296 PHYSIOLOGY 
physics. In certain directions present knowledge is almost or quite 
sufficient to permit the framing of physical and chemical explanations. 
In others the data of chemistry and physics are not yet adequate for this; 
and in still others it seems now quite improbable that the phenomena can 
ever be analyzed in terms of matter and force. It must not be forgotten, 
however, that this is the direction of all recent advances, and that what 
is hopelessly obscure often becomes beautifully clear as some new van- 
tage point widens the view. 
In its broadest sense, then, plant physiology includes the study of 
the behavior of plants of all sorts, and of all the ways in which this is 
affected by external agents of every sort. On the one hand it overlaps 
morphology, and on the other it includes a large part of ecology. In 
this book, however, it is restricted in the main to a consideration of the 
behavior of the larger plants, especially seed plants, though in certain 
cases reference is made to others. In this part no section on reproduc- 
tion will be found. That topic is relegated to Morphology (Part I), 
since the purely physiological processes are relatively simple, so far as 
known, and very much alike, whereas the reproductive organs are very 
different in different groups of plants and are most significant for their 
morphology. For convenience, also, the effect of external agents on 
plants is treated so as to develop and illustrate general principles, 
whereas the more extended account of specific cases will be found in 
Part III, on Ecology. 
