The Origin of the Desert Climax and Climate 97 



no longer present in the desert comprise 44, 20 of them annuals. 

 The greater number of annuals in the first group accords with 

 the general rule for the desert. The majority of the perennials 

 extend at least to the western edge of the Great Plains, but the 

 typical societies of the true and mixed prairies drop out long be- 

 fore reaching the borders of the desert, as would be expected. 



As may readily be inferred, aquatic forbs afford an exception- 

 ally high percentage of transads; in fact, there are but one or 

 two of this group restricted to the desert and only a few that 

 extend to either side. The number of transads still found in the 

 desert barely exceeds a dozen, and these are almost wholly con- 

 fined to the sedge family, to Juncus, Typha, and the duckweeds. 

 The aquatic transads that have disappeared from the desert are 

 twice as many and their relationships are much wider, the cos- 

 mopolitan genera of the figwort family being best represented. 

 Although the inference is plausible that many of these transads 

 have persisted since the remote days of extensive swamp and 

 meadow, the relative ease of migration among aquatics does not 

 exclude more recent colonization. However, they not only aid 

 in completing the details of the reconstructed picture, but they 

 also prove that desert climates do not exclude perennial ponds 

 and swamps with their characteristic water plants and animals. 

 Thus, in desert as in grassland generally, such edaphic com- 

 pensation for climatic dryness is more or less adequate, if 

 occasional, and the organisms concerned cannot be direcdy em- 

 ployed as climatic indicators. 



Shrub Transads 



These are likewise divided into desert transads and transads 

 proper, the former numbering three times the latter. However, 

 for the present purpose it is more convenient to arrange the 



