CHAPTER XX 
THE LEAF 
The leaf is the most important organ of the plant, for in 
it are carried on most of those processes which pertain to the 
life of the plant itself. Because it is so important an organ 
and because it has so many kinds of work to do, it has many 
adaptations in form, position, and structure which fit it 
to do its various kinds of work, besides giving individuality 
to plants. We shall study the work of the leaves first in order 
that we may more easily understand how the structure, form, 
and position of the various kinds of leaves adapt each to its 
work. 
237. The Work of a Leaf. — The life processes carried on 
by the leaf are (1) photosynthesis, peculiar to plants having 
chlorophyll, green coloring matter; (2) respiration, common 
to all living protoplasm; (3) digestion, (4) circulation, 
(5) assimilation, and (6) excretion. All but the first are 
much the same as the processes of the same names in animals. 
Transpiration which, like photosynthesis, is peculiar to 
green plants, while not one of the life processes, is made 
necessary by them. 
The work that a leaf does is of greater importance than 
anything else about it. We study its structure in order to 
understand how it is able to carry it on. For the same 
reason we study the arrangement of the leaves on the stem, 
and their form, which has a close relation to arrangement 
Although the work that leaves do for plants is the same in 
all kinds of plants the world over (with a few exceptions to 
be noted later), each particular kind of plant has its peculiar 
shape, size, and arrangement of leaves. This helps us to 
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