266 THE PLANT WORLD 



the leav^es of the mesquite and acacia, near which this plant 

 generally occurs. 



Late in the humid season, the fruits of the various 

 prickly pears ripen and develop their hues of red, purple and 

 crimson, the juicy, seedy pulp being most eagerly sought by 

 scores of animals. At times one may see great clusters of 

 the huge "June bug" {AUorhina mtitahilis) on the pads 

 clustered around a fruit which has reached the proper ripe- 

 ness. The rnpid maturity of these fruits is in striking con- 

 trast to those of the cylindrical forms of Opuntia, the acid 

 and solid fruits of which often remain attached to the 

 stems for many years, perhaps a decade in some instances. 



THE DRY AFTER-SUMMER. 



The latter part of the moist mid-summer has witnessed 

 the beginnintr of growth of a number of grasses of the 

 genera Trtodia, Bouteloua and Aristida, which ripen their 

 seeds and persist as tufted bunches of dry haulms and 

 leaves during the rainless season of October and November, 

 being eagerly eaten by grazing animals. During this season 

 an almost total cessation of vegetative activity ensues, and 

 continues until the double stimulation of the moisture of the 

 winter rains and of the increasing heat of the sun after the 

 winter solstice is received. Then the seasonal succession of 

 forms ensues as described, with various modifications due to 

 the wide departures from the normal or average conditions. 

 An example of this diversity is suggested by the records of 

 precipitation, which may vary from 5 to 25 inches per year. 



A general view of the entire region shows that it in- 

 cludes as the principal topographical features a series of 

 minor ranges of mountains trending in a northerly and 

 southerly direction, which have undergone tremendous erosion 

 in a desert climate, with the result that the intervening areas 

 are great valleys or bolsons filled with detrital material 

 brought down from the mountains. The soil is very poor 

 in humus, the drainage is undeveloped, oases of peculiar 



