OSMOTIC PRESSURE OF TISSUE FLUIDS 
Note also that in the instance of secondary parasitism noted above, 
the primary parasite is leafless (Dendrophthora) and the secondary 
parasite (Phthirusa) leafy. 
The problem of the relative magnitudes of the osmotic pressures of 
the fluids of parasites and host merits a short theoretical discussion. 
Let Ph be the observed pressure of the solution h in the tissues of 
the host, and Pp the observed pressure of the solution p in the tissue 
of the parasite. Then at first thought 
Pv > Ph 
would seem to be the necessary condition for the absorption of water 
from the host by the parasite. It is as a matter of fact the relation- 
ship almost always, but apparently not invariably, found. The view 
that it is the essential condition is of course quite fallacious, since p 
is not drawing water directly from h but the two are obtaining their 
water and its solutes from a common source, say t, the solution in the 
tracheae, with osmotic pressure P^, which must be assumed to be less 
than that of either the solutions in the leaves of the host or the tissues 
of the parasites. 
Now nothing is known concerning the concentration of the solu- 
tions in the xylem of the species upon which our determinations were 
based, but a priori, there should be no question in the minds of botan- 
ists that the osmotic pressure of the sap surrounding the haustoria of 
the parasite is distinctly lower than that of the leaf cells. Those who 
desire authority in support of this view may turn to the classic state- 
ment of Sachs that the water in the tracheae is an exceedingly dilute 
solution of nutritive salts which may be compared at once to ordinary 
drinking water. 
Fortunately direct evidences are available for trees of temperate 
regions. Dixon and Atkins (1914, 1915) have given the following 
values for sap centrifuged from the wood in comparison with those 
obtained from the leaves after treatment with liquid air. 
For Acer pseudoplatanus, in August 
from tracheae of root A = 0.070 
from tracheae of branch A 0.049 
from tissue of leaf A = 1.207 
For Populus alba, in August 
from tracheae of root A = 0.072 
from tracheae of stem A = 0.047 
from tissue of leaf A = 1.487 
