i86 BOTANICAL GAZETTE [September 



that, in general, local variation of present physical conditions of 

 the habitat governs to a considerable degree the distribution of 

 plant communities, but the need of at least recognizing these other 

 sets of factors should be emphasized. It must be further seen 

 that, in the invasion of a new habitat, representatives from more 

 than one plant community can be successful in establishing 

 themselves, resulting in mixed vegetation-types. In fact, probably 

 the greater part of the area studied is occupied by mixed associations 

 or mictia (Clements). Even areas of established vegetation are 

 usually open enough to permit the continual ecesis within them of 

 new plant immigrants from quite different communities. This 

 diversity is likely to be relatively enduring, for plant competition 

 usually does not here operate to exclude all but a single type of 

 dominants. The opposite relation between plants, which may be 

 called accommodation, is as greatly in evidence. The control 

 exerted by vegetation upon the physical environment is slight over 

 the generally xerophytic mountain-front region. 



A second factor contributing to the mixed effect is the frequent 

 extremely local variability of physical conditions within the habitat. 

 This might be called mosaic variability, and its effect a mosaic 

 mixture of vegetation-types. The influence of large surface rocks 

 partly imbedded in fine soil, allowing the growth of comparatively 

 mesophytic plants in a rather constant interspersal with xerophytes 

 over a considerable area, may be cited as an example. 



Vegetation-types and their distribution 



Since the plant communities have been described separately 

 in the two articles preceding this, their systematic characterization 

 here may be condensed very considerably. A tabular view of the 

 communities, giving some idea of their general character and of 

 their distribution in the main geographic divisions of the region 

 studied, is shown in table VII. 



Some of the more important features of the particular associa- 

 tions may now be noted. Details and references to other accounts 

 may be found in the articles preceding. The general appearance 

 of certain vegetation- types may be seen in fig. 17. 



