232 LEAVES 



known, but it appears that the chlorophyll is chiefly con- 

 cerned in the absorption of energy from the sunlight and 

 in the breaking up of the molecules of carbon dioxide, 

 while the protoplasm is more directly concerned in build- 

 ing up molecules of carbohydrates. The protoplasm of 

 the chloroplasts is also continually manufacturing chloro- 

 phyll, for the chlorophyll itself is constantly being 

 broken down by the effects of light. It needs constant 

 renewal. 



Having found that water and carbon dioxide are the 

 only substances used in carbohydrate manufacture, you 

 may be wondering what becomes of the various substances 

 which come up as solutes from the soil. They are used 

 in the manufacture of proteins, which contain several 

 different elements, and of other substances. Thus, for 

 example, the presence of iron is necessary to the manu- 

 facture of chlorophyll. If the roots absorb no molecules 

 of iron, chlorophyll does not appear. 



The first product of photosynthesis which can be readily 

 observed is starch. This is principally because starch is 

 insoluble; it collects in the chloroplasts in the form of 

 starch grains. The molecules of starch are much larger 

 than the molecules of sugar and other soluble carbohydrates ;■ 

 they contain many more atoms. Thus we note that the 

 solubiHty of a substance may depend somewhat on the 

 size of its molecules ; the larger the molecules, the less 

 likely they are to dissolve. Certain carbohydrate mole- 

 cules, much smaller than those of starch, are the first 

 products of photosynthesis, and these smaller molecules are 

 gradually built up into larger and more complex ones. 

 Starch molecules may contain hundreds of atoms, but the 

 proportion of two of hydrogen to one of oxygen is never 



