8 PLANT EELATIONS. 



the amount of work an active leaf can do will depend in 

 part upon the amount of surface it exposes. 



THE LIGHT-RELATION". 



12. The general relation.— The ordinary position of the 

 foliage leaf is more or less horizontal. This enables it to 

 receive the direct rays of light upon its upper surface. In 



this way more rays of 

 light strike the leaf sur- 

 face than if it stood ob- 

 liquely or on edge. It is 

 often said that leaf blades 

 are so directed that the 

 flat surface is at right 

 angles to the incident 

 rays of light. While this 

 may be true of horizon- 

 tal leaves in a general 

 way, the observation of 

 almost any plant will 

 show that it is a very 

 general statement, to 

 which there are numerous 

 exceptions (see Fig. 1). 

 Leaves must be arranged 

 to receive as much light 

 as possible to help in 

 their work, but too much 

 light will destroy the 

 green substance (chloro- 

 phyll), which is essential 

 to the work. The adjust- 

 ment to light, therefore, 

 is a delicate one, for 

 there must be just enough 



Pig. 1. The leaves of this plant (Ficus) are 

 in general horizontal, but it will be seen 

 that the lower ones are directed down- 

 ward, and that the leaves become more 

 horizontal as the stem is ascended. It 

 will also be seen that the leaves are so 

 broad that there are few vertical rows. 



