Volume 1 1 Number I 2 



The Plant World 



^ Tllripaitnr af (lirnrral iBiitany 



DECEMBER, 1908 



THE COURSE OF THE VEGETATIVE SEASONS 

 IN SOUTHERN ARIZONA. 



{Concluded) 



By D. T. MacDougal. 



THE HUMID MID-SUMMER. 



The daily maximum registered by the thermometer 

 rises irregularly during June occasionally touching ii';^ F., 

 relative humidity decreases and the surface layer of the soil 

 becomes heated to a temperature of over ioo° F. in the 

 daytime, with but little cooling at night. Thousands of seeds 

 of cacti, hard coated legumes and of grasses are lying quies- 

 cent in the soil, which contains too little moisture for swelling 

 and germination. The heat exposure at the beginning of 

 July has now reached a total of 160,539 H-D-F. units, and 

 all of the conditions are favorable for the awakening of 

 hundreds of species which have hitherto shown no activity. 



Snowy piles of cumuli begin to be seen on the mountain 

 summits late in June, and the earlier short showers are fol- 

 lowed by longer ones which spread out over the plain in 

 fantastic patterns, generally giving the greatest rainfall of 

 the year during July and August. As soon as sufficient pre- 

 cipitation has ensued to bring the soil-moisture up to the 

 critical point, millions of seedlings spring into activity and 

 forty-eight hours may see the entire face of the landscape 

 changed in appearance. 



Summer perennials. The great barrel cacti (Echino- 

 cacti) , which have hitherto remained practically dormant, 



