LEAVES 627 



9. LEAVES AS ORGANS OF ACCUMULATION OF WATER 



AND FOOD 



Food accumulation in leaves. Generally it would be disadvanta- 

 geous for leaves to serve as organs of food accumulation or " storage," 

 since this would impair their synthetic efficiency, partly because of the 

 space that such foods would occupy, and partly (in the case of soluble 

 foods, like the sugars) because increasing concentration retards further 

 manufacture. Leaves serve as organs of temporary accumulation, be- 

 cause in the daytime, foods usually are manufactured faster than they are 

 removed to other organs; during the night, however, removal continues, 

 so that the leaf is relatively free from accumulated foods by morning. In 

 various desert xerophytes in which the leaves remain for years, large 

 quantities of food and water may accumulate, as in Agave, whose 

 developing flower-stalk so drains the leaves of their contents that they 

 fall back limp and wrinkled (figs. 921, 922). 



General features of water-accumulating leaves. Water retention. 

 In leaves the accumulation and retention of water in conspicuous 

 amount is much more common than that of foods, and is especially 

 characteristic of succulent xerophytes, such as the Crassulaceae and 

 Chenopodiaceae. The water retentiveness of such plants is well shown 

 when attempts are made to dry them; Crassulaceae and Cactaceae 

 frequently grow for days or even for weeks while being dried under 

 pressure for herbarium specimens. This retentiveness is due in some 

 instances to high cutinization (Cactaceae), or to waxy coats (Crassula- 

 ceae) , or to both combined (Agave) ; most succulent plants, however, 

 exhibit weak cutinization. Another character favoring retentiveness in 

 succulent leaves is a small evaporating surface in proportion to the 

 volume, but many orchid leaves which evaporate slowly are thin and 

 slightly cutinized (as in species of Habenaria) . Again, in many succulents 

 which have a cell sap of high osmotic pressure, evaporation doubtless is 

 relatively slow. The best attested cases of such concentrated cell sap are 

 in the succulent plants of salt marshes, which may have an osmotic 

 pressure equal to twenty atmospheres, and in various desert plants, 

 largely shrubs, some of which may have a pressure as high as one hun- 

 dred atmospheres, as compared with a pressure of five to ten atmospheres 

 in plants of ordinary habitats. The osmotic pressure in cacti, on the 

 other hand, has been shown to approximate that in ordinary plants. 

 Probably upon this basis succulent plants may be divided into two classes : 



