3o 



BOTANY. 



pressure from within, but a point is soon reached beyond 

 which it will not stretch, and an equilibrium then tends to be 

 established between the pressure from within on the protoplas- 

 mic membrane, and the pressure from without by the elastic 

 cell wall. This state of a cell is turgescence, or such a cell is 

 said to be turgescenl, or turgid. 



Demonstration 12. 



60. Experiment to show diffusion through an animal membrane. — For 



this experiment use a thistle tube, across the larger end of which should be 

 stretched and tied tightly a piece of bladder mem- 

 brane. 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 immersed in water 

 within a wide-mouth bottle, the neck of the tube 

 being supported 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 

 membrane in the direction of the sugar solution, so 

 that this gain? more water than it loses. 



61 . How diffusion takes place. — 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 dif- 

 fuses through the animal membrane with 

 difficulty. The water, therefore, not contain- 

 ing any solvent, according to a general law 

 which has been found to obtain in such cases, diffuses 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 mem- 

 brane, since the diffusion currents travel through it. In this ex- 

 periment then the bulk of the sugar solution is increased, and the 



Fig. 15. 



