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THE AMERICAN NATURALIST [Vol. XLIII 



piration can not successfully occupy the same habitat 

 if their capacity for root absorption differs in the same 

 ratio. But supposing that with infinite patience and with 

 a reasonable approach to accuracy both sets of physio- 

 logical data have been determined, we are still, quite pos- 

 sibly, entirely in the dark as to the real cause of the 

 different behavior of the plants under investigation. It 

 may be in their case that the whole matter of absorption, 

 conduction, and transpiration is beside the mark, and 

 that certain plants can not succeed in the desert because 

 the intense insolation exerts directly a prejudicial in- 

 fluence to which they have not become inured. The intri- 

 cate nature of the subject is apparent, and it is also evi- 

 dent that there is little encouragement for any one to 

 take it up who has not had extended training and thorough 

 equipment for physiological research. Yet with all its 

 difficulties the problem is an attractive one, and the 

 abundance of material to be had in any desert city, to- 

 gether with the great mass of data that has accumulated 

 in the hands of horticulturists and at the experiment 

 stations, offers the best of opportunities for extended and 

 fruitful work. 



If, as we have seen, the different deportment in the 

 desert of plants growing, or having the opportunity to 

 grow, side by side in well watered ground, is an exceed- 

 ingly complicated matter, by how much are the difficul- 

 ties increased when we pass from a habitat of uniform 

 and highly favorable conditions, to the various and often 

 extremely trying conditions which prevail in different 

 neighboring habitats, such as the dry slopes underlaid 

 by caliche, the salt spots and others. If the case of a 

 plant growing in well watered soil may become desperate 

 because of the scorching winds or the intense insolation 

 to which its top is exposed, what hope is there for one 

 that assays to grow where both dry air and dry soil pre- 

 sent the supreme test of endurance? As a matter of fact 

 only relatively few species meet the test successfully, 

 yet there are some that do, and they present some of the 



