FOLIAGE LEAVES: THE LIGHT- RELATION 7 
do not develop this color. Even when leaves have devel- 
oped the green color they lose it if deprived of light, as is 
shown by the process of blanching celery, and by the effect 
on the color of grass if a board has lain upon it for 
some time. It seems plain, therefore, that the green color 
found in working foliage leaves depends upon light for its 
existence. 
We conclude that at least one of the essential life-rela- 
tions of a foliage leaf is what may be called the light-rela- 
tion. This seems to explain satisfactorily why such leaves 
are not developed in a subterranean position, as are many 
stems and most roots, and why plants which produce them 
do not grow in the dark, as in caverns. The same green, 
and hence the same light-relation, is observed in other 
parts of the plant as well, and in plants without leaves, the 
only difference being that leaves display it most conspicu- 
ously. Another indication that the green color is con- 
nected with light may be obtained from the fact that it is 
found only in the surface region of plants. If one cuts 
across a living twig or into a cactus body, the green color 
will be seen only in the outer part of the section. The con- 
clusion is that the leaf is a special organ for the light-re- 
lation. Plants sometimes grow in such situations that it 
would be unsafe for them to display leaves, or at least large 
leaves. In such a case the work of the leaves can be thrown 
upon the stem. A notable illustration of this is the cactus 
plant, which produces no foliage leaves, but whose stem dis- 
plays the leaf color. 
11. An expanded organ. Another general fact in refer- 
ence to the foliage leaf is that in most cases it is an expanded 
organ. This means that it has a great amount of surface 
exposed in comparison with its mass. As this form is of 
such common occurrence it is safe to conclude that it is in 
some way related to the work of the leaf, and that whatever 
work the leaf does demands an exposure of surface rather 
than thickness of body. It is but another step to say that 
