Tubular structures in developing plastids of three dicotyledonous species
Tubular structures were observed in the developing plastids of the meristematic regions of spinach, beet, and turnip leaves. These structures were located near the plastid periphery, were frequently in contact with the plastid envelope and (or) the internal plastic membranes, usually had a near-perpendicular orientation with their associated membranes, and were decorated with a distinct striated coating. Based on the high degree of structural similarity, it was suggested that these tubules represent a unique class of plastid inclusions with a common specialized function. A detailed examination of the spinach plastids provided evidence that the tubules are membranous structures and that the tubular lumen is confluent with that of the plastid envelope and also some internal plastidial compartments. It was also shown that the membranes of the tubules differed from the other plastidial membranes in that they were thinner and only lightly stained by osmium – potassium ferrocyanide postfixation. Key words: tubular, structures, developing, plastids, dicots.