A STUDY IN PLANT ADAPTATION. 



485 



It is not to be presumed that every variation in the structure of plant 

 organs is a direct result of adaptations taken on by the plant to protect 

 it from unfavorable factors in its environment. It is the natural, in- 

 herent tendency of plants to vary, and when the variation chances to be 

 in a direction that fits it better to its environment, the variation 

 is apt to persist in future generations. There is no apparent reason, 

 however, why in many instances structures may not be present in the 

 plant that are in no sense of direct aid. We should not expect to refer 

 every variation in plant structures to variation in environment. We 

 should, however, expect those species to do best that in their natural 



Fig. 1. The Tucson Plains, showing Range Cattle in the distance i eeding cpon 



THE FklIT of IHE CHULLA. 



tendencies to vary become so modified as to fit them most perfectly to 

 their surroundings. 



Each plant organ must not only be adapted for the kind of work 

 that it has to do, but is must be adapted for doing its best under the ex- 

 ternal influences which enable it to persist in any given form. The foli- 

 age leaf bears a definite relation to light and moisture; the leaves of 

 one plant, however, may have quite different requirements as to light 

 and moisture than the leaves of another. Every traveler in our arid 

 southwest has noticed that the leaves of the trees and shrubs are small 

 and thick, or, in some instances, entirely absent as foliage. The reason 

 for this is very clear. It arises from the necessity of the desert plant 

 to expose a comparatively small surface to the intense sunlight and the 

 desiccating action of tlie dry atmosphere. 



