OSMOTIC CONCENTRATION OF TISSUE FLUIDS 295 
each of the sub-habitats for both Hgneous and herbaceous forms. In 
our original paper (Harris, Lawrence and Gortner, 1916) the determina- 
tions for the Hgneous plants are further subdivided into trees and 
shrubs as one class and dwarf shrubs, half shrubs and woody twiners 
as the other. Such distinctions have not been so easily made in the 
rain forest. The two groups of desert ligneous perennials have, 
therefore, been combined to render them more comparable with the 
Jamaica ligneous perennials. 
The Arizona herbaceous plants were originally divided into the 
two very distinct groups, winter annuals and perennial herbs. These 
have also been combined to render them more nearly comparable with 
the herbaceou plants of the Blue Mountain region. 
Of course no one of the desert habitats is at all similar to those 
of the Blue Mountains. Those which are least of all comparable, 
the bajadas and the salt spots, have been set off from the others. 
The tables show at a glance that the concentrations of the desert 
are from fifty to nearly two hundred percent higher for individual 
habitats in the Arizona deserts than in the Jamaica Blue Mountains. 
The differences between the two regions are strikingly exemplified 
by a comparison of the herbaceous plants of the desert with the 
ligneous plants of the rain forest. The minimum osmotic concentra- 
tion in desert herbaceous plants (12.99 atmospheres in the arroyos) 
is practically as high as the maximum concentration for ligneous plants 
in the Blue Mountains (13.05 atmospheres in the ruinate). The 
mean concentration for herbaceous plants in the desert is 15.15 
atmospheres as compared with 11.44 atmospheres, the mean con- 
centration of ligneous plants in the Blue Mountains. 
While logically a comparison of the rain-forest vegetation of the 
Blue Mountains with the desert vegetation of the coastal deserts has 
no greater significance than that with the vegetation of the Arizona 
deserts it will, because of the relatively short distance separating 
the two Jamaican habitats, have a greater interest for most readers. 
The comparison with the coastal desert of the southern shore of 
Jamaica (Harris and Lawrence, 191 7) must be limited to ligneous 
perennials. The average of the 31 species means for arborescent and 
suffrutescent plants of the coastal desert, omitting only the herbaceous 
Sesuvium, Bromelia, Bryophyllum and the Cacti, is 30.05 atmospheres, 
as compared with 11.44 atmospheres for the montane habitats! 
Very high concentrations are also found in the mangrove swamps 
