ADJUSTMENT TO WATER 45 



Imbibition is a factor in absorption, since it affects both the 

 cellulose wall of the root-hair, and the lining membrane of pro- 

 toplasm. It not only carries the water into the plant by filling 

 wall and protoplasm Aivith it, but it also renders both wall and 

 protoplasm more porous by reason of the swelling due to the 

 separation of the molecules. For this reason it is a most important 

 aid to the process of osmosis, which controls the diffusion of liquids 

 through plant membranas. Imbibition takes place more rapidly 

 at high than at low temperatures. It is increased by the action 

 of acids and alkalies, provided they are not too strong. It is 

 greater in distilled water than in water containing salts in solu- 

 tion. The powerful force exerterl in imbibition is made evident 

 by the lifting power of swelling seeds or wood, as well as by the 

 fact that heat is given off. This evolution of heat is apparently 

 due to the movement of the molecules and also to the conden- 

 sation of the water imbibed. The turning and twisting of awns 

 when moistened, as in Stipa, is likewise an evidence of the energy 

 of imbibition. 



Experiment 8, Water of imbibition. Put 25 grams of dry peas and 

 an equal amount of distilled water in a small cylindrical jar with straight 

 walls. Cover the peas with a close-fitting cork float, through the center 

 ()f which is placed a thermometer. Weight the cork down with several 

 UX)-gram weights, at the same time marking its position. Note the 

 rise of the float as the peas swell, and also compare the temperature 

 with that of the surrounding air. Determine the volume of a swollen 

 and a dry pea by means of calipers, and compute the percent of increase 

 due to imbibition. 



Cut small pieces of hard wood and of soft wood to the same size. 

 Measure carefully, soak in distilled water, measure again, and compare 

 the amount of swelling in the two, 



58. Osmosis. Two solutions of unequal density when sepa- 

 rated by a porous membrane will gradually pass through the lat- 

 ter, and mix with each other. This purely physical process of 

 diffusion is termed osmosis. It depends primarily upon the dif- 

 fering density of solutions able to wet the limiting membrane. 

 r)smosi8 consists essentially of two currents which set in oppo- 

 site directions. The first current set up is toward the denser 

 or stronger solution, but this is regularly compensated by a second 

 current passing from the denser to the weaker solution. The 

 difference in density between the two solutions determines the 

 strength of the first current, as well as the length of interval that 



