HOW PLANTS GET THEIR CARBON FOOD. 75 



of the leaf, while the white part of the leaf is still uncolored. 

 This is well shown in fig. 59, which is from a photograph of 

 another coleus leaf treated with 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 



Fig. 58- Fig. 59. 



Leaf of coleus showing green and white Similar leaf treated with iodine, the starch re- 

 areas, before treatment with iodine. action only showing where the leaf was green. 



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

 which have been standing for some time in the sunlight where 

 the process of the absorption of CO., and the giving off of oxygen 

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

 or in leaves of plants which have stood for some time in the 

 dark, starch is absent. We reason from this that starch is the 

 product of the chemical change which takes place in the green 

 cells under these conditions. Because CO a is absorbed during 

 this process, and because of the chemical changes which take 

 place in the formation of starch, by means of which the carbon 



