INTRODUCTION 
PLANT BIOLOGY 
Botany may be divided into two branches : 
1. Morphology (Gr. morphe, form), which deals with 
the form, structure, origin, and development of plant- 
organs and their relations with one another. This branch 
includes—(a) external morphology, dealing with the out- 
ward form of organs; and (6b) internal morphology, 
dealing with the morphology of the minute cells and 
tissues of which the organs are composed. 
2. Physiology (Gr. physis, nature, inherent qualities), 
which deals with the functions of the organs. 
Thus we may consider a leaf with respect to its form, 
size, and structure ; we may trace its origin and develop- 
ment, and compare it with other leaves and organs. 
This is examining the leaf morphologically. Or we may 
direct our attention to the work or functions performed by 
the leaf, its vital activities—assimilation, transpiration, 
respiration—and consider how the particular leaf under 
notice performs them. This is looking at the leaf on the 
physiological side. 
In dealing with the form and structure of any organ 
it is important to associate its form with its function. 
Variation of form means variation of function, if only in 
degree. Thus, suppose we compare two leaves—a holly- 
leaf with a lime. They are very different. One is thick, 
tough, shiny, and evergreen, and the main veins are con- 
tinued as spines. The other is thin and deciduous. 
But modification in form implies modification in function. 
A green leaf has three main functions : it assimilates, it 
transpires, it respires. Each of these functions is modified 
as the form is modified, but which is the most important ? 
On reference to p. 43, we see that the tough evergreen 
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