1. Starch, containing 7.05 atom per cent C13 excess has been produced in the mesophyll cells of bean leaves, starting with C13O2 containing 7.26 atom per cent C13 excess. Approximately 67 per cent of the C13 taken up by the leaves was determined in the starch fraction.

2. The soluble carbohydrate, containing 6.72 atom per cent C13 excess, accounts for approximately 23 per cent of the C13 taken up by the leaves. The remainder was principally in the coarse tissue fragments fraction (9.73 per cent of the C13 utilized).

3. The apparatus and procedures used in this experiment are described.

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