Volume I I Number 9 



The Plant World 



^ ^Uariaztiu- uf Uirnrral ^ktanr 

 SEPTEMBER 1908 



THE COURSE OF THE VEGETATIVE SEASONS 

 IX SOUTHERN ARIZONA. 



By D. T. MacDougal. 



CLIMATIC AND VKGKTATIVE CONDITIONS. 



The climatic and physiographic features which char- 

 acterize deserts comprise combinations of meteoric and 

 orographic factors which produce a rainfall markedly less 

 than the possiijle evaporation, low relative humidity, com- 

 paratively small vertical increase in soil moisture, low humus 

 content of the soil, undeveloped drainage and comparativ'ely 

 great diurnal variations in both soil and air temperatures. 

 Many of the plants and animals of such regions exhibit dis- 

 tinctive habits anci specialized structures, but it is not to be 

 taken for granted that all of the organisms native or resident 

 within a region properly designated as a desert, show marked 

 xerophilous qualities. On the contrary it is quite possible 

 that species suitable by form and structure for existence under 

 conditions furnished by regions of great precipitation, may 

 Hnd wide distribution in arid areas. This occurs, howe\'er, 

 only when the scanty precipitation comes within a limited 

 part of the year giving rainy seasons or periods of maximum 

 precipitation, in which such forms may carry out their entire 

 cycle of acti\-ity and then pass into dormancy, remaining 

 quiescent in the form of seeds or modified shoots during 

 the intervening drier seasons. 



