CHAPTER VIII — ADAPTATION 



The problem of adaptation. — In the preceding chapters it has been 

 seen that most plant structures are more or less perfectly suited for the 

 conditions in which they live, and that their behavior is in most instances 

 advantageous. There have been various theories to account for the 

 origin of the obvious harmony between plants and their surroundings. 

 Originally it was supposed that plants were specially created with the 

 structures and behavior that they now possess, but it has long been known 

 that their characters are the result of evolution. 



The theory of adaptive response. — A common theory, prevalent es- 

 pecially in the past century, has been that plants possess an inherent 

 capacity to adapt themselves to their surroundings, being able as con- 

 ditions change to change their structure or behavior or both in an ad- 

 vantageous manner. Indeed, this doctrine has been formulated into a 

 so-called law, namely, that the cause of a need is at the same time the 

 cause of its satisfaction ; new conditions create new needs, and the new 

 needs are supposed to result in new organs.' For example, the abundant 

 development of root hairs in a moist air culture of maize seedlings has 

 been regarded as an adaptive response, the hairs being believed to grow 

 abundantly because they are needed, whereas water cultures of the same 

 species are thought to be hairless, because hairs are not needed in a water 

 medium. Such a philosophy, in which need is supposed to control re- 

 sponse, and in which, therefore, the pursuit of material causes is replaced 

 by purposive expressions, is denominated teleological. 



The theory of adaptive response has had two aspects; some investi- 

 gators hold that plant species remain plastic and thus able to adapt 

 themselves directly to changed conditions, while others hold to a theory 

 of original plasticity, with the subsequent fixation of adaptive structures, 

 the so-called inheritance of acquired characters. The continued plasticity 

 of amphibious plants and of beer yeast might be cited in favor of the 

 former view, while the facts of progressive variability (p. 759), the re- 

 tention of short vegetative cycles in plants from northern grown seeds, 



' A less crude modern statement of this theory is that new conditions result in new 

 functions, which in turn result in modified organs. 



