THE STEM AND BRANCHES. 185 



here into the nature of so important and uni- 

 versal a plant function. You will remember 

 that when we were discussing the nature of 

 leaves, I gave you a woodcut of a thin slice 

 through a leaf (Fig. 1) which showed the blade 

 as naturally divided into an upper and under 

 portion. The upper portion consisted of very 

 close-set green cells, containing living, chloro- 

 phyll, and covered by a single transparent 

 water-layer, which absorbed carbonic acid from 

 the air about, and passed it on to be digested 

 by the living chlorophyll-layer just beneath it. 

 But the under portion was sparse-looking and 

 spongy ; it was composed of cells loosely 

 arranged among themselves, and interspersed 

 with great empty spaces. I told you but little 

 at the time of the function or use of this lower 

 portion ; we must return to it now in the 

 present connection, as a component element in 

 the task of water-supply. 



The lowr portion of most leaves is the part 

 employeoin the great and necessary work of 

 evaporation. 



For this purpose the tissue at the under side 

 of the leaf is composed of loose and spongy cells 

 which have much of their surface exposed to the 

 empty spaces between them : and these empty 

 spaces are really air-cavities. The object of the 

 cavities, indeed, is to facilitate evaporation. 

 Liquid transpires into them from the various 

 cells through the wall that bounds them. How 

 fast water evaporates in the leaves of plants we 

 all know by experience in a thousand ways. 

 We know, for instance, that if we pick bunches 



