CARBON FOOD OF PLANTS. 6l 



of the chemical change which takes place in the green cells 

 under these conditions. Because C0 2 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 is 

 changed from its attraction in the molecule of carbon dioxide to 

 its attraction in the molecule of starch, the process may be 

 termed carbon conversion. 



This process has been termed carbon assimilation, but since it is not truly 

 an assimilatory process, and because sunlight is necessary in the first step 

 of the conversion, it has also been recently termed photosyntax, or photo- 

 synthesis. These terms, however, seem inappropriate, since the synthetic 

 part of the process is not known to be due to the action of light. In the 

 presence of chlorophyll light reduces the carbon dioxide, while the synthetic 

 part of the process may not be influenced by light. Since the process is 

 similar to that which chemists call conversion, and since the carbon is the 

 important food element derived from the air, for popular treatment the term 

 carbon conversion seems more appropriate. 



148. 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. 51 is 

 from a photograph of such a leaf. We should first boil it 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. 52, which is from a photograph of another coleus 

 leaf treated with the iodine solution. 



149. Translocation of starch. — It has been found that leaves of green 

 plants grown in the sunlight contain starch when examined after being in the 

 sunlight for several hours. But when the plants are left in the dark for a 

 day or two the leaves contain no starch, or a much smaller amount. This sug- 

 gests that starch after it has been formed may be transferred from the leaves, 

 or from those areas of the leaves where it has been formed. 



