MOVEMENTS OF VEGETATION IN THE SALTON SINK. 161 



and seeds into the saline lake which is increasing in salinity and toxicity as against the 

 constancy of composition of sea-water. The initial vegetation on beaches laid bare by 

 the recession of the lake appears on strands down-slope from localities bearing a halophytic 

 and xerophytic vegetation. These desert slopes receive but little rainfall and have nothing 

 but ephemeral streams, run-off currents (which rush down the shallow channels for a few 

 minutes or a few hours) being made by the precipitation of vagrant storms. Such run-off 

 currents furnish a special method of flotation of seeds and propagulse which would not be 

 encountered in any place except in arid regions. 



NATURE OF THE EVIDENCE. 



It is obvious that most of the conclusions with regard to the dissemination or dispersal 

 of plants in cases like those under discussion must rest largely upon circumstantial evidence 

 and that the reasoning employed is largely inferential. To emphasize this point it need 

 only be said that no one has actually followed an individual plant in its movement over 

 a wide distance with identification at all stages of the journey. The only part of the matter 

 which is amenable to direct observation and controlled experimentation is that which may 

 be brought into tests of the length of flotation and germination of seeds, fruits, and propa- 

 gative bodies. The importance of results bearing upon this subject is attested by the num- 

 ber of workers who have devoted attention to the matter. The positive evidence at hand 

 as to the relation of the size of seeds, form and structure of fruits, shape of appendages, 

 etc., is also of some value, while the presence of seeds in mud on the feet of birds, or attached 

 to their plumage, is highly suggestive of possibilities and allows the observer to account 

 for the presence of plants in localities otherwise unexplainable. 



The seeds of one-fourth of the species which were found on the newly emersed beaches 

 were treated experimentally in flotation and germination tests, and these represented 

 fairly well the size, structure, and general anatomical features of the entire lot, although 

 the untested remainder might well be expected to present some novel reactions. 



GERMINATION, FLOTATION, AND SURVIVAL. 



The first and main series of tests were made in equalized temperatures slightly lower 

 than a similar, treatment would have secured in the Salton region itself, but it is doubtful 

 whether much difference might be found in the minima encountered in the two cases. 

 Furthermore, the march of the seasons in the two instances was parallel, since the seeds 

 were put in Salton water in the middle or latter part of November at about the same time 

 as if they had been cast from the plant in the Salton area. The plantlets derived by the 

 germinations would have corresponded fairly well with others allowed to germinate on 

 the beaches where the seeds might have been deposited by the waves or carried by the 

 winds or birds. The seeds which might be expected to fall into the water in a free condi- 

 tion weighed from 0.013 to 6 mg. The fruits inclosing the seeds of Atriplex lentiformis 

 weigh 7 or 8 mg. Other fruits not tested, such as the large globular melons of Cucurbita 

 palmata, may weigh as much as 30 to 50 grams, even when dried, but by reason of their 

 comparatively great size are very buoyant on water. 



It seems to be generally conceded that structures or appendages are of small con- 

 sequence in dispersal of seeds over any great distance, and it is worthy of note that fully 

 80 per cent of the species occupying the beaches are liable to be moved by the wind. 



The initial plants on the bared strips were generally arranged in well-defined zones 

 or bands, which might give the observer the impression that they could have been deposited 

 in this strict order only by waves, but the recession of the water is followed by a concomi- 

 tant desiccation which leaves a moist strip which varies in width according to the slope 

 and texture of the soil in the separate localities and affords the only suitable conditions 



11 



