Electron microscopic examination of developing chloroplasts of sesame indicates that plastids in the shoot apex and leaf buds contain a membrane-bound inclusion with electron-dense, homogenous contents. The incipient grana of these young plastids have enlarged loculi filled with an electron-dense material, and the membranes exhibit a ‘negative’ image. The bounding membrane of the inclusion is frequently observed in intimate contact with, and possibly contiguous with, the grana-fretwork system of the developing chloroplast. As development continues, many of the inclusions are observed to contain partly or wholly crystalline contents, the locular content of electron-dense material decreases, and the ‘negative’ membrane image disappears. With further maturation, the inclusions become smaller, and eventually disappear.
It is suggested that, in accordance with other studies, the membrane-bound inclusion of developing sesame leaf chloroplasts is a storage centre for thylakoid precursor materials. The ‘negative’ image observed in the incipient grana may also be a reflection of incomplete assembly of the membranes associated with the accumulation of material within the loculus.