35 



The other four are probably responsible chiefly for the selection of a 

 sand flora from those species which are located within invading- dis- 

 tance of the area in question. They certainly can not account for 

 the sharp differentiation of the vegetation into definite associations. 

 In this the plants themselves are most concerned, through their modi- 

 fication and control of the physical features of the environment. If 

 their control is lost, a successional series begins. In the field study, 

 it was usually possible to recognize in the dynamic trend of the vege- 

 tation the underlying cause. The descriptions which follow take up 

 the subject from this aspect, discussing in more detail the effect of 

 the dynamic environment, especially the wind, and its partial or com- 

 plete control by the vegetation. 



General Discussion 



In the field study upon which this work is based the observational 

 method has been used almost exclusively. No apology or justification 

 for this method is necessary, for direct obsei-vation has led in the 

 past and will lead in the future to some of the most important results 

 of plant ecology, and must always be the method by which the first 

 ecological work in any region is done. The value of an exact knowl- 

 edge of some of the physical features of the environment is evident, 

 but their evaluation in an area of considerable size is a task not to be 

 undertaken by one man or completed in a single season. Undue 

 emphasis on the environment may lead to the partial neglect of the 

 most important feature of a region, the vegetation. The plant itself 

 is in many cases the controlling agent in the environment; the dif- 

 ferentiation of definite associations is mainly due to the interrelation 

 of the component plants ; and the physical environment is as often 

 the result as the cause of the vegetation. The relative importance of 

 the plant covering and the physical environment is happily expressed 

 by Spalding (ipop: 477, 479) : "But little reflection is needed to 

 arrive at the conclusion that the classical question regarding the rel- 

 ative importance of physical constitution and chemical composition 

 of the substratum to plant growth * * * (^oes not, and can not 

 reach the heart of the problem. * * * This being the case, it 

 would seem that in the future, investigations of the habitat relations, 

 of the desert species especially, must be directed mainly to the plant 

 itself. * * * fj-ig establishment of a plant in the place which it 

 occupies is conditioned quite as much by the influence of other plants 

 as by that of the physical environment." 



In the prairies of the sand deposits the two chief dynamic features 



