Volume 1 1 Number 1 



The Plant World 



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OCTOBER, 1908 



THE COURSE OF THE VEGETATIVE SEASONS 

 IN SOUTHERN ARIZONA. 



{Continued) 

 By. D. T. MacDougal. 



THE ARID FORE-SUMMER. 



The precipitation shows a decrease to an average of 

 .90 of an inch during February, and this with a further 

 diminution to .77 of an inch during March coupled with 

 the steadily rising temperature brings to an end the activities 

 of the lush and luxuriant vegetation of the moist winter 

 season. The ensuing dry fore-summer may be taken to in- 

 clude April, May and June with a total average precipita- 

 tion of .67 of an inch, while maximum temperatures, as high 

 as 95-112'^ F. are presented. The possible evaporation of 

 course exceeds the rainfall by a large ratio, and the propor- 

 tion of soil moisture as well as relative humidity decreases, 

 the latter sometimes falling as low as 6 or 8 per cent. The 

 summation of heat exposures has usually reached the large 

 total of 59071 hour degree units, with which amount a num- 

 l)er of spinose and succulent types start into rapid activity 

 in flower formation and growth. This applies especially to 

 the great sahuaro {Cerens ^U^cDitcns) the flat jointied opun'ias 

 and the arboreous forms of the same group as well as to the 

 echinocereuses. 



Spinose and Sncculejit Forms of the Dry Fore-summer. 

 The succulents comprise two general types, one of which 

 is represented by the cacti with atrophied foliar organs and 



