86 The Plant World. 



PLANT ASSOCIATIONS OF THE DESERT LABORATORY 

 DOMAIN AND ADJACENT VALLEY 



By V. M. Spalding. 



(Concluded.) 



(8) Superficial Soil Layers; Association of Annuals. 

 Thus far, attention has been directed wholly to perennial 

 species which, rooted in their places and remaining there for a 

 period of years, constitute the more permanent and conspicuous 

 elements of the vegetation. There are, however, a very consid- 

 erable number of annuals which, at certain seasons of the 3^ear, 

 become a highly characteristic and conspicuous constituent, and 

 whose habits and distribution involve interesting and impor- 

 ant problems. As in the case of the general association of per- 

 ennial plants of the hill, we are here dealing with a more or less 

 composite biological group, which, however, covers a wider area, 

 and is perhaps even more influenced by distinctively local con- 

 ditions. This, in its turn, will be studied to best advantage 

 under several groups, which, though not sharply delimited, are 

 sufficiently well defined. Taking the winter annuals first, three 

 divisions based on physiological requirements are recognized. 



(a) Shade-loving species. 

 A good representative of this group is Parieiaria dehilis, 

 which grows luxuriantly in the continuous shade of rocks on 

 northern exposures. The water content of this plant is very 

 high, the stems have the shining pellucid appearance of the east- 

 ern clearweed, and there are no indications of specially devel- 

 oped means for the prevention of excessive transpiration. 

 Though designated as shade-loving, because of its almost ex- 

 clusive occurrence in the deep shade of rocks, there is, neverthe- 

 less, evidence that its growth here is conditioned by moisture, 

 rather than lessened light intensity. It ventures here and there 

 a little beyond the shade, and though strikingly modified in 

 form and structure by exposure to full insolation for a part of the 

 day, it is nevertheless capable in this situation of maturing seeds. 

 It is apparently a plant of the same physiological rejuirements 

 as the so-called shade-loving species of mesophytic forests, and 

 in the one case as in the other it may well be questioned whether 



