PLANT PHYSIOLOGY AND ECOLOGY 



CHAPTER I 

 STIMULUS AND RESPONSE 



1. Fundamental relations. A plant is an organism capable 

 of nourishing itself under the control of external conditions, and 

 of modifying its form and structure in accordance with this fact. 

 Hence the most important matter in the study of plants as living 

 things is to find out how the making and the using of food take 

 place, and how the carrying out of these processes affects the 

 structure of the plant. In seeking to explain the behavior of 

 the living plant, i.e., its activities or functions, the first need is 

 to discover the external forces that control it. We must next 

 determine the effects which these produce. These are first seen 

 in the functions of the plant, and in some cases they become evi- 

 dent here alone. As a rule, however, many of them appear sooner 

 or later as a change in the minute structure or form of the plant. 

 The proper task of physiology is the study of the external factors 

 of the environment or habitat in which the plant lives, and of 

 the activities and structures which these factors call forth. The 

 former are causes, the latter effects. The sequence of study is 

 consequently factor, function, and form, and the primary object 

 to discover the nature and amount of this fundamental connec- 

 tion between the causative factors and the resulting functions 

 and forms. 



Physiology was originally understood to be an inquiry into 

 the origin and nature of plants. This is the view that pervades 

 the following pages, and in accordance with this the subject-mat- 

 ter of ecology is merged with that of ]-)hysiology. 



2. The nature of stimuli. Any factor of the habitat that 

 produces a change in the functions of a plant is a stimulus. The 



