166 BOTANY 



diffusion of both liquids will take place through the separating mem- 

 brane. If, however, the membrane is more easily permeated by one 

 of the solutions than by the other, then a larger quantity of the one 

 than the other will pass through it ; and, in case the membrane is only 

 permeable for one solution, that one alone will be drawn through it. 

 If a pig's bladder be filled with a solution of common salt and then 

 immersed in water, the flow of water into the bladder is more rapid 

 than the outflow of the salt solution, and, in consequence, an internal 

 pressure is exerted within the bladder sufficient to expand it to a hard, 

 rigid body. 



A pressure similar to that arising from the osmotic attraction of 

 the salt solution is produced in plant cells by the substances, particu- 

 larly organic and inorganic acids, salts and sugar compounds, held in 

 solution in the cell sap. The living protoplasm of the cell does not 

 allow any of the substances dissolved in the sap to pass out, except 

 such as escape through the diffusion taking place between the cells 

 themselves. In this process a constant transmutation and transforma- 

 tion of the cell substances occurs, but, as may be observed in cells 

 with coloured cell sap, these are held in by the protoplasm, and in 

 particular by the protoplasmic membrane (p. 51). These substances, 

 however, draw in water through the cell walls and the protoplasm, 

 and so set up a pressure within the cells often as high as 3 atmo- 

 spheres. In some instances this pressure may amount to 10, 12, 15, 

 and 20 atmospheres (e.g. cells of the cambium and medullary rays of 

 trees). Thus a tension is created which frequently exceeds that 

 exerted by the steam of the most powerful locomotives. Through 

 the force of such a tension the cell walls become so distended, that 

 cells under the influence of this pressure or turgidity become longer 

 and larger than in their unexpanded condition. 



When, from any cause, the quantity of water in such a turgescent 

 cell is diminished the internal pressure is naturally decreased, and the 

 distended cell walls shrink together again. The cell grows smaller, 

 and, at the same time losing its rigidity and elasticity, becomes soft 

 and flaccid. 



This condition occurs from natural causes when a succulent plant 

 loses more water by evaporation than it can replace, and, in conse- 

 quence, becomes flaccid. Such a flaccid plant plainly shows that the 

 rigidity is not maintained by its framework of cell walls, but by the 

 hydrostatic pressure within the cells, for with a- more abundant water 

 supply it returns to its original condition. 



In addition to loss by evaporation, water is also withdrawn from 

 cells by the same molecular force which causes the internal or endos- 

 motic pressure. In cases where the cells are surrounded by a solution 

 which exerts an attraction upon water, the turgidity of the cells is 

 proportionally weakened, and, if the force of the exosmotic pressure is 

 sufficient, it may be altogether overcome. On account of the consequent 



