OSMOTIC PRESSURE OF TISSUE FLUIDS 
453 
pressure of the cell came into equilibrium with the tension of the 
surrounding cell wall. The rate of absorption would then be limited 
by this tension, and p would be free from the competition of h so long 
as the volume of t was sufficient for both. In just such a region as the 
Blue Mountains of Jamaica, with its heavy and well distributed 
precipitation ; one would expect the water brought up from the soil 
to be continually adequate. 
There now seems ample comparative evidence for the soundness 
of the reasoning here involved. Dixon and Atkins in a series of 
studies (1912, 1912a, 1912&, 1915a) have shown by very exact work 
on Ilex aquifolium, Hedera Helix and Syringa vulgaris that the osmotic 
pressure of the young leaves is generally lower than that of the older 
ones. We have in our own unpublished data constants for a wide 
range of species from rain forest, desert and mesophytic habitats. 
These show that in general lower osmotic pressure characterizes the 
sap of the young leaves. Nevertheless it is quite obvious that these 
young leaves are drawing water in competition with the old ones. 
It is important to note that in the cases in which determinations 
were based upon the tissue fluids of young leaves as well as upon those 
of the old leaves of the host, the osmotic pressure of the juices of the 
parasite has always been found to be higher than that of the young 
leaves. 
Most convincing evidence of the same kind is furnished by our 
studies of prolification of the fruit in Passiflora (Harris, Gortner and 
Lawrence, 191 5). In this remarkable abnormality a whorl of incom- 
pletely fused carpels is formed inside the trimerous or tetramerous 
ovary wall. Extensive observations have shown that for electrolytes, 
measured by electrical conductivity, and for both electrolytes and 
non-electrolytes, measured by specific gravity and by freezing point 
lowering, the concentration of the fluids of the abnormal structure is 
almost invariably lower than that of the normal wall, in competition 
with which it must draw its solutions from the conducting system of 
the plant. 
V. Recapitulation 
In this paper, which is one of a series on various physiological, 
ecological and phytogeographical problems involving the investigation 
of the physico-chemical properties of vegetable saps, we present data on 
the osmotic pressure of the tissue fluids of tropical mistletoes parasitic 
on various ligneous hosts. 
