situations. But many plants show the thick cuticle, hairy covering and sunken stomata, etc., of typical xerophytes and yet do not show the power of resisting drought, which is the characteristic of xerophytes. It would appear that the ability of the protoplasm to suffer considerable water loss without injury, rather than the reduction of transpiration, is the feature which permits an xerophyte to exist sucoessfully under dry conditions. Some xerophytes, notably lichens, show no important xeromorphic features and yet are able to recover from prolonged desiccation. In conclusion, it must be said that the phenomenon of xeromorphism is not thoroughly understood, and that there are many aspects of the subject which have yet to be explained.