METABOLISM 



53 



of the CO2 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. 

 The presence of 

 starch may be easily 

 detected by testing 

 with a solution of 

 iodine, which colors 

 it a bright blue. 



FIG. 20. Experiment showing the 

 function of sunlight in the synthesis of 

 starch in the green leaf. A slice of cork is 

 fastened over the leaf and the rest of the 

 leaf exposed to the sunlight. The figure 

 to the right shows the result when the cork 

 is removed and the leaf dipped in solution 

 of iodine. (Bailey and Coleman.) 



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. 



