Light Microscopic Analysis of the Three-Dimensional Structure of Higher Plant Chloroplasts. Position of Starch Grains and Probable Spiral Arrangement of Stroma Lamellae and Grana

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Abstract
Light microscopic observations of grana-containing chloroplasts in living cells of leaves of numerous species of plants (26 spp. including 15 Nicotiana species) including both monocotyledons and dicotyledons, have allowed specification of numerous parameters of chloroplast structure. Chloroplasts are thin and saucer-shaped, with the convex surface facing the cell wall and the concave surface facing the vacuole. The thickness of the grana-containing and starch-containing region of chloroplasts does not exceed 2 .mu.m although the length may reach more than 15 .mu.m in chloroplasts containing 150 grana. The grana do not overlap each other and are in a plane of focus above that of the starch grains. The grana are tilted with respect to each other and are located at varying levels with respect to the convex surface of the chloroplast. In slightly disrupted, isolated chloroplasts, the grana are sometimes arranged in rows and serially connected to each other by a fine thread. In living cells, some chloroplasts exhibit a distinct spiral arrangement of the grana. Using these observations and the dimensions derived from them, a new conception of the 3 dimensional structure of the grana-containing region of the chloroplast has been obtained. In this conception, the grana are uniformly thin, nonoverlapping cylinders, connected in a single series as in a string of beads and wound into a slightly raised spiral (a helix). Starch grains, when present, are located in the concavity of the helix. The length of the string of grana determines the area of the grana-containing region of a chloroplast.

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