Fig. 7.

Tangential sections and tracings of serial sections through a grana stack. a The non-stacked stroma thylakoids are angled with respect to the stacked grana thylakoids and show evidence for a step-like ascent (arrows) corresponding to the frequency of the grana thylakoids. b Three evenly spaced stroma thylakoids marked by arrows appear angled to the plane of the stacked grana thylakoids in both the plane of the section and in the third dimension. c Tracings from three selected sections in a series of ten serial sections of a granum and associated stroma thylakoids from a bean chloroplast. The median section is reproduced in all three images, with the tangential sections from the near side (uppermost image) and far side (lowermost image) of the granum superimposed upon it. Stroma thylakoids in the tangential sections are shown in red. The helical structure of the stroma thylakoids can be inferred within a series of sections by comparing the tilt of the membranes as seen on the near versus the far tangents. Stroma thylakoids that could be followed completely around the grana stack are numbered (1–4). a, b From Paolillo and Falk (1966); c based on Figs. 1–10 in Paolillo and Reighard (1967); a, b Bars 0.1 mm