6o8 



ECOLOGY 



leaf or even a leaf in which the basal asymmetry is reversed, while leaves commonly 

 symmetrical in like manner may be induced to become asymmetrical. Anisophylly 



is well illustrated by such a plant 

 as Selaginella (fig. 896), which 

 has two sorts of leaves, large 

 and small, more or less regularly 

 placed in relation to each other, 

 and also by the leafy liverworts, 

 which have two dorsal rows of 

 green leaves and a ventral row 

 of small, colorless leaves (am- 

 phigastria). Anisophylly, like 

 asymmetry, is believed to be due 

 to the unequal action of various 

 factors, among which light and 

 gravity appear the more impor- 

 tant. In many plants anisophylly 

 characterizes the leaves of hori- 

 zontal shoots, where the influence 

 of such factors is unequal, while 

 the leaves of erect shoots are 

 equal. In some cases at least, 



FlG. 896. A plant of Selaginella apus, illus- 

 trating anisophylly (i.e. inequality in leaf size); 

 note that large and small leaves adjoin one another 

 in regular fashion ; the structures depending from 

 the horizontal stems are rootlike organs known as 

 rhizophores (r) ; note the dichotomous branching. 



the primordia of anisophyllous 

 leaves are equal, and it has been 

 shown that upon emergence from 



the bud, one leaf of a pair has a more favored position and hence manufactures 

 more food than does the other, making possible its greater development. In some 

 plants anisophylly seems unrelated to external factors. Both asymmetry and ani- 

 sophylly have been considered advantageous from the standpoint of light-reception, 

 since they favor the development of leaf mosaics and the maximum utilization of space. 



7. THE ABSORPTION OF WATER AND OF NON-GASEOUS 

 SOLUTES BY LEAVES 



General remarks on leaf absorption. While roots are the chief 

 regions of entry for water and salts, there are many plants in which 

 these substances enter directly into the chlorophyll-bearing organs. In 

 such cases the division of labor between absorptive and synthetic organs 

 is not pronounced, and there may be little or no transportation of water 

 and salts through a specialized conductive system. As a class, however, 

 leaves are disadvantageously placed for water absorption, since they are 

 in much less frequent contact with water than are roots, and generally 

 are subject to transpiration rather than to absorption; the cutinized epi- 

 dermal walls of most leaves, which are advantageous in checking tran- 



