Phytoferritin in plastids of the cambial zone of willow.

"Crystalline" and "paracrystalline" arrays of electron-opaque granules have been found in plastids of the cambial zone and its immediate derivatives in crack willow (Salix fragilis L.). These granules have a diameter of 55 to 70 Å and, when in crystalline arrangement, show a centre to centre spacing of 100 Å with adjacent, slightly curved, or linear rows running parallel. The 70 Å particles have a substructure of four to six subunits 15 Å in diameter. These units are arranged around an electron-translucent core 20 Å diameter. It is suggested that this complex is phytoferritin. It is assumed that the electron-translucent area around the opaque granules represents the proteinaceous shell characteristic of both plant and animal ferritin as described by other authors. The phytoferritin is commonly found spread in a thin, regular, array over the surface of plastoglobuli in the plastids.It is further suggested that the phytoferritin is an iron-protein complex which allows the plant to store iron in non-toxic form. This theory would be in accord with the presence of phytoferritin in plastids which appear to be morphologically mature but which, on account of their position within the stem, would not be expected to be photosynthetically very active.