PROBLEM I. The Part Leaves Play in Maki?ig Food i^j 



white? Explain. (Note: You use the starch test because it is easy to test 

 for starch in a leaf. The plant makes sugar first and later changes it to 

 starch). 



5. Demonstration by the teacher. The release of oxygen by a green 

 plant can be demonstrated if advantage is taken of the fuming of white 

 phosphorus (DANGER) in the presence of oxygen. (The reaction forms 

 phosphorus pentoxide.) A piece of white phosphorus is fastened to a 

 cork which is then used to close one end of a large tube about one inch 

 in diameter. Water is poured into the tube, leaving an air space of three 

 or four inches. Elodea plants are introduced and the tube then closed 

 bv means of a second cork at the other end. When the tube is inverted 

 so that the phosphorus is in the air space, fuming occurs. After a time 

 all the oxygen will have been removed from the air. The glass tube is 

 then turned so that the phosphorus is in water. After having been kept in 

 the bright sunlight the tube is inverted again. Fuming occurs a second 

 time showing that the elodea plant must have released oxygen. The dem- 

 onstration may be repeated several times. 



6. Read again the account of Van Helmont's experiment with the 

 willow twig, page 137. Van Helmont concluded that the plant made its 

 substance from water. This was a reasonable conclusion considering the 

 knowledge available to him. List the facts you know that make Van Hel- 

 mont's conclusion unacceptable today. 



Further Activities in Biology 



1. What is the effect of a constant electric light upon the leaves of a 

 plant? Start some bean seedlings in moist sawdust, then transplant to 

 soil. Use plants of the same size. When the cotyledons have shriveled 

 place half of the plants under a strong electric light at night. Keep all 

 of the plants in the light (sunlight, if possible) during the day. Compare 

 the leaves in the two groups of plants. Has the electric light made any 

 measurable difference? Try the starch test on leaves from each of the 

 two groups of plants. What do you find? Explain. 



2. Can you detect any difference between the gases that diffuse from a 

 plant not engaged in photosynthesis and one in which photosynthesis is 

 going on? Devise an experiment to demonstrate this. 



3. Devise an experiment to show what effect darkness has on chloro- 

 phyll formation in growing plants. 



