Plant Associations at the Desert Laboratory. 87 



in the complex of physical factors necessarily involved, light in- 

 tensity plays as important a role as has been attributed to it. 

 In any case, it is to be understood that the classification here 

 adopted is retained chiefly for convenience in discussing observed 

 facts. 



A number of species of winter annuals grow luxuriantly on 

 northern slopes and in the shade of rocks and bushes that are 

 hardly met with on full southern exposures. Such are 

 Bowlesia lohata and, to a less marked degree, species of Phacelia, 

 Amsinckia, and others. Study of the habits of these plants, 

 however, indicates quite as plainly as in the preceding case, that 

 the water relation is of primary importance. Bowlesia, for ex- 

 ample, which has grown thickly in the shade of a creosote bush 

 or palo verde, continues, when they are destroyed, to produce 

 new crops year after year on the same ground, where the accumu- 

 lation of humus insures a better water supply. This and the other 

 plants just named are entirely ca]Dable of successful growth in 

 the full light of the sun, provided the root-system is still in soil 

 that supplies a sufficient amount of water. The light relation, 

 notwithstanding the apparent choice of shade on the part of 

 these species, appears, therefore, to be subordinate to water sup- 

 ply as a factor determining local distribution. 



(6) Light-loving species. 



Many of the winter annuals of Tumamoc Hill grow every- 

 where in the open, where they are fully exposed to the sun, and 

 to a greater or less extent, also, where there is some shade; such 

 are species of Harpagonella, Pectocarya, Plantago, Daucus, 

 Erodium, and other genera. These also are far more numerously 

 represented on northern than on southern exposures, but from 

 the fact that the range of light intensity under which they habitu- 

 ally accomplish their normal development is so great, there 

 can hardly be any doubt that the local distribution of these 

 species, like that of the preceding sections, is correlated first of 

 all with water supply; and far less, if indeed to any appreciable 

 degree, with light intensity. 



In its ultimate analysis, therefore, the local distribution of 

 the winter annuals here represented is fundamentally based on 

 the water relation. So broad a conclusion should receive con- 

 firmation from the determination of physiological constants 



