Harris: DESERT LORANTHACEAE 315 
The average freezing-point lowering of the tissue fluids of the 
parasite is 2.383? as compared with 2.041? in the fluids of the host, 
or a difference of + .342°. 
In terms of osmotic pressure, the average for the sap of the 
parasite is 28.63 atmospheres, that of the host is 24.50 atmospheres, 
and the average difference between them is + 4.13 atmospheres. 
The four determinations of P. macrophyllum on Populus are 
consistent in indicating higher osmotic concentration in the 
parasite. The excess ranges, roughly, from 1 to 5 atmospheres. 
The average for the four comparisons is 23.85 atmospheres for the 
parasite and 20.33 atmospheres for the host, a difference of 3.52 
atmospheres. 
Thus there are for the leafy desert Loranthaceae 14 deter- 
minations in which the-concentration of the parasite exceeds that 
of the host against three cases in which the reverse is true. 
Of the five determinations based on P. californicum in which 
comparison with the host is possible, four show an excess for the 
parasite, but the difference is extremely slight in one case. Far 
more work must be done before any final conclusions can be 
drawn concerning the relationship of the sap concentration of 
parasite and host in the leafless forms. 
IV. RECAPITULATION 
This paper presents data toward the solution of the problem 
of the water relationships of the desert Loranthaceae. 
Three species of the genus Phoradendron, the leafless P. cali- 
fornicum and the leafy P. Coryae and P. macrophyllum, have been 
investigated on a number of hosts. 
The osmotic concentration of the tissue fluids of the Loran- 
thaceae of the Arizona deserts is, roughly speaking, twice as great 
as demonstrated by similar methods in the tissue fluids of the 
species investigated in the montane rain-forest of the Jamaican 
Blue Mountains. | 
In desert Loranthaceae, as in those of the montane rain-forest, 
the osmotic concentration of the tissue fluids of the parasite is 
generally, but not invariably, higher than that of the host. 
These studies will be continued. 
