304 BOTANICAL GAZETTE [OCTOBER 
long periods of deprivation; (2) the rainfall in the Jamaican coastal 
desert region is not distributed uniformly throughout the year. 
During periods of heavy rainfall the salts would be highly diluted 
or even largely washed out of the superficial soil layers in which 
the absorbing roots of the cacti lie, thus permitting water intake 
in quantities quite sufficient to maintain the plant until conditions 
again become favorable for water absorption. Thus species may 
differ very greatly in the relationship of their sap properties to 
environmental factors. Two species may be rooted in the same 
substratum, but because of differences in root penetration or in their 
capacity for water absorption or retention in reality they may be 
living in very different environments, or reacting quite differently 
to the same environment. 
Whether the hypotheses just advanced in explanation of the 
great diversity of the constants determined on the sap of particular 
species of plants growing in the same habitat be correct, can only 
be determined by intensive observational and experimental studies 
in the field. In the meantime they seem consistent with the 
available facts of desert plant physiology. 
Recapitulation 
In the present paper, which is one of a series on the physico- 
chemical properties of the tissue fluids of the plants of typical 
vegetations, we have presented the results of determinations of the 
freezing-point lowering of the tissue fluids of the plant species of the 
Jamaican coastal deserts; have compared the constants secured 
with those already available for the Arizona deserts and for meso- 
phytic habitats; and have offered tentative suggestions concern- 
ing the proximate causes of certain of the observed peculiarities of 
individual species. 
The deserts investigated constitute a small area on the southern 
coast of the island, where not merely the reduction in the rainfall 
due to the interception of the trade winds by relatively high moun- 
tains, but peculiarities of the substratum, contribute to the rigor 
of conditions limiting plant growth. 
Two sub-habitats have been recognized, low-lying coastal flats 
of finely ground detrital material, to a considerable extent impres- 
