Plant Growth and Vegetation. 



3 



other of our species at this season, when moisture is commonly 

 abundant both in the soil and the atmosphere, is extremely 

 rapid. 



The distribution or rather limitation of these short-lived 

 plants in our region is exceedingly interesting. Neither group 

 grows in abundance in the extreme western and southwestern 

 parts of the Territory, except, perhaps occasionally, in depres- 

 sions where storm-water collects from heavy showers, on account 

 of the greatly restricted rainfall in this section, this portion of 

 Arizona, as is well known, being one of extremely low alti- 

 tudes. On the other hand, neither group becomes a character- 

 istic feature of the flora above altitudes of 4500 to 4800 feet; 

 in the case of winter annuals this is believed to be due to the 

 fact that the lower temperatures of the winter and early spring 

 months at these altitudes retard in no small degree their growth 

 during this period of abundant moisture, after which droughty 

 conditions set in. With summer annuals the case is quite 

 different; the rainfall during the summer and fall months at 

 altitudes of 4500 to 4800 feet is usually sufficient to maintain 

 a good growth of the perennial bunch grasses which are perma- 

 nently rooted, summer-growing species that tend to hold in 

 check the germination and development of small annual plants. 

 Where these bunch grasses are killed out as a result of over- 

 grazing and tramping, summer annuals or some more resistent 

 perennial form assert themselves and occupy the ground. For 

 reasons given above, winter annuals and summer annuals are 

 at their best, therefore, between altitudes of 1500 and 4500 

 feet. Even within these limits, especially in shallow soil, sum- 

 mer annual growth is greatly embarrassed by the presence of 

 such species as creosote bush and the various cacti, the well- 

 developed root systems of which appropriate much of the 

 available moisture before the shallow-rooted annuals can take 

 it up; this, in connection with the extremely rapid surface 

 evaporation commonly results in the seedlings of annual plants 

 drying up between showers. 



These two groups of annuals, viz., winter annuals and sum- 

 mer annuals, growing at different seasons and under different 

 temperature conditions are for the most part small, rapid- 

 developing, short-lived plants; they are, in brief, mesophytes 

 growing within mesophytic or semi-mesophytic periods amidst 



