OSMOSIS AND DIGESTION 35 



3. Potato sections, like those of all parts of living plants, 

 are composed of a large number of living cells, each one 

 inclosed by a cell membrane (Fig. 9). 

 Call to mind what you learned in 45, and state why the 

 cells become even more flabby in one solution and more 

 rigid in the other. 



47. Will starch pass through a membrane ? Laboratory 

 Study No. 25. 



Thistle tube No. 3. Put into a third thistle tube a mixture of 

 starch and water, cover the bulb with a membrane, and invert in a 

 bottle of water, as already directed for the first thistle tube. See 

 that the level of the liquid is the same in all of the experiments. 



1. In what respects does the preparation of thistle tube 



No 3 resemble that of No. 1? How do the two 

 experiments differ? 



2. At the end of a few hours test the liquid in bottle No. 3 



by removing a sample to a test tube (as already 

 directed in 45), and adding iodine solution. 



a. Is starch present? How do you know? 



b. What is your conclusion as to the possibility of starch 



passing through a membrane? 



3. What have these experiments in osmosis taught you as 



to one difference between starch and grape sugar? 



48. Definitions and applications. The experiments we 

 have been performing have most important relations to the 

 study of all living plants and animals. We may give the 

 following as a definition of the process we are considering: 

 Osmosis is the interchange of liquids of different density that are 

 separated by a plant or an animal membrane, and in this pro- 

 cess the greater flow is always from the less dense to the more 

 dense. 



We shall constantly refer to this principle of osmosis, and 

 we shall find that it explains in large measure the absorption 

 of soil water by roots, the transfer of sap from one part of a 



