288 PLANT LIFE ON THE FARM. 
they are in general very numerous. So far, then, there 
is little difference to be noted -between the structural 
elements of a leaf and those of a root. The root is more 
or less cylindric, the leaf is more or less flat ; but the es- 
sential structures, though differently arranged, are pretty 
much the same, with one or two notable exceptions. 
The root has no breathing pores or stomata, and the 
contents of its constituent cells are so far different from 
those of the leaves that they contain no green coloring 
matter. . 
Chlorophyll.—The main and specially important char- 
acteristic of the leaf (and of all the green parts of plants), 
so far as their life-work is concerned, is the presence in 
the cells of the green matter called ‘‘ chlorophyll.” 
With and by its agency the leaf can do work impossible 
to be done otherwise; work, the measure of which de- 
termines the health and vigor of the plant, the default 
of which ensures its death. It is true that chemists and 
physicists have not yet unravelled all the mysteries of 
chlorophyll, and there remains a doubt whether it is the 
potent agent it has hitherto been supposed to be, or 
whether the power does not reside in some other agent 
mixed with it.. Into these questions we cannot here 
enter. Whatever be the actual truth of the matter, the 
transcendent importance of chlorophyll, and of all that 
its presence implies, is universally admitted. It must 
suffice to say that chlorophyll is a green, waxy substance, 
occurring in certain of the cells mixed with their proto- 
plasmic contents. In amount and appearance it varies in 
different cases and under varying circumstances. It 
does not occur in all the cells of the leaf, but chiefly or 
only in those on the upper surface, and which are there- 
fore the most directly exposed to the action of the rays 
of the sun. 
