190 
Walter Stiles 
(1-015 M KN 0 3 ) as that of Fagus sylvatica (1-017 M KN 0 3 ), while 
the mean values for epidermis and spongy parenchyma were higher 
in both Urtica dioica and Helleborus fceiidus than in Fagus sylvatica. 
That there might be some fundamental relation between the 
osmotic concentration of the cells of parasitic plants and their hosts 
was suggested by MacDougal and Cannon (191Q) and MacDougal 
(1911 a, 1911 b). Since then observations by Senn (1913), and a 
number of observations by Harris and Lawrence (1916) and Harris 
and Valentine (1920) on montane rain-forest, and desert Loranthacese 
indicate that the osmotic concentration of the cells of the parasite is 
in general higher than that of the leaves of the host. The values set 
out in Table XXII, taken from Harris and Valentine, indicate the 
degree of difference in the osmotic pressures of the cells of host and 
parasite. 
Table XXII 
Osmotic Pressures of the Cells of Host and Parasite 
(Data from Harris and Valentine) 
Osmotic pressure 
of leaf sap of host 
Parasite and host in atmospheres 
Phorodendron Californicum 
on A cacia greggii 
on Olneya tesota 
Phorodendron Cockerellii 
on Populus wislizeni 
on Salix wrightii 
on Fraxinus attenuata 
Osmotic pressure 
of sap of parasite 
in atmospheres 
26-57 33-66 
25-24 26-98 
22- 04 23-05 
20-88 24-98 
23- 47 26-47 
Harris (1918) has shown that epiphytes have an abnormally low 
osmotic pressure in their cells, the osmotic pressure in the epiphytic 
species examined by him being 37 to 60 per cent, lower than that of 
the expressed sap of herbaceous forms, and 28 to 45 per cent, lower 
than that of ligneous forms. 
Thus the evidence available appears to indicate that the osmotic 
pressure of plant fluids depends in part on the ecological type of 
plant. Harris and Popenhoe (1916) even claim to have shown a 
slight difference in the osmotic pressures, as indicated by freezing 
point lowering, of the expressed sap of the leaves of different horti¬ 
cultural types of the avocado (Persea americana). 
