Ultrastructural Modifications in Cotyledonous Leaves ofRicinus CommunisL. During Aging

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Abstract
The ultrastructural variations observed in Ricinus communis L. cotyledonous leaves during aging have been studied. Various samples up to the 65th day of age, shortly before the fall of the cotyledon itself, were taken. The ultrastructural variations found in various cellular organelles are described. As already observed by other authors in various plants, the most impressive modifications concerned the chloroplasts. Amongst these of particular note is the presence in the senescent cotyledons of vesicles the membranes of which are clearly continuous with those of the thylakoids. These vesicles, which seem to include portions of matrix, draw near to one another absorbing the contacting membranes and seem to give origin to very elongated vesicular formations. The presence inside the vesicles of starch, plastoglobules and DNA fibers supports the hypothesis that portions of matrix are included in them. The plastoglobules of the last samples have various degrees of electron-opacity. It is suggested that also this fact is related to senescence. DNA fibers were found up to the most senescent stages in chloroplasts and mitochondria. This datum does not seem to be present in the research of other authors, who have concerned themselves with cellular aging from the morphological point of view.