162 



COLLEGE BOTANY 



membrane is permeable to both the solvent and the dissolved 

 substance, there is always movement of both water molecules 

 and solute molecules through the membrane. But if the mem- 

 brane is semi-permeable the solvent only is involved. Plant 

 membranes are usually permeable to some solutes and imperme- 

 able to others. If in an open container the denser solution will 



increase in volume 

 and if in a closed con- 

 tainer it will exert a 

 very strong pres- 

 sure (see Turgor, 

 page 165). 



Demonstration of 

 Osmotic Pressure. 

 This phenomenon is 

 very easily demon- 

 strated by soaking a 

 semi-permeable mem- 

 brane, such as an ani- 

 mal bladder or parch- 

 ment, in water and ty- 



Fio. 96. Apparatus for demonstrating osmosis; thistle 1X1 S ^ over the large 

 tube on left and egg on right. > * .-, .-, , -, 



end ot a thistle tube 



by means of a waxed thread. The bulb part of the thistle 

 tube is filled with a thick sugar solution and is then placed 

 in a glass jar of distilled water so that the two liquids will 

 be at the same level. The liquid in the thistle tube will rise to 

 considerable height due to the passage of water through the 

 membrane (Fig. 96). 



Osmosis and Diffusion in the Living Plant. In the living 



