917] HARRIS & LAWRENCE—TISSUE FLUIDS 305 
nated with salts, and rocky limestone hills incapable of retaining 
moisture or of deriving it by capillarity. 
The vegetation of the coastal flats comprises a number of hard- 
leaved trees, among which is a mesquite very similar to that of the 
deserts of the southwestern United States, some thin and some 
succulent-leaved halophytes, and a number of genera and species of 
cacti which form a luxuriant-stand. The vegetation of the rocky 
"hills is of a more arborescent type, consisting chiefly of dwarfed 
broad-leaved trees with a number of small dwarf or half shrubs 
which have few purely structural characteristics which would ally 
them to desert plants. 
Taken as a whole, the species of the Jamaican coastal deserts 
show a concentration of their tissue fluids quite as high as, if not 
slightly higher than, that of as nearly as possible comparable 
growth forms in the Arizona deserts. The concentration of the 
leaf sap of the ligneous forms averages about two or three times 
that demonstrated in mesophytic regions. 
ile the plants of the rocky slopes show high concentrations, 
higher indeed than do those of the rocky slopes of the Arizona 
deserts, their constants are distinctly lower than those of the species 
of the coastal flats. 
The sap of the cacti has only a fraction of the osmotic concen- 
tration of that of the hard or succulent leaves of the trees and half 
shrubs among which they are rooted. The succulent Bryophyllum 
pinnatum and the terrestrial bromeliad Bromelia Pinguin show far 
lower concentrations than do the other species. Furthermore, 
Prosopis juliflora exhibits sap concentrations distinctly lower than 
those of certain other of the arborescent species. These form the 
extreme illustration of the fact that species of the same habitat 
show marked differences in sap properties. Suggestions concerning 
the underlying causes of such differences are offered. 
STATION FoR EXPERIMENTAL EVOLUTION 
Cotp Sprinc Harzor, Lone Istanp, N.Y. 
