STARCH AND SUGAR: CHLOROPHYLL. 67 



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. The CO 2 which is absorbed by the 

 plant mixes with the water (H 2 O) in the cell and immediately 

 forms carbonic acid. The chlorophyll in the leaf absorbs radi- 

 ant energy from the sun which splits up the carbonic acid, and 

 its elements then are put together into a more complex com- 

 pound, starch. This process of putting together the elements 

 of an organic compound is a synthesis, or a synthetic assimila- 

 tion, since it is done by the living plant. It is therefore a syn- 

 thetic assimilation of carbon dioxide. Since the sunlight sup- 

 plies the energy it is also called photosynthesis, or photosynthetic 

 assimilation. We can also say carbon dioxide assimilation, or 

 CO 2 assimilation (see paragraph on assimilation at close of 

 Chapter 10). 



139. Starch is formed only in the green parts of variegated 

 leaves. If we test for starch in variegated leaves like the leaf of 

 a coleus plant, we shall have an interesting demonstration of the 

 fact that the green parts of plants only form starch. We may 

 take a leaf which is partly green and partly white, from a plant 

 which has been standing for some time in bright light. Fig. 68 

 is from a photograph of such a leaf. We should first boil jt in 

 alcohol to remove the green color. Now immerse it in the 

 potassium iodide of iodine solution for a short time The parts 

 which were formerly green are now dark blue or nearly black, 

 showing the presence of starch in those portions of the leaf, 

 while the white part of the leaf is still uncolored. This is well 

 shown in fig. 69, which is from a photograph of another coleus 

 leaf treated with the iodine solution. 



3. Chlorophyll and the Formation of Starch. 



140. In our experiments thus far in treating of the absorption 

 of carbon dioxide and the evolution of oxygen, with the accom- 

 panying formation of starch, we have used green plants. 



