190 THK PLANT Wl^KLD 



1 he region considered in the present paper is one which 

 has probably had an arid climate since Pleistocene times, 

 and a large number of the plants now inhabiting it must 

 have originated within its limits. In the progressive desicca- 

 tion of this or anv other region the forms present would 

 necessarilv restrict their acti\ity to the periods of greatest 

 precipitation in the manner described above. Later, by some 

 method of evolutionarv de\elopment, species capable of 

 carrying on growth and other activity during the drier 

 periods must ha\"e arisen, and it is these which characterize 

 the ciesert. A consicieration of all a\ailable data leads to 

 the conclusion that all xerophilous or desert species have 

 originated within comparati\'ely recent time. 



The newlv originated xerophilous forms and the mois- 

 ture requiring species mav li\'e intermingled over the greater 

 area of anv desert. In the desiccation which has resulted 

 in the formation of a desert, howe\er, there are general 

 included topographical features, such as mountain ranges, 

 in which the special conditions fa\oral)le to condensation, 

 and the lowered temperature due to ele\ation may combine 

 to furnish suitable habitats for moisture lo\Ing species with 

 more or less alpine characteristics. Thus included within 

 the sub-tropical Arizona deserts which ha\e an annual pre- 

 cipitation of 10 to 12 inches are manv of these elevated meso- 

 phvtic islands receiving 30 to 40 inches of rainfall annually, 

 and which sustain a vegetation including many forms of the 

 temperate or even arctic zone. 



These generalizations are clearU' illustrateci by the facts 

 to be obtained bv an analvsis of the flora of the Tucson 

 region, where two maxima of rainfall occur, one m :mid- 

 winter anci the other in mid-summer. 



1111. WIXIKR WKT SKASOX. 



The secondarv maximum of precipitation is in December 

 and Januar\-, and amounts to 2 to 3 inches during these two 

 months. On the higher levels this comes in the form of snow, 

 and generallv melts slowh- in the glare of the noondav sun 



