104 Field, Forest, and Wayside Flowers 
air causes some of this water to evaporate from 
the foliage. More comes up to supply the loss, 
Fic. 19.—Starch-grains of the potato (a) and of wheat (0). 
(Much magnified.) 
(From the Vegetable World.) 
and this also dries out slowly through the sunlit 
hours. As the water evaporates the mineral sub- 
