162 



COLLEGE BOTANY 



membrane is permeable to both tbe solvent and the dissolved 

 .substance, there is always movem.ent of both water molecules 

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

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

 membranes are usually permeable to soime solutes and imperme^ 

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



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- 

 vment, in water and ty- 



FlQ.96.— Apparatus for demonstrating osmosis; thistle ^^S ^^ °''^®^ ™® large 

 tube on left and egg on riglit. ^ p j.-\ * i_i j. i 



-K, end 01 a thistle tube 



by means of a wa:xed thread. The biilb part of the thistle 

 1 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). 

 I ; Osmosis and Diffusion in the Living Plant. — In the living 



