2 



The Plant World 



While all our native plants respond readily as regards 

 growth to favorable conditions of moisture and temperature 

 those vegetation forms of our flora which are not in any sense 

 xerophytic in habit nor suggestive of desert and semi-desert 

 conditions appear to be most pronounced in this respect, which 

 may be due to their growth being restricted to a definite period 

 or season. These species are for the most part represented by 

 annuals which are confined largely to the lower altitudes already 

 noted, and perennial herbs which constitute a large percentage 

 of the flora of the higher altitudes. The annual species may be 

 divided conveniently into two groups, viz., winter annuals 

 which begin and end their existence within the season of the 

 winter and early spring rains when continuity of moisture in 

 the soil is more certain than at other periods of the. year; and, 

 summer annuals which grow only during the summer rainy sea- 

 son, i. e., July to September inclusive. In the vicinity of Tucson 

 there are approximately one hundred and fifty species of winter 

 annuals and about one-third that number of summer annuals. 

 The former are members chiefly of the borage, chicory, evening 

 primrose, grass, mustard, pea, phlox, poppy, plantain, sunflower 

 and waterleaf families, while the latter belong mostly to the 

 amaranth, carpet-weed, caltrop, four-o-clock, grass, morning 

 glory, purslane, spurge and sunflower families. 



The seeds of winter annuals germinate at relatively low 

 temperatures, the young plants passing the cold winter months 

 in a seeming inactive rosette stage from which they emerge 

 uninjured by the occasional severe frosts incident to our winter 

 weather, and begin rapid growth with the warm days of late 

 February. It needs hardly be noted that the Southwestern 

 winter and spring season is admirably suited to their require- 

 ments of growth. Summer annuals, on the other hand, grow 

 only during the prevailing higher temperatures of our summer 

 months when sufficient moisture obtains. They develop no 

 rosette stage, and from the germination of their seeds to the 

 time of seed maturation, a period of four or five weeks or longer 

 with exceptionally heavy rainfall, their growth is notably rapid. 

 One is led to believe at times that the uncertainty of the summer 

 showers together with the lack of continuity of moisture in the 

 soil at this season has reacted upon the growth of these plants. 

 I£ must not be overlooked, however, that growth with many 



