Structure of amyloplasts and endoplasmic reticulum in the root caps of Lepidium sativum and Zea mays observed after selective membrane staining and by high-voltage electron microscopy.

The structure of plastids in the root cap of cress and maize was studied by low- and high-voltage electron microscopy after staining their membranes with a mixture of zinc iodide and osmium tetroxide. In plastids of both species electron-opaque membranes were found in the plastid interior while membranes of lesser electron-opacity comprised the outer envelope and vesicles and cisternae underlying it. Electron-opaque tubules, often in groups attached to the inner membrane of the amyloplast envelope, were found in cress but not in maize. The internal, less-opaque membranes were often found associated with the starch grains. No specific association could be seen between amyloplasts and endoplasmic reticulum (ER); their surfaces showed no regular contact or connexion, though the amyloplasts clearly indented the underlying ER. The ER in statocytes was predominantly tubular in cress but predominantly cisternal in maize.