22 



Guayule. 



the vegetations of the north and south facing slopes (Lloyd, 1909), and 

 the peculiar distribution of certain plants, notably epiphytic species. 

 A most instructive example is offered by Tillandsia ciliata, which is to be 

 found epiphytic chiefly on the ocotillo (Fouquieria splendens) , on slopes, 

 mostly steep, where the drainage of cool air of high relative humidity 

 passes downward from higher levels. The ocotillo itself grows in the 

 more arid soil of southerly slopes. The Tillandsia C'pastle") occurs on 

 other shrubs also wherever the most favorable humidity conditions are 

 to be found, namely , in arroyos and narrow cafiadas receiving air-drainage 

 from adjacent high land, and I have seen a small amount in open flats 

 many miles from the mountains, where, during the rainy season, water 

 stands for some time over large areas, 1 thus producing similar conditions 

 in less marked degree. 



BO 



*o 



70 



60 



50 



3 



zo 



JAMk FEB^ MAR- APR. MAY JUNE JU 



Fig. 3. Monthly precipitation at Cedros, and 



LY AUG. SEPt OCT. NOV. OEC. 



relative humidity at Zacatecas city. 



We may therefore conclude that the atmospheric humidity in this 

 region is for a desert markedly favorable for vegetation, and may be 

 called into account to explain the denser total growth of this desert as 

 compared with the region immediately about Tucson, Arizona. What 

 biological relations between plant structure and the conditions described 

 above may be found is a problem for the future, the importance of which 

 I have elsewhere pointed out (Lloyd, 19086). Ross (1908) refers to the 

 occurrence of dews in the guayule region and suggests that the dense tri- 

 chome structure may be related to the absorption of atmospheric mois- 

 ture, but offers no evidence. At the present time we may do little more 



1 As in the "laguna" in the Camacho bolson, east from that place. 



