4 PLANT PHYSIOLOGY AND ECOLOGY 



sponse is dependent upon the intensity of the stimuhis, and it 

 is in many cases proportional to it. The same stimuhis may 

 not produce the same response in two different species, or neces- 

 sarily in two plants of the same species. It does have this effect 

 in individuals and species that are equally plastic. The study 

 of response is facilitated by distinguishing two kinds, viz., func- 

 tional and structural. Many reactions to stimuli are functional 

 alone. In a large number of cases a structural change also occurs, 

 and this is the rule when the functional change is pronounced. 

 Consequently, it becomes convenient to distinguish functional 

 response as adjustment, and structural response as adaptation. 



5. Adjustment and adaptation. The adjustment of a plant 

 to the stimuli of its habitat is taking place constantly. It is 

 seen daily in the processes of nutrition and growth. As long as 

 the stimuli are normal for the habitat, the adjustment of the 

 plant is restricted to its ordinary activities. But when the stimuli 

 become unusual in amount or in kind, either by a change of habitat 

 or by a modification within it, the consequent adjustment becomes 

 more evident, and is then usually recorded in the plant's struc- 

 ture. Adjustment may be expressed in the movement of parts 

 or organs, such as the closing of stomata or a change in the posi- 

 tion of leaves, or in growth or modification of structure. Slight 

 or periodic adjustment usually concerns function alone. Adjust- 

 ment is profoundly affected by the nature of the factor, and is 

 in direct relation to the intensity of the latter. Adaptation com- 

 prises all structural changes resulting from adjustment. It in- 

 cludes both growth and modification. The latter is really growth 

 in response to unusual stimuli, a fact that furnishes the clue to 

 all evolution. Growth is periodic and quantitative: it is the 

 result of the normal and continuous adjustment of the plant to 

 the stimuli of its own habitat. On the contrary, modification 

 is relatively permanent and qualitative: it is the response to 

 stimuli of an unusual kind or intensity. A good knowledge of 

 the way in which growth occurs is indispensable to the under- 

 standing of modification. In endeavoring to find the connection 

 between habitat and plant, however, it is in the modification 

 of the plant and not so much in its growth that the significant 

 responses to stimuli are to be sought. 



In the following survey of the relation between the stimuli, 

 functions, and structures of the plant, the physical factors of the 



