HYDROHARMOSE 121 
The development of sclereids below the epidermis likewise hinders the 
escape of water. Finally, the character of the cell sap often plays an im- 
portant part, since cells with high salt-content or those containing mucila- 
ginous substances give up their water with reluctance. 
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 con- 
dense 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 develop- 
ment takes place in the leaf. The epidermis frequently serves as a reser- 
voir 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 
