72 BOTANT. 



condition must not be forgotten, viz., that the water iiv 

 plant-cells contains many substances in solution, and con- 

 sequently evaporates less rapidly than pure water, in ac- 

 cordance with well-known physical laws. Moreover, the 

 attraction of the substance of the cell-walls for the water 

 counteracts, to a considerable extent, the tendency to evapo- 

 ration; and in the same manner, even to a greater extent, 

 the water is prevented from passing off by the " imbibition- 

 power" of protoplasm. It is, in fact, impossible to deprive 

 cellulose and protoplasm of all their water in dry air at 

 ordinary temperatures. 



137. In all the aerial parts of higher plants the epidermis 

 offers more or less resistance to the escape of the water of 

 the underlying tissues. This is mainly accomplished by 

 the thick outer wall of the epidermal layer; in many cases, 

 especially in plants growing naturally in very dry regions, 

 the epidermis consists of several layers of cells, which offer 

 still more resistance to evaporation by being themselves 

 filled with moist air only. 



138. Among the lower plants, the single reproductive 

 cells (spores) are guarded against the loss of water by hav- 

 ing their walls greatly thickened. Even in the lowest 

 plants, the Slime Moulds, the naked masses of protoplasm, 

 when placed in dry air, will contract into rounded masses, 

 which then become covered with a somewhat impervious 

 envelope. 



139. The breathing-pores of the green and succulent 

 parts of higher plants control to a great extent the amount 

 and rapidity of their exhalation. Breathing-pores are 

 placed over intercellular spaces, which are in communica- 

 tion with the intercellular passages of the plant. These 

 spaces and passages are filled with moist air and gases, 



