20 PHYSIOLOGY. 



investigated, have the power of taking up water under press- 

 ure. 



40. Cell-sap a solution of certain substances. — From these experiments we 

 are led to believe that certain substances reside in the cell-sap of plants, which 

 behave very much like the salt solution when separated from water by the 

 protoplasmic membrane. Let us attempt to interpret these phenomena by 

 recourse to diffusion experiments, where an animal membrane separates two 

 liquids of difierent concentration. 



41. Diifusion through an animal membrane. — For this experiment we 

 may use a thistle tube, across the larger end of which should be stretched and 

 tied tightly a piece of a bladder membrane. A strong sugar solution (three 

 parts sugar to one part water) is now placed in the tube so that the bulb is 

 filled and the liquid extends part way in the neck of the tube. This is im- 

 mersed in water within a wide-mouth bottle, the neck of the tube being sup- 

 ported in a perforated cork in such a way that the sugar solution in the tube is 

 on a level with the water in the bottle or jar. In a short while the liquid 

 begins to rise in the thistle tube, in the course of several hours having risen 

 several centimeters. The diffusion current is thus stronger through the mem- 

 brane in the direction of the sugar solution, so that this gains more water than 

 it loses. 



42. We have here two liquids separated by an animal membrane, water on 

 the one hand which diffuses readily through the membrane, while on the other 

 is a solution of sugar which diffuses through the animal membrane with diffi- 

 culty. The water, therefore, not containing any solvent, according to a 

 general law which has been found to obtain in such cases, diffiises more 

 readily through the membrane into the sugar solution, which thus increases in 

 volume, and also becomes more dilute. The bladder membrane is what is 

 sometimes called a diffusion membrane, since the diffusion currents travel 

 through it. 



43. In this experiment then the bulk of the sugar solution is increased, and 

 the liquid rises in the tube by this pressure above the level of the water in the 

 jar outside of the thistle tube. The diffusion of liquids through a membrane 

 is osmosis, 



44. Importance of these physical processes in plants. — Now if we recur 

 to our experiment with spirogyra we find that exactly the same processes take 

 place. The protoplasmic membrane is the diffusion membrane, through which 

 the diffusion takes place. The salt solution which is first used to bathe the 

 threads of the plant is a stronger solution than that of the cell-sap within the 

 cell. Water therefore is drawn out of the cell-sap, but the substances in 

 solution in the cell-sap do not readily move out. .\s the bulk of the cell-sap 

 diminishes the pressure from the outside pushes the protoplasmic membrane 

 away from the wall. Now when we remove the salt solution and bathe 



