10 OSMOTIC PRESSURE 



membranes cannot be completely impermeable, since most of 

 the substances found dissolved in the soil-water can be detected 

 withm the plant. Moreover, plants will thrive for months or years 

 in a water-culture solution, ^ from which analysis shows that 

 mineral salts have been absorbed (cf. also below, p. 15). 



Were it not for the restraining influence of the wall, which 

 is only slightly elastic, but possesses considerable strength, a 

 plant-cell placed in pure water would increase in size until it 

 became ruptured. This actually occurs when the root-hairs of 

 salt-marsh plants are suddenly transferred to water, for these, 

 like other marine plants, have a highly concentrated sap. Owing 

 to the restriction on the dimensions of the cell, under normal 

 circumstances, the intake of water is arrested when the wall 

 has reached the limit of its stretching capacity. The more con- 

 centrated the sap the greater its attraction for water, and hence, 

 when a cell is surrounded by water, = the cytoplasm will be forced 

 against the cell-wall with a pressure proportional to the strength 

 of the sap. This pressure is spoken of as osmotic pressure, and 

 in many plant-cells is very considerable, as is well illustrated by 

 the following figures, which give the osmotic pressures in atmo- 

 spheres in a few selected cases ; 



Leaves of Acer pseudoplatanus . . I4'52 



,, Hedera helix . . . lyGS-iS'y 



,, Pteris aquilina . . . 7^44 



Rhizome of Iris germanica . . . 9^97 



Owing to the small size of the cell, it is not possible to estimate 

 the osmotic pressure of the sap directly, but an indirect measure 

 is afforded by the strength of the solution necessary to bring 

 about plasmolysis. By using artificially prepared semi-permeable 

 membranes (see Appendix IV), the osmotic pressure of any given 

 solution can be calculated in terms of atmospheric pressure. In 



1 Cf., F. it S., pp. 125, 126, sec footnote on p. 9. 



2 Even when a cell forms one of a tissue, it will receive water from all 

 sides as long as the water-supply is normal, since all parts of a healthy 

 plant imbibe water (cf. p. 26). Until a cell has reached the limit of its 

 stretching capacity, water will be withdrawn into its vacuole from any 

 of the surrounjiag cells which are more turgid. 



