i02 THE TLANT 



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 

 imusual 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 afifords 

 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 movem.cnt 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; 



