Plant Associations at the Desert Laboratory. 33 



been taking place, broadly speaking, since early Pleistocene 

 times. 



It can hardly be doubted that in this region, during the 

 period from the Tertiary to the present, two distinct floras of 

 different origin have existed side by side as they do today; the 

 one including the desert plants of the arid plains and lower ele- 

 vations, the other the mesophytes of the mountains. As just 

 stated, the species belonging to the former have apparently orig- 

 inated where they now live, or at least have undergone migrations 

 of very limited extent compared with those which many of the 

 latter have made as representatives of plants which in earlier 

 days migrated southward from the arctic regions, and which still, 

 at the present time, exhibit features of the old Eurasian stock. 

 There is, of course, more or less intermingling of the elements of 

 these very diverse floras, where climatic and soil conditions make 

 this possible, but one has only to place side by side the plants 

 of Tumamoc Hill and those of Mount Lemmon, a few miles away, 

 to realize how essentially different in their fundamental character- 

 istics the two floras are. 



In dealing with the former, it is found that with it are 

 closely associated, though in more or less distinct contrast, 

 the plants of the neighboring valleys, and it has seemed neces- 

 sary, therefore, to include with the Laboratory domain the nat- 

 ural setting of Tumamoc Hill, that is, the ground immediately 

 adjacent, extending from the bed of the Santa Cruz River on 

 the east to the boundary of the Laboratory reservation on the 

 west, and from the flood-plain of the river on the north to the 

 same topographic horizon on the south, in this way delimiting 

 the area studied by natural rather than artificial boundaries. 

 Thus bounded, this area includes approximately 4 square miles 

 and exhibits all of the characteristic plant associations within 

 the immediate neighborhood of the Desert Laboratory. 



(1) The River and Irrigating Ditches; Association of 



Hygrophytes. 

 The Santa Cruz River has the general character of streams 

 in the Southwestern United States. For months at a time its 

 bed is empty, but at the period of summer and winter rains it is 

 not infrequently filled with a raging torrent which has measured 

 no less than 4,012 cubic feet of water per second at the Tucson 



