72 BOTANY. 
condition must not be forgotten, viz., that the water in 
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 regiotis, 
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, 
