SECT, ii PHYSIOLOGY 179 



stretched and the tube longer and thicker. By this process the tube 

 becomes just so much stiffer and firmer the greater the internal pressure 

 and the more elastic and thinner its wall. By the similar tension of 

 their elastic cell walls arising from internal pressure, the rigidity and 

 elasticity of thin-walled plant cells, and organs composed of them, are 

 maintained. The internal pressure results from the osmotic attraction 

 and interchanges between the substances dissolved in the cell sap and 

 the water outside the cell. 



When two substances in solution are separated by a membrane 

 which is permeable to both, a mixture of the two solutions will take 

 place through the dividing wall. 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. 

 If the membrane is semipermeable, i.e. 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 sugars, 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 the 

 constant translocation and exchange of substances within the plant. As 

 may be directly observed in cells with coloured cell sap, the dissolved 

 substances are held in by the protoplasm. These substances, however, 

 draw in water through the cell walls and the protoplasm, and so set 

 up a pressure within the cells of at least 3*5 atmospheres, and often as 

 great as 5-10 atmospheres. In some instances this pressure may 

 amount to 15, 20, or more atmospheres. In the cambial cells of trees 

 a pressure of 25 atmospheres, and in the "nodes" of grasses, of 40 

 atmospheres is attained. 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 dis- 

 tended, that cells under the influence of this internal 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 

 cell walls, the distension of which may have amounted to 10-20 

 per cent, again contract. 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 



