ADAPTATION TO WATER 151 



spaces to mere lines, and thereby greatly decreases the amount 

 of transpiration. The conversion of sponge tissue into palisade 

 tissue further diminishes water loss by placing the chloroplasts 

 in such a position that they mutually shield each other, and thus 

 reduce the transpiration caused by light. The development of 

 layers or masses of stone fibers, or sclereids, beneath the epidermis, 

 though primarily for support, likewise hinders the escape of 

 moisture. Such modifications are frequent in needle leaves, 

 especially those of evergreen trees, pines, spruces' etc. The 

 cell-sap sometimes plays an important part, by holding water in 

 cells which have a high salt content, or contain more or less 

 mucilage. 



171. Increase and storage of water supply. The amount of 

 water supplied to the lea\'es by the roots can be increased only 

 by increasing the area of the absorbing surface, or by changing 

 its location. The production of more root hairs, accompanied 

 by the growth and branching of the roots, is the usual response of 

 plants to moderate drouth. Plants of dry habitats increase their 

 absorption by extending the absorbing surfaces of the root into 

 the deeper portions of the soil, as well as by their branching within 

 this area. Water loss from the root surfaces in contact with the 

 dry upper soil is prevented by means of a well-developed cortex. 

 For these reasons xerophytes are often characterized by the 

 possession of tap roots. Some plants, chiefly epiphytes, absorb 

 rain water and dew by means of their leaves. A few desert plants 

 seem able to condense the moisture of the air by means of hygro- 

 scopic salts, or in other ways, and to absorb it through the epi- 

 dermis of the leaf. For all plants with roots, however, the amount 

 of absorption by stem or leaf is inconsiderable, and can play no 

 important part in increasing water supply. 



There is a limit to the increase of water supply l)y the extension 

 of the root surface. In consequence, many xerophytes have 

 developed structures for storing water. Modifications for water 

 storage are occasionally found in roots and stems, such as those 

 of many fleshy plants. Storage devices occur chiefly in the leaves, 

 where they are of great importance. They increase the water 

 supply by storing the suri:)lus of absor])ed water against a time of 

 need. Moreover, they often retain the stored water with great t oiiac- 

 ity, and thus tend to offset the pull exerted by evaporation. 

 The epidermis is frequently modified to form reservoirs for water. 



