38 PLANT BIOLOGY 



1. In what respects does the preparation of thistle tube 



No. 4 resemble that of thistle tube No. 1 ? How do the 

 two experiments differ ? 



2. Allow the experiment to stand for several hours, and then 



remove with a glass tube a sample of the liquid in 

 bottle No. 4, and test it by adding nitric acid and boil- 

 ing. Is protein present ? How do you know ? 



3. Do you conclude, therefore, that protein will or will not 



pass through a membrane ? 



53. Digestive ferments. We have stated that proto- 

 plasm secretes a substance called diastase, and have shown 

 that this diastase will change insoluble starch to soluble grape 

 sugar, which will pass from one cell to another by the process 

 of osmosis. Diastase is a substance known as a digestive 

 ferment. Now protoplasm produces other digestive ferments, 

 some of which will change insoluble protein to a soluble sub- 

 stance known as peptone. The latter will readily pass by the 

 process of osmosis through a membrane. 



Fats, also, like starch and protein, are insoluble and can- 

 not, therefore, pass by osmosis through cell walls. To make 

 these food substances available for use they must also be 

 changed by the plant cells into such forms that they may be 

 readily transferred from one part of the plant to another. 

 These changes are caused by other chemical ferments pro- 

 duced by protoplasm. 



