28 PLANT LIFE otf 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. 



