NUTRITION 373 



(4) The Products ahd the Process 



The products. — The first product of photosynthesis is not known with 

 entire certainty, and the process, therefore, cannot be described ac- 

 curately. The product of later synthesis which is most general and has 

 been longest known is starch. The fact that it is so generally present 

 and that it is so universally used as evidence of photosynthesis because 

 it can be so easily detected, tend to confirm the common impression that 

 starch is the product of photosynthesis. But there are many plants in 

 which starch is either not formed at all, or appears only under excep- 

 tional conditions, and in no plants is it the exclusive product. Thus, in 

 most fungi no starch is formed when they are fed on carbohydrates; in 

 the kelps fucosan takes its place, and in many monocotyledons, oil; 

 while even in the plants which produce starch abundantly, much of the 

 earlier product is diverted into amides and possibly other nitrogenous 

 compounds. 



In any event starch is a secondary product, and represents the surplus 

 in the manufacture of primary carbohydrates over immediate use, re- 

 moval, transformation into amides, etc. That starch does not appear 

 under certain conditions, in a leaf in which it is usually formed, is no 

 evidence, therefore, that no photosynthesis has occurred, but only that 

 it has not gone on at a rate rapid enough to yield enough excess to appear 

 as starch. 



Amount of product. — A method of estimating the amount of photo- 

 synthesis under various conditions is based upon the relative weight 

 of equal, but necessarily small, areas of leaves, taken at the be- 

 ginning and end of the experimental time, allowances being made 

 for migration ' and use by data from other experiments. The results 

 at best cannot be exact, and the introduction and multiplication of 

 small initial errors make the calculations based on these data quite 

 unreliable.^ 



When accurate data for photosynthesis are needed, the only reliable 

 method is to determine the amount of CO2 used. This requires rather 

 complicated apparatus, skillful manipulation, and accurate gas analysis. 

 This method is obviously independent of the products and their use or 

 migration. 



■ Or this may be rendered impossible by severing the leaf from the plant. 

 " The results obtained by this method are two to three times as large as those in the 

 table on the following page. 



