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THE AMERICAN NATURALIST [Vol. XLIII 



just beneath the surface of the ground, where they take 

 up and promptly pass on to the storage cells of the trunk 

 the water brought to them by every light rain. The roots 

 of the palo verde, on the other hand, extend much more 

 deeply into the ground, and are in a position to utilize 

 the water which soaks down to lower levels after heavier 

 rains. Thus the sahuaro profits by all rains, light and 

 heavy alike, while its constant companion, the palo verde, 

 is free from all competition on its part for the water 

 which penetrates to lower levels. Much the same thing 

 is seen on the flood plain of the Santa Cruz and other 

 rivers, where the mesquite, with its deep roots reaching 

 to the water table, is associated with Bigelowia and other 

 plants, the roots of which extend to relatively shallow 

 depths. In short, it appears that just as in a tropical 

 forest the vegetation occupies successive "stories," so 

 here the root systems of various plants habitually reach 

 to different depths, and thus enable at least some species 

 that would otherwise compete with each other to live in 

 close and advantageous association. 



From what has been said, it is evident that in the suc- 

 cessful occupation of a desert habitat the mutual rela- 

 tions of the associated species play a highly important 

 part. It is not quite easy at this stage of progress to 

 point out the exact steps by which these complicated re- 

 lations are to be determined and estimated; meantime 

 the homely and effectual method of patiently gathering 

 the data that are obtainable by careful observation is 

 open, and as far as it has been pursued has yielded val- 

 uable results. 



The broad general problem of the local distribution of 

 desert plants is necessarily approached along the several 

 lines that have been indicated. As we have seen, atmos- 

 pheric conditions, whether of intense insolation or ex- 

 treme dryness, that obtain in arid regions are limiting 

 factors which many plants successfully meet, but to 

 which many others succumb. There has been great need 

 of more practical methods of determining and estimating 



