Developmental changes occurring in isolated intact etioplasts.

When dark-grown plants are illuminated, the characteristic morphology of the etioplast changes. The tubules of the paracrystalline prolamellar body disperse as the membranes reorganize and 'grow out' as 'perforated' thylakoids. Similar changes are expected during the greening of the isolated etioplasts of Avena. Under certain preparative conditions the morphological changes vary from those previously reported for intact tissue. Following low illumination at a low temperature the process of outgrowth is slowed down, resulting in the outgrowth of regularly spaced tubules from the prolamellar body into the stroma. These tubules appear to develop slowly into normal thylakoids by a process of tubule fusion to form perforated thylakoids. A diagram is included to show how this is believed to take place. The outgrowing tubules have an outer diameter of 25—28 nm with a lumen of 7-0-7-6 nm diameter. Two types of protein body are observed, the ' stromacentre' which is a regular feature of Avena plastids and a 'crystallite' which is only observed under certain conditions. The 'stromacentre' is a fibrillar spherulite with fibrils (diameter 9 nm) in bundles up to 200 nm long which in turn are aggregated to form the characteristic spherulite of the stromacentre. The crystallite is composed of fibrils 7-5-8-5 nm in diameter lying parallel to each other with a regular spacing of 125 nm. The crystallite appears to develop when the etioplast is damaged and has started to vesiculate. Particles which are larger than either plastid or cytoplasmic ribosomes have also been observed. These vary in diameter from 18 to 245 nm and are arranged in rows adjacent to the thylakoids.