216 BOTANICAL GAZETTE [September 



Light also is of course a factor even with the plants of the mesa. 

 I tried to grow some Yucca and Fallugia in the slight shade of 

 some box-elder trees, but they all died. It is probable that light 

 is quite as important indirectly through its acceleration of tran- 

 spiration as directly through its relation to photosynthesis. 



Furthermore, these different factors may be of a very diverse 

 importance in different groups. What may be an effective barrier 

 for one form of life may have little influence on others. The sum 

 total of heat during the season of reproduction may well be more 

 of a barrier to mammals than to plants. Banks has remarked 

 in a recent publication that it would seem to be necessary to have 

 a different arrangement of zones for at least every family of insects. 



Summary 



i. In North Central New Mexico the arid climate of the south- 

 west meets (in the mountains) the more humid one of the north 

 and east. 



2. Corresponding with this abrupt change of climate there is 

 an abrupt change of plant life. 



3. The genera and some of the species of the mountains are 

 identical with those of the east; those of the mesa are entirely 

 different. There is a greater difference between the flora of the 

 yellow pine association and that of the mesa, less than a mile away, 

 than between the former and Ohio and probably even Europe or 

 Japan . 



4. The chief factor determining this change is moisture, the 

 supply of which is largely determined by precipitation, ability to 

 hold it, and protection from drying winds and sun, as shown by the 

 following facts : 



a) The same plants (Fallugia, Erodium, oaks) occur throughout 

 a great range of altitude and temperature, but in soil of about the 

 same degree of humidity. 



b) Spruces and pinons will grow with their branches almost 

 touching if the roots of the former have access to an unfailing 

 water supply. 



v * J? 



c) A spring will change "Upper Sonoran" to "Lower Sonoran. 



