=164. The increase of water supply.= Plants of dry habitats can increase their absorption only by modifying the root system so that the absorbing surfaces are carried into the deep-seated layers of soil, and the surfaces in contact with the dry soil are protected by means of a cortex. Exception must be made for epiphytes and a few other plants that absorb rain water and dew through their leaves, and for those desert plants that seem to condense the moisture of the air by means of hygroscopic salts, and absorb it through the epidermis of the leaf. The storage of water in the leaf is a very important device; it increases the water supply by storing the surplus of absorbed water against the time of need. Modifications for water-storage are occasionally found in roots and stems, but their chief development takes place in the leaf. The epidermis frequently serves as a reservoir for water, either by the use of the epidermal cells themselves, by the formation of hypodermal water layers, or by means of superficial bulliform cells. The water cells of the chlorenchym regularly appear in the form of large clear cells, scattered singly or arranged in groups. In this event, they occur either as transverse bands, or as horizontal layers, lying between the palisade and sponge areas, and connecting the bundles. A few plants possess tracheid-like cells which also serve to store water. In the case of succulent leaves, practically the whole chlorenchym is used for storing water, though they owe their ability to withstand transpiration to a combination of factors. =165. Modifications due to an excessive water supply.= Water plants with aerial leaf surfaces are modified in such manner as to increase water loss and to decrease water supply, but the resulting modifications are rarely striking. There is a marked tendency to increase the exposed surface. This is indicated by the fact that, while the leaves of mud and floating forms become larger, they change little or not at all in thickness. The lobing of leaves is also greatly reduced, or the lobes come to overlap. Leaves of water plants are practically destitute of all modifications of epidermis and stomata, which could serve to hinder transpiration. The stomata are usually more numerous on the upper surface, and in the same species their number is greater in the forms grown in wet places. These facts explain in part the extreme development of air-passages in water plants, though this is, in large measure, a response to the increasing difficulty of aeration. The increase of air-spaces is correlated with reduction of the palisade, and a decided increase in the sponge. An increase in water supply is indicated by the absence of storage tissues, and the reduction of the vascular system, which, however, is more closely connected with a diminished need for mechanical support. [Illustration: Fig. 32. Mesophyll of _Pedicularis procera_ (chresard, 15%, light, 1). × 130.]