Plant Associations at the Deskrt Laboratory. 31 



We have noted the occurrence of teratological flowers, 

 fruits, and leaves, which not only exhibit very interesting char- 

 acters, but also appear to be worthy of further study when re- 

 viewed in the light of the following considerations: (1) The fact 

 that they were of frequent occurrence in 1909 among the walnut 

 trees in Brea Can yon, thus indicating either some general external 

 factor or an unstable condition in a certain proportion of the 

 individuals of the species. (2) The probability that trees grown 

 from seeds produced from the abnormal flowers will have cha- 

 acters strikingly different trom those of Juglans calif ornica par- 

 ticularly in their leaves and habit of growth. 



PLANT ASSOCIATIONS OF THE DESERT LABORATORY 

 DOMAIN AND ADJACENT VALLEV.* 

 By V. M. Spalding. 



Tumamoc Hill, on the northern slope of which the Desert 

 Botanical Laboratory of the Carnegie Institution of Washington 

 is situated, is an outlying peak close to the eastern slope of the 

 Tucson Range, which latter extends from the neighborhood ot 

 the old mission of San Xavier del Bac, south of Tucson, in a gen- 

 eral northwesterly direction upwards of 25 miles, parallel to 

 the v^ alley of the Santa Cruz River. Its geographical position 

 and topographical features are alike favorable for an investiga- 

 tion of factors by which the distribution and present associations 

 of plants here represented have been determined. 



Standing, as it does, on the border-land between the plateau 

 region of central and eastern Arizona, on the one hand, and the 

 great desert plain that stretches westward to the Colorado River 

 and its delta, on the other; connected, moreover, by the valley 

 of the Santa Cruz and by broken ranges of mountains and hills 

 with the highlands of northern Mexico, there mingle within the 

 narrow limits of this one hill and the adjacent valleys many plants 

 represented in the widely different floras of California, New 

 Mexico, and Texas; and along the north and south pathway in 

 which it stands may be noted the northern limit of various 

 Sonoran species, as they drop out one by one. Its altitude, 



♦Reprinted in part, with revision, from Publication No. 113 of the Carnegie Institution of 

 Washington. 



