DESEET CLIMAXES. 



231 



"The relation of Yucca radiosa to the sand dunes is unusually interesting. 

 A group of four small yucca shoots standing about 3 feet high to the tip of 

 the highest leaf was found upon the summit ridge of a 30-foot dune. We dug 

 the trunk out to a depth of 14 feet. All four plants were from branches of 

 the same trunk, the lowest branch arising about 16 feet from the base of the 

 dune; the main trunk and the branches bore marks of rosettes of leaves at 

 intervals all the way to the lowest point reached. The trunk was thicker 

 here, about 4 inches, than at any point above. The strata in the cut showed 

 that the yucca once stood on the front slope of the dune. The trunk sloped 

 in the direction in which the dune was moving. In the plain in front of the 

 dunes were occasional low plants of the same species of yucca. Considering 

 all the evidence, the conclusion is irresistible that the yucca originally grew 

 on the plain, was engulfed by the sand, and gradually grew through each 

 successive layer of sand that drifted over it until the siunmit of the dune was 

 reached. In the vicinity, at the rear of the dime, were other long trunks 

 partly denuded by the passing of the dune." 



MacDoicgal (1914 : 115) has presented in detail the comprehensive results of 

 his study of succession on the bare areas formed by the recession of the Salton 

 Sea in southern California. The critical examination of the emersed strands 

 and islands each year from 1907 to 1913 has produced a wealth of material to 

 which no abstract can do justice. However, the following extract from the 

 author's own smnmary (166) will be of service to those to whom the original 

 is not accessible: 



"SUCCESSIONS AND ELIMINATIONS. 



"The successions or transitions in the vegetation of arid shores of bodies 

 of either salt or fresh water are very abrupt, as has been foimd by the examinar 

 tion of great stretches of the coast of the Gulf of California. The tidal zone 

 may bear such plants as Laguncularia and other tide-marsh plants, but imme- 

 diately above the action of the waves the vegetation of the desert finds place. 



"The ephemeral character of Salton Lake with its rapidly sinking level 

 called into action a set of conditions entirely different from those to be met on 

 the shores of a body of water fluctuating about a fixed level. In the Salton 

 the water receded at such rate that during the time of maximum evaporation 

 in May or June a strip more than a yard in width would be bared permanently 

 every day and seeds of all kinds in motion at that time might fall on it and 

 germinate. All other physical conditions now were minor to the fact that 

 the soil began to desiccate toward a soil-moisture content equivalent to that 

 of the surrounding desert. Occasionally small flat places or shallow depres- 

 sions in the soil would be occupied by a growth of Spirulina, which with the 

 drying of the soil would, with the surface layer of the soil a few millimeters in 

 thickness, break into innumerable concave fragments, but this was not fol- 

 lowed by any definite procedure. 



"The main facts of interest on the shores centered about the survival of 

 the initial sowings on the beaches, the later introductions being for the most 

 part only of minor importance. The chief features of the endurance of the 

 initial forms and of the appearance of additional species on the beaches after 

 the first year may be best illustrated by a recapitulation of the observations 

 on the two beaches taken for the discussion of initial occupation, the Imperial 

 Junction Beach and the Travertine Terraces. 



" The emersion of 1907 at Imperial Beach bore Atriplex canescens, A . fasdcu- 

 lata, A. linearis, A. polycarpa, Amaranthus, Baccharis, Cucurbita, Distichlis, 

 Lepidium, Leptochloa, Heliotropium, Oligomeris, Pluchea sericea, Seswoium, 



