PROBLEM 71 87 



Observations. After several days notice the position of the 

 blades of the leaves. Notice also the leaf stalks. 



Conclusion. What is the effect of light on leaves and stems? 



NOTE. Evidently sunlight has something to do with the life of a green plant ; 

 for a plant growing in complete darkness is yellow or bleached (for example, sprout- 

 ing potatoes kept in darkness). Let us see if we can find out by experiment just 

 what is the relation between light and green leaves. 



Problem 7O : To determine the relation of light to the pres- 

 ence of starch in a green leaf. 



Materials. Green plant, black alpaca, alcohol, iodine. 



Method. Place any small green plant in a dark room for 

 24 hours. Then cover parts of several different leaves with 

 strips of black alpaca. Expose to direct sunlight for an hour or 

 more. Pick off the leaves partly covered with the black cloth, 

 take off the cloth, and place the leaves in hot methyl alcohol. 

 Next wash the leaves and place them in a solution of iodine. 



Observations. What happens to the leaves after placing them 

 in the alcohol? What happens to the leaves placed in iodine 

 solution ? 



Conclusion. 1. Why do we place the plant in the dark at the 

 beginning of this experiment? 



2. What effect does sunlight have upon green leaves? How 

 do you know? 



3. What effect does absence of light have? 



NOTE. Evidently a green leaf under certain conditions (light is one) manufac- 

 tures starch. Let us find out another. 



Problem, 71 : Is a part of the air a factor in starch making in 



leaves ? 



Materials. Green potted plant, vaseline, iodine, alcohol. 



Method. Treat the plant as in the last problem. After 

 removing the plant from the dark room, vaseline both sides of two 

 or three leaves. Place the plant in the direct sunlight for an hour 

 or two, then pick off the vaselined leaves and some others, marking 

 them so that you may know them. Place in hot methyl alcohol. 

 After the chlorophyll is removed, test both vaselined and un- 

 vaselined leaves for starch. 



