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, or be ready for use in the cells. Diastase is a 

 substance known as a digestive ferment. Now protoplasm 

 produces other digestive ferments, some of which will change 

 proteins to soluble substances that will readily pass through 

 cell-walls by the process of osmosis, and be in such a condi- 

 tion that it can be used by protoplasm. 



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. 



