Electron Microscope Studies on the Morphogenesis of Plastids

To know about the mechanism of interconversion of plastids, electron microscopic observations were made on Cryptomeria leaves which aquire a reddish brown color in winter and recover their green color in coming spring to summer. In normal green leaves, two different kinds of plastids have been observed, viz, the chloroplasts having well organized grana structure in mesophyll cells and those completely lacking grana lamellae in bundle sheath cells. General feature of plastids in reddish brown leaves, may be summarized as follows: (1) The presence of red granules of rhodoxanthin (a carotenoid), and a well developed lamellar system involving grana- and intergrana-lamellae. (2) The plastoglobules, osmiophilic granules in plastids, increase in number and size as compared with the chloroplasts in normal green leaves. (3) Shrinkage which is one of the characteristic features of senescent plastids is not observed. (4) RNA content in plastid stroma is almost unchanged throughout an entire leaf stage ranging from normal green to subsequent regreening. Basic structure of the plastids in regreened leaves is quite similar to that in winter leaves, except for some increase of thylakoid membrane and decrease of osmiophilic granules. Accordingly, it is presumed that the plastids appearing in reddish brown leaves are not mere chromoplasts but those having an incipient nature of the chloroplast, since real chromoplasts can not be converted into the chloroplasts. It seems that “monotrope Plastiden-Metamorphose” is plausible in this case.