ADAPTATIONS 251 



nate different types of response to altered conditions of growth. 

 Inasmuch as there seems to be no generally accepted usage of these 

 different terms, only one of them, namely, the word "adaptation" 

 will be used here; and different manifestations of this phenomenon 

 will be distinguished by using appropriate adjectives, as " phys- 

 iological adaptations," " chromatic adaptations," " morphological 

 adaptations," etc. 



Two markedly different types of responses to altered conditions, 

 or of adjustment to environment, may be recognized. In the first 

 of these, for which we will use the term " physiological adaptation," 

 the species of plant simply acquires the ability to exist and grow 

 normally under conditions which formerly inhibited its growth. 

 Thus, we may speak of the phenomena mentioned above as 

 " acclimitization " as the physiological adaptation of the crop to 

 the new conditions of ,growth. In general, physiological adapta- 

 tions include such variations in the characters or habits of growth 

 of plants as results in differences in resistance to heat or to cold, 

 relations to water, aggressiveness in competition with other plants, 

 etc. In such cases, no modification of the morphological charac- 

 ters of the plant can be observed, the changes which take place in 

 the structure of the plant (if, indeed, there be any such changes) 

 must be only minor adjustments of the protoplasm to meet the 

 new environmental needs. 



In the second type of adaptations, for which we will use the 

 term " morphological adaptations," the structure, or color, or 

 some other morphological character of the plant is actually 

 changed in some easily recognizable way, in order that the plant 

 may be better adjusted to its environment. As examples of 

 morphological adaptations, there may be cited the change in color of 

 sea-weeds with increasing depth in the sea, and other examples of 

 chromatic adaptation which are discussed below ; the development 

 of fewer, or a larger number, of buds on the above-ground stems of 

 plants, hi response to decreases, or increases, in the available 

 supply of food; the alteration in the size and shape of the leaves 

 of many plants when they are grown in shade; the dwarfing of 

 plants at high altitudes, or under conditions of severe drought; 

 the development of underground storage organs for certain species 

 of shrubs and trees which grow in regions that are subject to 

 periodical burning-over, in such a way as to destroy the above- 

 ground storage stems, etc. 



