Loading of assimilates in wheat leaves. II. The path from chloroplast to vein

An improved method is described for micro‐autoradiography of water‐soluble substances alter freeze‐substitution of plant tissue in which water is rigorously excluded. Resin sections are cut and flattened dry, and dry photographic emulsion is mounted on them. When the location of 14 C in wheat leaves after assimilation of 14 CO 2 was studied with this method, it was found that 14 C entered the intermediate veins before the laterals and entered both types of veins along the flanks of the veins adjacent to the phloem. High concentrations of 14 C were found in small spaces in the cells of mesophyll and vein parenchyma; these spaces coincide with the nuclei. The concentration of 14 C in these nucleus‐associated spaces was as high as that reached at later times in the sieve tubes. Water washed the 14 C out of these spaces of the sections and the label in the washings was predominantly in sucrose. The high 14 C concentrations of the nucleus‐associated spaces were particularly easily leached. It is concluded that the raising of the sugar concentration to the high levels found in sieve tubes can take place in these leaves in a special space in each mesophyll cell not, or not only, at the boundary of the sieve tubes.