170 THE STORY OF THE PLANTS. 



you a woodcut of a thin slice through a leaf (Fig. 

 i) which showed the blade as naturally divided 

 into an upper and under portion. The upper por- 

 tion consisted of very close-set green cells, con- 

 taining living chlorophyll, 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 be- 

 neath it. But the under portion was sparse-look- 

 ing 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 connec- 

 tion, as a component element in the task of water- 

 supply. 



The lower portion of most leaves is the part 

 employed in 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 emp- 

 ty 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 of 

 flowers and leave them in the sun without water, 

 they fade and dry up in a very short time. We 

 also know that if we forget to water plants in 

 pots, the plants similarly dry up and die after a 

 few hours' exposure. Leaves, in fact, are pur- 

 posely arranged in most cases so as to encourage 



