METABOLISM 



53 



of the CO 2 has disappeared and has been replaced 

 by an equal volume of oxygen. The total volume 

 of the gas has not been altered, but we find that 

 the carbon has disappeared from the tube. And since 

 this change will not- take place in the dark, even 

 if the other conditions be similar, we conclude that 

 the sunlight has 

 supplied the neces- 

 sary energy. What, 

 then, has become 

 of the carbon ? 



Green plants al- 

 ways have a certain 

 amount of food ma- 

 terial stored up in 

 the leaves, usually 



in the form of Starch. FIG. 20. Experiment showing the 



rpi f function of sunlight in the synthesis of 



starch in the green leaf. A slice of cork is 

 Starch may be easily fastened over the leaf and the rest of the 



leaf exposed to the sunlight. The figure 



detected by testing to the right shows the result when the cork 

 with a solution of is removed and the leaf dipped in solution 



of iodine. (Bailey and Coleman.) 



iodine, which colors 



it a bright blue. If we keep such a plant in 

 the dark for a while, it will exhaust this store 

 of starch, as may be shown by the leaves giving a 

 negative test with the iodine. 1 If, however, we 

 pin a strip of cork across part of a leaf from which 

 the starch has been exhausted, and expose the 



1 In such an experiment the chlorophyll must be dissolved out in hot 

 alcohol before the iodine is applied, in order that its green color may not 

 mask the starch reaction. 



