Healthy chloroplasts of Hordeum vulgare are compared with chloroplasts subjected to abnormal stresses such as in situ disruption, isolation, isolation plus washing in 0.5 m sucrose, and isolation plus washing in 0.5 m sucrose and distilled H 2 O. Normal chloroplasts resemble those of Nicotiana rustica and Phaseolus vulgaris in being composed of compartmented grana connected by an anastomosing fretwork system. They differ in having a somewhat greater incidence of parallel frets and double partitions. Under conditions of stress both grana and fretwork undergo varying degrees of swelling, and the double partition maintains its structural integrity. Grana are more resistant to abnormal stresses than the fretwork. Fret connections with more than 3 grana do not generally occur, but in some micrographs a single pathway may be traced through several grana. Washing isolated chloroplasts in distilled water results in an enlargement involving compartments of 2 or more grana together with the associated fretwork membranes. These results indicate that the grana in mature chloroplasts of Hordeum vulgare, like those of Nicotiana rustica and Phaseolus vulgaris, are compartmented structural units and not a series of localized aligned thickenings in regular extensive discs. These enlargements are complex structures comprising the membranes and spaces of both grana and frets. The swelling indicates an increase of locular and fret channel substance and possibly an enlargement of membrane surfaces. Dried down on grids, the compartments and frets appear as flat discs with radial appendages.

The partitions and frets of chloroplasts of Aspidistra, Phaseolus vulgaris, and Pisum sativum may be resolved after successful fixation (KMnO4 glutaraldehyde followed by Dalton's or OsO4, acrolein followed by Dalton's or OsO4, and by OsO4 alone) into a series of subunits which are interpreted as globular proteins with associated lipids. The fret membrane, about 75 Å wide consists of a single row of subunits. Two rows of subunits form the partitions, which are 135 Å wide. They are less than twice the width of the two membranes that form them. The subunits consist of a light core with an average diameter of 37 Å surrounded by dark rims about 28 Å wide. The arrangement of the subunits in the partitions gives the partitions a superficial appearance of being five-layered membranes formed of three dark components about 40 Å wide separated by light leaflets at 19 Å in width. The central dark component (40 Å) because of its relationship with the outer components of the fret and margin membranes should be twice the width of the two outer dark components, or 80 Å. The structure of the plastid membrane thus resolved by electron microscopy is compared with its structure as derived from studies with polarized light, X-rays, chemical information, and electron microscopic studies of whole membranes. The concept of membranes formed of spherical subunits is considered in the light of the unit membrane as it relates to the permeability studies of Danielli and the myelin sheath of nerve cells.

Further evidence from shadow casting techniques of the empty cells of Scenedesmus quadricauda confirms the structure of the pectic layer presented in an earlier work which was based entirely on reconstruction from thin sections. The structure of the net and props and their relationship are clearly seen and the inner boundary of the pectic layer is demonstrated by staining techniques.