452 
J. ARTHUR HARRIS AND JOHN V. LAWRENCE 
Clearly the values found for the leaves are enormously greater 
than those for the sap from the non-Hving conducting tissue of the 
stem. Further determinations for the wood of stem and root in Acer 
pseudoplatanus, Cotoneaster frigida, Fagus silvatica, Ilex aquifolium, 
Populus alba and Salix hahylonica indicate that while the concentra- 
tion of the fluids centrifuged from these stems varies rather widely 
during the year, concentrations comparable to those of the foliage 
leaf are never found. Indeed, the observed depression of such fluids 
is only a fraction of that prevailing in the leaf tissue of the typical 
arborescent plant. 
Thus if the trunks of the ligneous hosts from which our parasites 
were taken present no higher concentrations in their fluids than do 
those of the trees studied by Dixon and Atkins,^^ there would seem to 
be no reasonable question of the adequacy of the osmotic pressures 
demonstrated in both the leaves of the host and the tissues of the 
parasite for any role which may be logically assigned it in maintaining 
a flow of water from the wood cells to the chlorophyll bearing tissues 
of both host and parasite. 
Thus the condition 
Pn > Pt, > 
where h represents the leaves and / the conducting system of the trunk 
of the host, may be reasonably assumed to obtain universally in the 
case of shrubby parasites on the aerial parts of the ligneous hosts. 
Returning to the question of the relative values of P^ and Pp it 
is now evident that these merely draw from t in competition with each 
other. Were the supply of t inadequate then Pp > Ph would be a 
necessary condition for the development of the parasite, but if the 
supply of t is not limited it seems theoretically quite possible for a 
parasite to flourish on a host which has leaves of a higher osmotic 
pressure drawing water from the vascular system in competition with it. 
In such competition h would draw water from t until the osmotic 
1^ Incidentally it may be pointed out that if the conclusion reached by Dixon 
and Atkins, that sugars (monosaccharides, disaccharides or both) are found at all 
times in the tracheae of trees, be found to apply generally to ligneous plants, new 
light is thrown upon the much discussed question of the degree of parasitism in the 
Loranthaceae. Even if they draw their substance entirely from the xylem they can 
hardly be regarded as merely water and mineral parasites, provided the tracheae 
contain throughout the year sugars supplied from the cortex by way of the medullary 
rays and the wood parenchyma, as urged by Dixon and Atkins. The discussion of 
this point falls quite outside the scope of the present paper. 
