8 
PLANT STUDIES 
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 
"1 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. 
