ACROSS PAPAGUERIA. I 23 



Lastly, where some 500 feet higher the general slope begins 

 to merge into the foothills, the mesquite still persists, the only 

 species of the four, while Ccllis paUida becomes prominent 

 and well distributed on open topography devoid of rocks, 

 similar to that of the Rincons. 



Thus we have in these five species a very decided 

 tendency with lowering elevation to confine themselves to 

 water courses and to certain other places offering good soil 

 moisture conditions, and a certain degree of protection from 

 the drouth and heat of the desert. The above facts con- 

 cerning the nine species mentioned are too few to permit of 

 generalization, but they furnish pointers for research. They 

 appear to indicate that the factors of pre-eminent influence 

 are the intimately associated ones of moisture and tempera- 

 ture, and less so, that of light. The feeling can hardly be 

 escaped that the establishment of centers of distribution, and 

 lines of dispersal, belongs to an exceedingly remote past and 

 therefore these are hardly traceable. On the other hand, 

 there is clearly evident a very high degree of adjustment be- 

 tween plant and habitat, in distribution no less than in 

 structure and in form. In other words, equilibrium between 

 flora and environment has long been established. 



Tucson, Ariz., March 16, 1908. 



ACROSS PAPAGUERIA. 



{Continued.) 



By Dr. D. T. iMacDougal. 



The opuntias w^ith flattened stems as well as the cylin- 

 drical forms force themselves persistently on the attention 

 of the traveler in Papagueria, and great discomfort or even 

 serious injury may result from an unguarded encounter with 

 these armed plants. The heavy, rigid spines on the semi- 

 rigid joints, may penetrate deeply into the flesh, and the 

 glochids which form short tufts at numerous places on the 

 surface, are easily detached and quickly work into the skin 



