290 



The Plant World 



The data in the table suggest some interesting observations. 

 It may be taken as almost axiomatic that biennial species are not 

 a success in our region. The intense conditions prevailing regu- 

 larly over a considerable portion of the year, together with oc- 

 casional prolonged droughts, render their existence almost im- 

 possible. Either our plants develop root-systems extensive 

 and permanent enough to endure for some years, as in the case of 

 many perennial herbs and all the suffrutescent and woody forms; 

 or else their growth is completed within the course of a few 

 weeks, or at most a few months, during those portions of the year 

 when mesophytic conditions prevail to some extent ovei the 

 country. There appears to be little middle ground in this matter. 

 In areas I and II , where the most pronounced xerophytic condi- 

 tions obtain, the woody species, including trees, shrubs, dwarf 

 shrubs, half shrubs, and woody twiners, constitute 30 per cent 

 of the plants. It needs hardly be said that to these plant types 

 belong our characteristic desert forms. 



The short-lived winter annual and summer annual species, 

 on the other hand, make up 43 per cent of the plants of these 

 same two areas. That these plants are unable to withstand arid 

 conditions may be inferred from the fact that with their first 

 approach they cease further growth and begin dying. Such 

 species as Phacelia distans, Ellisia torreyi, Pectocarya linearis^ 

 and Harpagonclla Palmeri die off in early spring, except in the 

 shade of bushes, even with the presence of considerable moisture 

 in the soil. As opposed to these, plants like Phacelia crenulata, 

 Plagiobothrys pringlei, Amsinckia intermedia, and Cilia floccosa 

 are able to endure considerable drought by virtue of a better de- 

 veloped root-system. They are, nevertheless, species of short 

 duiation. As heietofore noted, growth obtains in these plant- 

 groups during the moistei portions of the year, after which they 

 disappear entirely from the foothills and mesa country. Those 

 of the winter annual type, which are by far the more numerous, 

 begin to grow in the fall with the advent of winter rains and pre- 

 vailing lower temperatures, such temperature conditions being 

 essential to their germination, * and continue until well in April, 

 after which time they cease to be a factor in the floral covering. 



♦The writer germinated successfully seeds of winter annual species last summer with proper 

 conditions of moisture in a refrigerator; while seeds from the same lots with the prevailing 

 summer temperatures remained unchanged. 



