188 SURFACES AND MEMBRANES 



experiments on the middle nerve leaf of Tradescantia discolor are shown 

 in Fig. V-6. In diagram A note how the sap fills the normal cell until 

 its bulging sides touch the cellulose walls. When such a structure was 

 immersed in a 10 per cent sugar solution, the protoplasm was found to 

 shrink gradually and draw away from the walls, as shown in B. When 

 the cell was immersed in a strong solution of potassium nitrate, the sap 

 was observed to shrink into globular masses like those shown in C. 



A 



Fig. V-6. Cells from the middle nerve leaf Tradescantia discolor. A, normal 

 cell. B, plasmolysis started, cytoplasm shrinks from cell wall. C, strong plasmolysis 

 due to 1.0 M potassium nitrate. (After H. de Vries [1888].) 



De Vries found that such a shrinkage was produced by the passage 

 of the water content of the cell through the cell membrane while the leaf 

 was immersed in a sugar or salt solution. 



He also discovered that many plant cells, after having undergone what 

 he termed -plasmolysis, could be restored to their natural fluid conditions 

 by placing them in pure water. This experiment proved that the mem- 

 brane was permeable to water in either direction. 



He concluded that the dead cellulose walls were freely permeable to 

 the sugar and salt solutions, but that the membrane boundary of the cell 

 protoplasm was impermeable to the crystalloids but permeable to water. 



A familiar example of membrane permeability is the swelling of seeds 

 when steeped in pure water. They will increase as much as 70 per cent 

 in weight from the transmission of water through the semi-permeable 

 covering with which they are surrounded. 



The apparent " selective " permeability of seed coverings to certain 

 substances is illustrated by the use of copper sulphate as a fungicide for 

 wheat. This salt, though highly poisonous for the fungus spores adher- 

 ing to it, will not affect the vitality of the wheat seed because the seed 

 covering is impermeable to it. Other examples of selective permeability 

 are furnished by the many marine forms in which the sodium-potassium 

 ratio of the cell content is very different from that found in sea water. 

 The marine plant Valonia, for instance, was found by Osterhout [1936] 



