66 



PHYSIO LOG Y. 



The leaves are made transparent by the chloral hydrate so that 

 the starch reaction from the iodine is easily detected. 



The following is a convenient and safe method of extract- 

 ing chlorophyll from leaves. Fill a large pan, preferably a 

 dishpan, half full of hot water. This may be kept hot by a 

 small flame. On the water float an evaporating dish partly 

 filled with alcohol. The leaves should be first immersed in 

 the hot water for several minutes, then placed in the alcohol, 

 which will quickly remove the chlorophyll. Now immerse the 

 leaves in the iodine solution. 



138. Green parts of plants form starch when exposed to 

 light. Thus we find that in the case of all the green plants we 

 have examined, starch is present in the green cells of those which 



Fig. 



Leaf of coleus showing green and white 

 areas, before treatment with iodine. 



Fig. 69. 



Similar leaf treated with iodine, the starch 

 reaction only showing where the leaf 

 was green. 



have been standing for some time in the sunlight where the proc- 

 ess of the absorption of CO 2 and the giving off of oxygen can 

 go on, and that in the case of plants grown in the dark, or in 



