189 



APPENDIX. 



THE PHYSICS OF THE ABSORPTION OF WATER, SALTS, 

 AND GASES BY THE CELL. 



The question of the absorption of raw food-materials by the 

 living cell is a most important one, and involves, as has been seen 

 on p. 172, the consideration of osmosis. This process, vehich 

 must be studied a little more in detail, is partly vital and partly 

 physico-chemical, and concerns not only the absorption into the 

 cell of water and salts, but also of the gases COg and 0.., which 

 are used during assimilation and respiration respectively. If a 

 bladder composed .of moistened vegetable parchment be filled 

 with a solution of sodium chloride in water and immersed in 

 distilled water, water will pass into the bladder, and a certain 

 small amount of salt will also escape in the opposite direction. 

 The inward diffusion of water is known by the term endosmosis, 

 whilst the exit of salt is known as exosmosis. After a certain 

 time a condition of equilibrium is reached, when neither endos- 

 mose nor exosmose occur, and this state is known as osmotic 

 equilibrium, the bladder being turgid. 



If the concentration of two solutions of a salt separated by a 

 permeable membrane is the same, no permanent interchange of 

 water and salt occurs, but if the dilution of one of the solutions 

 be altered ever so little by the addition of more solvent, then 

 osmosis occurs. Nevertheless, even when the state of balance 

 has been reached, it is assumed that a constant interchange of 

 equal quantities of salt is taking place, so that a condition of 

 rest never obtains. 



In the case of the living plant-cell (root-hair, &c.), the state 

 of things is somewhat modified by the fact that the membrane 

 separating the two solutions (which are here on one side, the 



