MOVEMENTS OF VEGETATION IN THE SALTON SINK. 149 



water. The only place in which the plant was found near the margin of the lake was on 

 newly bared areas in the archipelago of Big Island and the weight of probability lies with 

 the conclusion that the fruits were carried to this place by winds. The fact that the fruits 

 do not wet readily would operate against their being carried by adhesion to the legs or 

 feathers of birds. The stranding test was arranged to bring the plantlets in contact with 

 a substratum much like that of the shores of the lake and at a time when the seedlings 

 had not been weakened by prolonged flotation. 



Prosopis glandulosa is the "mesquite" of the Colorado desert. It is widely variant 

 in habit, being no more than a shrub on the upper parts of the bajadas or rocky slopes, 

 while it becomes a tree of some size in the Delta, where it is the dominant woody plant. 

 Its abundance in the alluvial lands and along streamways gives especial significance to its 

 relations to water. The pods are compressed and indehiscent, 10 to 12 cm. long, ripening 

 in the autumn, and the seeds are surrounded by a mass of dry, spongy tissue which seems 

 to contain a fermentable sugar. A large share of the pods are so completely bored by some 

 insect that perhaps not more than one or two perfect seeds may be found in each. Flota- 

 tion tests were made by placing whole pods in Salton water and others were broken into 

 fragments of various lengths. 



The first trials were begun in January 1913, with the species native to the Tucson 

 region. Naked seeds sank like pebbles. Pods and fragments, however, sustained various 

 positions in the water, some floating, some a short distance below the surface, and some on 

 the bottom, according to the buoying capacity of the gases resulting from the fermenta- 

 tion of material in the pods. No germinations were obtained in the first lot of seeds, prob- 

 ably by reason of the unfavorably low temperatures; some of the seeds became swollen, 

 but the decay of the pods made it advisable to discard the preparation. Next, a lot of 

 seeds was put into Salton water on January 25. A month later some were removed to a 

 vessel containing only one-fourth Salton water added to tap water. This seemed to make 

 no difference in their activity, however, as both lots germinated in about equal numbers 

 by March 10, at which time the temperatures were rising. The seedlings did not become 

 free from the seed-coats within a fortnight, but remained at the bottom until deteriora- 

 tion set in. Next, a lot of pods collected in the Salton region, early in March 1913, by 

 Mr. S. B. Parish, was put into water. The fermentation of the material in the fleshy 

 part of the pods made a strong odor, but no germinations ensued. These tests make it 

 clear that the mesquite may not be disseminated by flotation of the seeds or seedlings to 

 any great distance. The fleshy pods are attractive as food to many animals, perhaps to 

 the larger birds, although the author knows nothing as to the extent to which they are so 

 used. 



The mesquite appeared on the beaches of 1908 and 1909 at Travertine Terrace, but 

 has not formed a member of the beach communities of later date. The single specimen 

 which was among the pioneers on Obsidian Island was transported to that place as a 

 living branch, which was half buried in the sand of the beach and gave rise to the tree, 

 the appearance of which has been described. (Plate 21 a.) 



Prosopis pubescens, or screwbean, has played the most prominent part of any tree 

 on the beaches of the Salton. The indehiscent pods are rolled up into compact cylinders, 

 which mature in late summer, but remain attached to the tree indefinitely. The first 

 tests with the seeds were made with material taken from herbarium specimens of the 

 University of Arizona. These weighed about 6 mg., and sank to the bottom of a vessel 

 of Salton water in which they were placed on January 19, 1913. Swelling began within 

 a week, and one was germinating by the 31st; fifteen more, representing the entire number 

 of perfect seeds, germinated within the next ten days. The cotyledons were released from 

 the seed-coats somewhat slowly; and four rose to the surface of the water on February 15, 

 two weeks after germination began. These carried on a slow but normal development for 



