NUTRITION 373 
(4) THE PRODUCTS AND 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 1 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. 2 
When accurate data for photosynthesis are needed, the only reliable 
method is to determine the amount of CO 2 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. 
1 Or this may be rendered impossible by severing the leaf from the plant. 
2 The results obtained by this method are two to three times as large as those in the 
table on the following page. 
