THE 
NEW PHYTOLOGIST 
Vol. XXI, No. 5 December 18, 1922 
PERMEABILITY 
By WALTER STILES 
CHAPTER X 
THE INFLUENCE OF EXTERNAL CONDITIONS ON THE INTAKE 
AND EXCRETION OF WATER BY PLANT CELLS AND TISSUES 
T he discussion of the water relations of the plant cell in 
Chapter IX involved a consideration of the internal conditions 
which may influence the rate of passage of water through the cell 
wall and protoplasm. These conditions are the concentration and 
nature of the substance dissolved in the cell sap which between 
them determine the osmotic pressure of the cell. The permeability of 
the protoplasm to solutes will also influence the rate of intake of 
water, inasmuch as the passage of solutes into or out from the cell 
will tend respectively to raise or to lower the osmotic concentration 
of the cell sap and hence the rate of intake of water through the 
protoplasm and cell wall. Lepeschkin (1908 a) appears to suggest that 
the osmotic pressure of a solution is a function of the permeability 
of the membrane, but the error of this view is obvious and has 
already been indicated by Brooks (1917 a), Hofler (1920) and V. H. 
Blackman (1921). Apart from electrical effects, the osmotic pressure 
which can be developed with a permeable membrane must con¬ 
tinually fall with time until equilibrium of the penetrating solute 
on the two sides of the membrane is attained. 
External conditions which are known to influence the rate of 
passage of water into and out from the cell are temperature and the 
composition of the external medium. It has to be noted that changes 
in the rate of passage of water into and out from the cell are not 
necessarily to be attributed to changes in the permeability of the 
protoplasm. For, neglecting the possible effect of limiting plasma- 
membranes, the rate of intake or excretion of water is the resultant 
Phyt. XXI. V. 
16 
