The Origin of the Desert Climax and Climate 95 



it during occupation by the grassland climax, but may or may 

 not occur within its borders today. Here again the grasses are 

 of the first importance, though perennial forbs and woody plants 

 also possess much significance. Strictly speaking, transads are to 

 be regarded as single species, but groups of closely related lin- 

 neons have similar if somewhat less meaning, and this is also 

 true of certain paired genera. The annuals have value, but it is 

 often difficult to determine, and they receive small considera- 

 tion, except for the group of desert endemics. Furthermore, 

 hydrophytes and halophytes not only possess peculiar ability to 

 migrate but they also evade climatic control in varying degree. 

 Their presence, therefore, is not of great significance. Finally, a 

 large number of species occur in the higher altitudes of moun- 

 tains on both sides of the desert, but these have moved from the 

 north along parallel systems and probably have never stretched 

 across the desert region. 



In consequence, the species listed below comprise only those 

 of which there is clear evidence that they once extended across 

 some part of the cUmax desert, as represented by the region from 

 Death Valley southward. There was desiccation to the east all 

 along the Sierra-Cascade axis, but this was not sufficient north 

 of Owens Lake to maintain the Larrea desert. 



Grass transads still present in the desert 



Oryzopsis hymenoides Phragmites communis 



Stipa speciosa Bouteloua gracilis 



Aristida divaricata Bouteloua racemosa 



Aristida d-parishi Andropogon saccharoides 



Aristida purpurea Andropogon glomeratus 



Poa scabrella Sporobolus airoides 



Poa s-nevadensis Sporobolus asperifolius 



Elymus sitanion Hordeum jubatum 



Distichlis spicata 



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