3. The vascular bundles of corn leaves are surrounded by a single layer of bundle sheath cells possessing plastids differing in size and shape from the chloroplasts of the mesophyll cells. The plastids of the mesophyll cells contain no starch; the sugars they produce are moved into the bundle sheath cells and there transformed to starch. Starch increasingly accumulates in the bundle sheath plastids in the day; during the night the starch is changed to soluble carbohydrates and translocation occurs. The plastids of the bundle sheath cells are usually devoid of starch by morning. These plastids contain a green pigment, presumably chlorophyll, but are of a lighter green color than are the chloroplasts of the mesophyll. Photosynthesis may occur in the bundle sheath plastids. However, the green color of the bundle sheath plastid is similar to that of the guard cells of the stomata. Sayre found that the guard cells of Rumex contained a light green pigment which was not chlorophyll. In view of the above facts it will be of interest to ascertain whether or not the green pigment in the bundle sheath plastids is chlorophyll.
Each of the bundle sheath plastids contains numerous, discrete regions, which may be likened to pyrenoids, in which the starch is deposited. It is surprising that the structure and functions of these unusual plastids have not been adequately described. Kiesselbach (1916 and cited in Weatherwax 1923) noted their abnormal size and shape but did not mention their function in starch synthesis. He believed these plastids had different shapes in fixed from those in living material. We have observed, however, the same variation in size and shape in both fixed and living cells.
M. M. Rhoades and Alcides Carvalho