948 ECOLOGY 



and the rigidity of many apparently adaptive characters, as in the xero- 

 phytic structures of conifers, may be cited in favor of the latter view. 



Apparently favoring the theory of adaptive response are the facts 

 previously cited in connection with the development of cutin, cork, and 

 air spaces, all of which are best developed where most useful. Cutin 

 and cork are wanting in submersed aquatics, and they develop increas- 

 ingly as the atmosphere becomes more desiccated, where their protec- 

 tive advantages become greater. Air spaces are best developed in sub- 

 mersed aquatics, where the difficulties in oxygenation are, perhaps, 

 greatest, while they are most poorly developed in xerophytes, where large 

 air spaces would tend to facilitate excessive transpiration. 



Associated with the adaptation theory is the doctrine of use and 

 ' disuse, it being held that an organ develops most when most used , and 

 least when least used. The best illustration that may be given of this 

 is in the conductive tissues, where an abundant flow of material occa- 

 sions maximum development, and where a slight flow, as in hydropl ytes, 

 occasions minimum development. There is no necessary association, 

 however, between the adaptation theory and the theory of use anc dis- 

 use; the conductive tracts that develop under the stimulus of parasitism 

 might illustrate development through use, but they are very far from 

 being adaptations; air spaces, on the other hand, might be cited as illus- 

 trating adaptation, but they develop most where they are least used and 

 least where they are most used. 



The theory that plants are able to adapt themselves to new conditions 

 is no longer tenable, being invalid a priori and disproven empirically. 

 The hypothesis of adaptive response rests upon the same foundation 

 as does the doctrine of vitalism, which postulates that there is something 

 inherently different between lifeless and living matter. Each year the 

 list of " vitalistic activities " of plants becomes more and more restricted 

 through the establishment of a definite physical or chemical cause for 

 what had been thought to have a vitalistic explanation, while never in 

 the history of science has any phenomenon once explained on a physi-. 

 cal or chemical basis been found later to be vitalistic. The same is true 

 of adaptations; for example, tubers at one time were thought to be a 

 provision made by plants for their vegetative offspring, but now it is 

 known that they arise as a definite reaction to specific external conditions 

 (p. 744). Similarly, many organs and structures now are known to 

 result from definite conditisns, and comparable explanations may be 

 expected in the case of various organs whose cause is still unknown. 



