Permeability 191 
The Influence of External Conditions on the Osmotic 
Pressure of Plant Cells 
This question is naturally connected with that discussed in the 
last section, for differences correlated with the ecological type of a 
plant may possibly be traced back ultimately to differences in the 
habitat. Thus Harris, Gortner, Hofman and Valentine (1921) found 
abnormally high osmotic values for the expressed sap of plants 
growing in the neighbourhood of the Great Salt Lake, the sap in two 
plants of the salt desert half shrub Atriplex confertifolia growing on 
rocky cliffs possessing osmotic pressures of 82*9 and 94*7 atmospheres 
respectively while three plants of the same species growing on low 
ridges in salt flats gave values of 74-2, 118*5 and 153*1 atmospheres 
respectively. The great difference in the osmotic pressure of the sap 
of plants of rain forest and of desert, even when the plants are of 
similar growth forms, may be correlated with differences in the 
environmental factors. Harris and Lawrence (1917 h) found the 
average osmotic pressure of the sap of ligneous forms in the rain 
forest of the Blue Mountains of Jamaica was about 11*44 atmospheres 
as compared with 14*40 for ligneous plants growing in Long Island 
and 24*97 atmospheres in the case of plants growing in the deserts 
of Southern Arizona (Harris, Lawrence and Gortner, 1916). Similar 
differences were found with herbaceous forms, the average osmotic 
pressure of the sap of Blue Mountain, Long Island and Arizona 
desert forms being 8*8o, 10*41 and 15*15 atmospheres respectively. 
High values of osmotic pressure in leaves of desert plants have also 
been observed by Fitting (1911) and Keller (1913). 
Iljin, Nazarova and Ostrovskaja (1916) measured the osmotic 
pressure of cells of the roots and leaves of a number of swamp, 
meadow and steppe plants by means of the plasmolytic method, and 
compared the values obtained with the conditions of the surrounding 
medium, soil or air, as the case might be, as regards water content. 
In roots they found the highest osmotic pressures in steppe plants, 
lower values in meadow plants, and lowest values in swamp plants, 
the values obtained for these three groups of plants being respectively 
0*40-0*48, 0*19-0*30 and 0*13-0*20 of the osmotic pressure of a 
normal solution of sodium chloride. The water content of the soil or 
swamp was in inverse order, so that it appears that the osmotic 
pressure of root cells is less in roots growing in a soil with a higher 
water content. Observations on a number of plants common to 
steppe and meadow confirmed this. Thus in the cases of Poa pra- 
tensis, T riticum repens , Kceleria gracilis , Slip a capillata, Festuca 
