Histological distribution of orange G-specific cytoplasmic globules in Ipomoea purpurea

Orange G-specific, spherical bodies, varying widely in size with some being relatively large, i.e., larger than the nuclei, and descending to the limit of resolution of the light microscope, were found in paraffin sections of shoot apices of Ipomoea purpurea Roth (Linn.) 'Heavenly Blue' when stained with safranin O followed by aniline blue-orange G. These globules only appeared when fixed in a noncoagulant, pH neutral fixative such as 5 percent glutaraldehyde in either a calcium acetate or a phosphate buffer. Visualization of the globules was improved in tissue treated with osmium tetroxide following primary fixation in gluteraldehyde and embedded in epoxy plastic. The globules appeared dense black when the osmium-treated tissue sections were stained with toluidine blue. In paraffin sections the globules were found only in dense files of cells of unknown function located in well-differentiated mesophyll and cortex tissues. In plastic they were located in the exterior layer of the pith just interior to the vascular cylinder as well as in the dense files of cells as in paraffin sections. Single giant cells throughout the mesophyll of differentiating leaves, not associated with the orange G colored globules, were also found. The cellular location of the globules was in the vacuoles. Although the globules reacted strongly with acidic buffered orange G and with osmium tetroxide, they gave negative results to various histochemical tests for protein, lipids, nucleic acids, insoluble carbohydrates and tannin.