454 
J. ARTHUR HARRIS AND JOHN V. LAWRENCE 
Osmotic pressure has in every case been calculated from the 
depression of the freezing point of saps extracted by pressure from 
previously frozen tissues. The conclusions are based upon 42 pairs 
of determinations distributed among seven species belonging to three 
genera of Loranthaceae parasitic on 19 species of host. All determina- 
tions here published were made on materials collected in the neighbor- 
hood of Cinchona, in the Blue Mountains of Jamaica. 
These studies demonstrate with reasonable certainty: 
(a) That the sap extracted from the tissue of the green stems of 
the leafless species belonging to the genus Dendrophthora are char- 
acterized by a lower osmotic pressure than that obtained from the 
leaf tissue of the leafy species of Phoradendron and Phthirusa. These 
are of the order 1.170° and 1.350° freezing point lowering or about 14.2 
and 16.2 atmospheres pressure respectively. 
{b) That the osmotic pressure of the sap extracted from the 
chlorophyll bearing tissues of the parasite is almost always, but 
apparently not invariably, greater than that expressed from the mature 
leaves of the host. 
While higher osmotic pressure of the sap of the parasite is a general 
condition, we have shown that it is not a necessary condition for at 
least the temporary success of the parasite. The parasite should be 
able to draw from the relatively dilute solutions in the stem in com- 
petition with organs of actually higher osmotic pressure, except at 
periods when the supply of soil moisture is limited, just as young 
leaves are able to draw water in competition with old leaves of higher 
osmotic pressure. 
In conclusion we have to express our hearty thanks to Professor 
Bower and the other members of the British Association Committee 
having in charge the Tropical Laboratory at Cinchona for its pro- 
tracted use, to Dr. MacDougal, director of the Department of Botan- 
ical Research, for material assistance in field operations, and to Dr. 
Britton, for the opportunity of having the materials upon which this and 
other unpublished physiological studies on Jamaican plants were based 
critically examined at the Herbarium of the New York Botanical 
Garden. To Mr. Percy Wilson of the Garden we are indebted for 
the painstaking care with which this work was carried out. To Mr. 
Wm. Harris, F.L.S., Superintendent of Public Gardens and Planta- 
tions, Jamaica, we are indebted for many courtesies which contributed 
