Review. 
283 
the climatic variations eliminated or negligible, owing to the uniform 
topography of the Sink, the question of distribution is left dependent 
directly upon the chemical and mechanical character of the soil and its 
water-content. Five “formations” are distinguished—hydrophytic, 
helophytic (always printed“heliophytic”), halophytic, mesophytic and 
xerophytic. The hydrophytes are very feebly represented, the water 
of the rivers and canals being so heavily silt-loaded as to prevent the 
growth of aquatics, and the amount of clear water being very limited; 
only in one place was a submerged seed-plant observed, the bed of a 
shallow stream being filled with a thick growth of Ruppia maritima. 
The helophytic formation is considered under two associations, 
that of springs at the upper or northern end of the Sink consisting 
of Typha latifolia and Scirpus olneyi, that of the rivers and canals 
at the lower or southern end of Typha with Scirpus paludosus and 
Cyperus erytlirorhizos. The halophytic associations are much more 
extensive and cover large areas of the alkaline mud-flats; the 
dominant plants are almost everywhere Chenopods. Those parts 
of the flats where the alkaline content exceeds the amount 
tolerated by the A triplex spp., are occupied mainly by species of 
Suceda and Spirostacliys. As may be inferred from the climatic 
conditions, mesophytes are poorly represented and show certain 
xerophytic features; they are mainly cottonwood (Populus 
macdougalii), Salix spp., and Baccharis glutinosus —all small—or 
narrow-leaved forms. The xerophytic vegetation of the Sink is a 
part of the general flora of the Colorado Desert, differentiated 
mainly by the great preponderance of species of Atriplex in its 
composition, and this formation occupies an area exceeding the 
combined areas of all the others, comprising, indeed, the greater 
part of the whole Sink. The xerophytic flora of the area is 
remarkably uniform, though showing three associations dependent 
on the nature of the several soils and distinguished as the associ¬ 
ations of detrial slopes, of the clays, and of the mounds of loose 
drifted materials found in different parts of the Sink. The total 
flora of the Sink consists of 202 species, of which 179 are seed- 
plants. Of the 79 xerophytes found, 51 are common in some 
parts of the Sink, and two are abundant everywhere in the arid 
soils and in the physiologically dry alkaline soils—these two, 
Atriplex canescens and Isocoma veneta, probably equal in number of 
individuals the united total of all the other plants above the rank 
of herbs. Six species of seed-plants are endemic in the Sink. 
The longest chapter (pp. 115-172) is that by Dr. MacDougal, 
dealing with the movements of vegetation in the area. The chief 
interest in this chapter “centres in the fate of organisms over¬ 
whelmed by floods, in the physical changes which follow emersion, 
and in the biological mechanism of re-occupation of sterilised areas 
as they emerge from the water—episodes which must have been 
duplicated in their main features innumerable times in the history 
of the surface of the earth.” As the author points out, opportunities 
for studies of this kind, and for analysis of the means and manner 
by which organisms colonise a sterilised area, are not common. 
The phenomena observed on the emersed lands of the Salton area 
are widely different from those investigated for Krakatau (Treub, 
Ernst, Campbell), the lavas of the Hawaiian Islands (Forbes), newly 
made land about the mouth of rivers (Oliver, Flahault and Combes), 
