The three-dimensional arrangement of intergranal lamellae in chloroplasts.

The intergranal lamellae, or frets, are helically arranged around each granum. All helices within a plastid are co-directional. The helices wind in the same direction from plastid to plastid, from cell to cell, and from species to species, and are right-handed. The continuity of the intergranal membranes in 3-dimensional space is facilitated by the fact that all the helices within a plastid wind in the same direction. Because there are multiple frets around each granum, the fretwork consists of a highly interlocked series of membranes. The incompleteness of the integration of the fretwork suggests that continuity of the membranes from granum to granum is obtained, to a large degree, by fusions of membranes rather than by continuous growth and ramification of a simpler membrane system.