The grana of chloroplasts of starch-free leaves of Nicotiana rustica are described in detail. Leaf sections were fixed in 2.5 per cent KMnO4 and embedded in mixtures of butyl and ethyl methacrylate. Chain length of the polymer was modified by use of a transfer agent. The grana are composed of compartments consisting of electron-scattering partitions and electron-transparent loculi. Compartments are not open to the stroma so that the grana are distinct subplastid organelles. Adjacent grana are connected by an anastomosing fretwork system composed of flexuous channels bordered by electron-scattering membranes. Ten different kinds of granum margins are described. These marginal variations depend upon grana-fretwork connections and internal marginal connections between adjacent loculi. A study of serial sections suggests, at least in some plastids, the occurrence of a possible orderly spiral arrangement of compartment-fretwork connections. Adjacent grana may have common compartments. Grana may branch. Variations in shape may depend upon the angle the section bears to the axis of the cylinder. This should also influence the relative thickness and sharpness of the partitions. Since all shapes and variations in partition thickness and sharpness cannot be accounted for on the basis of the orientation of the grana, such variations probably occur naturally. Grana vary in size, ranging from those few which have a single partition to those having 50 or more compartments which extend completely through the width of a plastid. Relationships between grana of different sizes and between compartments and frets indicate the possibility of growth of grana from union or extension of compartments and formation of compartments from the union of frets.

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