Establishment Behavior of the Palo Verde. 291 



which a piece of cleared desert becomes repopulated. Here is 

 no question of the clearing having made the conditions for re- 

 popiilation any less favorable than they are at any unoccupied 

 spot in a virgin stand of desert plants. For example, there has 

 come under my notice a block of ground within the city limits 

 of Tucson, which is occupied by a stand of Covillea of the aver- 

 age density, through which there formerly ran three roadways. 

 Ten > ears ago the block was fenced off and has not been used as 

 a highway since. The roads had never been worked, but were 

 simply made by the clearing of the ground and the wear of 

 wheels. They are still conspicuously visible, and in them have 

 not grown any young plants of Coiil/ca. 



The rate at which new individuals are added to natural 

 stands of desert plants is also extremely slow in very many 

 species. In the spring of 1906 Prof. V. M. Spalding surveyed 

 and carefully charted an area near the Desert Laboratory on 

 which there were growing, along \\ith other plants, some 72 in- 

 dividuals of Encelia farinosa. In the spring of 1910 I went over 

 this area and located each of the 72 plants and searched care- 

 fulh for new individuals, finding only one, which died in the 

 following month. 



My interest in these phenomena l^d me to inaugurate work 

 looking to the determination of: (a) the character of geimina- 

 tion and seeding in different types of desert plants; (b) the na- 

 ture of the vicissitudes which control germination and estab- 

 lishment, (c) the possible differences of establishment rate in 

 different types of deseit perennials. The first two of these ob- 

 jects are designed to secure knowledge as to the importance of 

 the early ontogeny of the peiennial types in determing their 

 local distribution and in contributing to the characteristic com- 

 plexity and openness of desert vegetation. The third of the ob- 

 jects is looking not only to the phenomena which may be opera- 

 tive in changing the character of the desert vegetation, but to- 

 ward a possibility that secular changes of climate are rendering 

 the conditions less suited to some of the desert plant types and 

 more suited for others. I have undertaken two lines of work as 

 sources of evidence on these problems: statistical work on adult 

 populations and observational woik on the actual occurences of 



