PARTICLE ARRANGEMENTS IN PROPLASTIDS OF TRITICUM VULGARE L. SEEDLINGS

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Abstract
Particles having ribosome-like characteristics are described in proplastids of dark-grown wheat seedlings as the membranes of the prolamellar body become transformed, under the influence of light, into grana and fret membranes. Three arrangements of particles were noted: a random distribution of discrete particles; particles occurring in helices or parallel rows; and particles arranged in rough squares with 6-8 particles per side. It is possible that the 3rd type of particle is a cross-section of long parallel rods. A particle ranges in size from 170 to 220 A, those of group 3 being somewhat smaller. The particulates vary from diamond shaped with smooth surfaces to circular with irregular surfaces. These particles have the characteristics of ribosomes as visualized by the electron microscope: they are preserved by glutaraldehyde and osmium tetroxide, they stain intensely with uranyl acetate, and are digested by RNase. Their properties do not coincide with those of viruses, smog-induced particles, stromacenter particles, or phytoferritin. They are frequently adjacent to membranes but never attached to membranes. The involvement of ribosomes in membrane development is discussed.