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THE AMEBIC AN NATURALIST [Vol. XLIV 



harbors between four and five hundred species which are by no 

 means all xerophytes, but species adjusted to the most diverse 

 moisture requirements. To be sure, this is duo in a measure to 

 the inclusion of a river and its flood plain in the Laboratory 

 domain. Still, a surprising number of species manage to exist 

 on the arid situations without being marked by any special 

 xerophytic structures. 



Chapter II takes up in detail an analysis of the distribution of 

 certain of the most prominent desert species. As to the factors 

 determining the local distribution and association of species, 

 these are, just as everywhere else, chiefly topographic and soil 

 conditions. Manifestly it is primarily the water supply as af- 

 fected by conditions of soil and topography which is the critical 

 factor, but soil drainage or aeration is found to play a large 

 role and, indeed, is the determining factor in the distribution 

 of certain species. 



Special attention may be called to that portion of Chapter II 

 which presents the facts and conclusions as to aspect preference 

 as exhibited on the laboratory domain. The question, in brief, is 

 to account for the differences in the vegetation covering of slopes 

 having a general southerly exposure as compared with those hav- 

 ing a general northerly one. Here is found fully justified the 

 claim advanced by certain American plant ecologists, that only 

 by exact quantitative study of the items involved may one expect 

 to arrive at an explanation of distribution, association and move- 

 ments of plants. In this connection the study of the root sys- 

 tem of Cercus giyantcus sustains its status as a model piece of 

 ecological investigation. 



Another thing which we learn from this chapter (II) is that 

 while, as we supposed, the chief struggle of plants in the desert 

 is against the physical environment, there is still a right marked 

 element of competition amongst desert species with its conse- 

 quent features of accommodation (commensalism?) or of exter- 

 mination and succession. Since, however, the problems of distri- 

 bution, association and movements arc so little complicated by 

 the factor of crowding as compared with mesophytic forest so- 

 cieties, for example, it has seemed to the reviewer that the desert 

 furnishes about the simplest as well as the most inviting point 

 of attack for the ecologist. 



In chapter VI under Review and Discussion and in Chapter 



