HYDRO HARMOSE lil 



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 theif 

 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 



