PART II PHYSIOLOGY 
THE relation between the form and structure of a plant and its behavior 
is very intimate and to a large extent reciprocal. Form and structure 
in general determine behavior, and behavior, especially as it is itself 
controlled by external agents, to a great degree determines form and 
structure. It is not possible at present to discover all these reciprocal 
relations, much less to describe them in terms of physics and chemistry. 
Nor is the behavior of plants sufficiently known to be explained in these 
terms. 
Morphology, concerned with form and structure, is particularly in- 
terested in how each plant comes to be what it is in the short history 
of its own life (ontogeny), and also seeks to form a conception of how 
plants have come to be what they are in the long course of their history 
since they began to develop on the earth (phylogeny). The former 
topic is clearly open to experimental study and constitutes the field of 
experimental morphology. But the latter is much less open to experi- 
ment ; scarcely at all, indeed, except for the determination of the laws 
of heredity, a field which has been called " experimental evolution." 
Obviously such experiments, whether in the field or laboratory, cannot be 
wisely planned or executed without a thorough knowledge of plant 
physiology. 
A wide range of facts is open also to mere observation, because the 
ordinary changes in climate and soil, some of which are produced by 
other plants and animals, affect the form and structure of plants. This 
field is part of that distinguished from physiology proper as Ecology 
(Part III). Naturally even the most careful observations need to be 
confirmed or corrected by experiments. Thus this portion of ecology 
and experimental morphology are mutually related, and both really 
form a part of physiology in the broadest sense, and depend upon it. 
Physiology, in its turn, seeking to expound the phenomena of plant life 
in terms of matter and force, depends upon the data of chemistry and 
age 
