The occurrence of RNA in plastids from etiolated and green maize leaves was demonstrated cytochemically, with both the light and the electron microscope. Etiolated leaves were allowed to incorporate tritiated cytidine for several hours and were subsequently fixed in formalin. Radioautographs of leaf sections 2 µ thick showed silver grains over the regions of the cytoplasm containing plastids. Plastids in these sections appeared intensely basophilic when stained with azure B. Both the basophilia and radioactivity were removable with ribonuclease, clearly demonstrating the occurrence of RNA in these organelles. Examination under the electron microscope of similar plastids which had been fixed in formalin revealed a particulate component in the plastid measuring approximately 170 A in diameter. This particulate component was completely removable with ribonuclease. Thus,it was concluded that RNA occurs in a particulate form in plastids from etiolated leaves. Mature plastids, when stained with azure B, did not appear basophilic under the light microscope. Nevertheless, when formalin-fixed tissues were examined with the electron microscope, the mature plastids were seen to contain particles in the stroma, identical in appearance with those visible in the plastids in etiolated leaves. Osmium tetroxide-fixed tissues were also examined with the electron microscope. Particles similar to those seen in plastids fixed with formalin were observed, although the results obtained with this fixative were variable. It is concluded that plastids from etiolated and green maize leaves contain RNA in a particulate form which resembles ribosomes.

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