102 THE PLANT 
response be distinguished. The chief value of this distinction lies in the 
fact that many reactions are functional alone; it serves also to emphasize 
the absolute interdependence of structure and function, and the imperative 
need of considering both in connection with the common stimulus, For 
these reasons, the logical treatment is to connect each stimulus with its 
proper functional change, and, through this, with the corresponding modi- 
fication of structure. For the sake of convenience, the term adjustment 
is used to denote response in function, and adaptation, to indicate the re- 
sponse in structure. 
147. Adjustment and adaptation. The adjustment of a plant to the 
stimuli of its habitat is a constant process. It is the daily task, seen in 
nutrition and growth. So long as these take place under stimulation by 
factors which fall within the normal variation of the habitat, the problems 
belong to what has long been called physiology. When the stimuli become 
unusual in degree or in kind, by a change of habitat or a modification in it, 
adjustment is of much greater moment and is recorded in the plant’s struc- 
ture. These structural records are the foundation of proper ecological 
study. Since they are the direct result of adjustment, however, this affords 
further evidence that a division of the field into ecology and physiology 
is illogical and superficial. Slight or periodical adjustment may concern 
function alone; it may be expressed in the movement of parts or organs, 
such as the closing of stomata or changes in the position of leaves, in 
growth, or in modifications of structure. This expression is fundamentally 
affected by the nature of the factor and is in direct relation to the intensity 
of the latter. Adaptation comprises all structural changes resulting from 
adjustment. It includes both growth and modification. The latter is 
merely growth in response to unusual stimuli, but this fact is the real 
clue to all evolution. Growth is periodic, and in a sense quantitative; it 
results from the normal continuous adjustment of the plant to the stimuli 
of its proper habitat. In contrast, modification is relatively permanent and 
qualitative; it is the response to stimuli unusual in kind or intensity. A 
definite knowledge of the processes of growth is indispensable to an under- 
standing of modification. In the fundamental task of connecting plant and 
habitat, it is the modification of the plant, and not its growth, which records 
the significant responses to stimuli. For this reason the discussion of 
adaptation in the pages that follow is practically confined to modification 
of structure. This is particularly desirable, since growth has long been the 
theme of physiological study, while modification has too often been con- 
sidered from the structural standpoint alone. The comparatively few 
studies that have taken function into account have been largely empirical; 
