Experi-ments on photosynthesis in C14O2 with Lemna gibba and Lemna minor showed that apiose occurs only in the fraction insoluble in alcohol and water and may be released by hydrolysis. The portion of radioactivity in apiose increases in the course of photosynthesis to about the same extent as that in xylose. This differs markedly from starch (glucose) in which the portion of radioactivity decreases during photosynthesis. Darkening, following a longer period of photosynthesis in C14O2, leads to a change in the ratio apiose-C14/glucose-Cl4 and xylose-C14/glu-cose-Cl^ respectively, in favour of apiose and xylose. The same is obtained if labeled glucose is fed for a longer period. It is concluded that apiose is not part of storage material but like xylose part of a cell wall component.