296 J. ARTHUR HARRIS AND JOHN V. LAWRENCE 
on the southern shore of Jamaica (Harris and Lawrence, 191 7&). 
Thus Rhizophora Mangle shows concentrations ranging from 29.2 to 
30.9 atmospheres, Laguncularia racemosa shows concentrations ranging 
from 24.6 to 34.8 atmospheres and Avicennia nitida yields values from 
41.5 to 54.4 atmospheres. 
IV. Recapitulation 
The Blue Mountains of Jamaica, intercepting as they do the 
trade winds in their sweep across the Caribbean Sea, exhibit a con- 
spicuous differentiation in the flora and especially in the vegetation 
of the windward (northern) and the leeward (southern) sides of the 
range. 
The windward slopes, and especially the windward ravines, exhibit 
all those features of vegetation and of structure of the constituent 
species which are called to the mind of the botanist by the term Rain 
Forest. In the higher mountains the leeward ravines share many of 
the characteristics of the windward ravines and slopes, but the leeward 
slopes, and especially the scrub formation known as ruinate, are far 
more xerophytic in their botanical characteristics. 
The subalpine ridges, while lacking some of the most characteristic 
and typical xerophytic species of the ruinate, are nevertheless clearly 
far more xerophytic than either the windward slopes or ravines or the 
leeward ravines. 
These differences have long been known to botanists, and have 
recently been splendidly described and illustrated by Shreve. 
The purpose of the investigations described in this paper, which 
is one of a series on the sap properties of the plant species of diverse 
vegetations, is to present the results of an extensive series of cryoscopic 
determinations of osmotic concentration of leaf sap in the species of 
the Blue Mountains, to compare these habitats among themselves on 
the basis of the average osmotic concentration of their leaf tissue 
fluids, and to compare the region as a whole with other areas, meso- 
phytic and xerophytic, which have been investigated in a similar 
manner. 
The results of the present study confirm the conclusions concerning 
the existence of a higher osmotic concentration in the tissue fluids of 
the leaves of ligneous than in those of the tissue fluids of herbaceous 
plants, drawn from the investigation of the deserts of southern Arizona. 
The difference between the concentration of the sap of the two groups 
