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Division of the guard mother cell (GMC) in Allium cotyledons has been examined in epidermal slices viewed with Nomarski optics and electron microscopy. Special attention has been directed towards elucidating the process by which the dividing cell determines its plane of division. In normal development, the cell plate formed during GMC division ultimately lies along the longitudinal axis of the cotyledon, in contrast to the transverse planes formed in other epidermal divisions. Our observations reveal that the final plane of division is not determined by the orientation of the spindle at metaphase but instead is established during late anaphase-telophase as a result of directed reorientation movements of the spindle-phragmoplast and associated daughter nuclei. The metaphase plate may lie at an oblique angle, even as great as 90°, from the final plane of the plate. Thus, daughter chromosomes separate into opposite corners of the cell. During late anaphase-telophase, movement of the spindle is activated; the daughter nuclei move along the sides of the cell while the interzone rotates. Movement continues until daughter nuclei reach positions opposite each other along the sides of the cell and the midzone or cell plate is positioned in the longitudinal orientation. Movement requires 15–20 minutes for completion, is highly directional, and does not overshoot the correct alignment. Following movement cytokinesis proceeds to completion forming two young guard cells. Possible mechanisms for reorientation are discussed, including one that suggests that interzone microtubules may interact with a cortical site on the plasmalemma adjacent to the end and paradermal walls. Such a site may be related to and governed by the same properties which controlled the prior formation of the preprophase band of microtubules in these cells.

Similar changes occurred in the ultrastructure of host cell organelles in the rust-infected tissues of both flax and sunflower. Up to the onset of sporulation at 8 days after inoculation the only major alteration in cell structure was a marked tendency for the cell organelles to be aggregated around the intracellular fungal haustorium and an accumulation of starch in the plastids. During sporulation, however, some striking changes occurred in the structure of plastids, mitochondria, and microbodies. Eleven days after inoculation, in a zone of host tissue adjacent to the rust pustule, the plastids no longer contained starch and varied in structure even within a single cell. Some were similar to normal chloroplasts, others had larger grana, and some bore a close resemblance to chromoplasts. In sunflower, but not in flax, mitochondria contained atypical plate-like cristae in addition to the usual vesiculate type. The micro-bodies of both plant species all contained crystalline cores in contrast to earlier stages in the disease where crystals were only occasionally detected. The plastids were also altered considerably in another zone of tissue situated beneath the rust pustule. In some plastids the thylakoids extended along their long axis, forming several large grana, and in others the large grana were interconnected by lamellae arranged in a vesicular or tubular configuration. Frequently the starch grains in these plastids were broken down into small rosettes of darkly staining particles with an appearance similar to that of animal glycogen.

The parasitic phase of the uredial stage of rust fungus infection is characterized by the formation of a specialized structure—the haustorium. The haustorial neck contains a band of darkly staining wall material, the length of which is dependent on the fungal species. In older infected tissues a collar sometimes develops at the site of penetration. In such cells, the host plasmalemma appears to double back on itself between the collar and the haustorial neck. The mitochondria of the haustorium contain many plate-like cristae and, less frequently, concentric whorls of membranes. Cytoplasmic microtubules are also observed in both the neck and body of the haustorium. Centriolar plaques are found appressed to the nuclear envelopes in both the hyphae and haustoria. They frequently consist of two amorphous regions which are connected by a rod-shaped, layered structure. The septal pore of the intercellular hyphae contains a plug of electron-transparent material. Surrounding it is a matrix of dense amorphous cytoplasm devoid of organelles which is bordered by a hemispherical array of microbodies containing crystals. Axenic cultures of Melampsora lini mycelium were examined and found to contain similar septal pore regions. These cultures also show some differences from the parasitic hyphae. The septa-associated microbodies in these cells appear to derive from an aggregate of tubular endoplasmic reticulum cisternae. Their mitochondria differ from those of the intercellular hyphae in having more and longer, plate-like cristae and a denser staining matrix. They therefore represent a type intermediate between those of the intercellular hyphae and the haustoria. The pseudoparenchymatous stroma of the axenic cultures contains cells of widely divergent ages. Some are young as determined by their dense cytoplasmic contents and large lobed nuclei. However, others are older, with large vacuoles and many lipid bodies. In some cases they have thick cell walls consisting of many layers.

The ultrastructure of the primary sieve elements of several papilionaceous legumes was studied using hypocotyl and young internode segments fixed in glutaraldehyde followed by osmium tetroxide. In particular, the study sought to determine whether the crystalline “flagellar inclusions” characteristic of these species are developmentally related to the P-protein bodies present in the phloem of these and other legumes and of angiosperms generally. The crystalline inclusions consist of a central body terminated at one or both ends by a gradually tapering tail. The central body is usually spindle shaped in longitudinal section and square in cross section. In all species examined, the inclusion is first seen as a small, thin crystal in the cytoplasm of young sieve elements. The crystal enlarges and acquires tails as the sieve element develops. In certain species, exemplified byDesmodium canadense, numerous tubules are formed in the cytoplasm near the crystal and appear to be concerned in its growth. The observations on the structure and interactions of these two components, tubules and crystalline inclusions, suggest that both represent forms of P-protein: the tubules are continuous with the crystal and are striated like the crystal near the tubule-crystal junction, suggesting that they are adding onto the crystal body; the tubules closely resemble the P-protein tubules described in the literature in that they measure 157 A in diameter, accumulate in spindle-shaped bundles, and disperse into striated fibrils late in the ontogeny of the sieve element; and finally, the crystal also disperses into fine filaments. The crystalline inclusion therefore probably represents still another aggregation state of P-protein, one that is characteristic of papilionaceous legumes. The different stages of crystal aggregation and the diverse forms of P-protein now known are discussed briefly in relation to the control of macromolecular assembly and subunit packing.

The fine structure of plastids and their starch deposits in differentiating sieve elements was studied in bean (Phaseolus vulgaris L.). Ultrastructural cytochemistry employing two carbohydrases specific for different linkages was then used to compare the chemical nature of "sieve tube starch" (the starch deposited in sieve elements) with that of the ordinary starch of other cell types. Hypocotyl tissue from seedlings was fixed in glutaraldehyde, postfixed in osmium tetroxide, and embedded in Epon-Araldite. Treatment of thin sections on uncoated copper grids with alpha-amylase or diastase at pH 6.8 to cleave alpha-(1 --4) bonds resulted in digestion of ordinary starch grains but not sieve element grains, as determined by electron microscopy. Since alpha-(1 --6) branch points in amylopectin-type starches make the adjacent alpha-(1 --4) linkages somewhat resistant to hydrolysis by alpha-amylase, other sections mounted on bare copper or gold grids were treated with pullulanase (a bacterial alpha-[1 --6] glucosidase) prior to digestion with diastase. Pullulanase did not digest sieve element starch, but rendered the starch digestible subsequently by alpha-amylase. Diastase followed by pullulanase did not result in digestion. The results provide evidence that sieve element starch is composed of highly branched molecules with numerous alpha-(1 --6) linkages.

Bundles of microfilaments very similar in appearance to actin are present in cytoplasmic suspensions obtained from Nitella flexilis . The microfilaments bind rabbit heavy meromyosin in arrowhead arrays similar to those produced on muscle actin. The arrowheads are removed with ATP. The results provide evidence that actin is present in green plants, probably in the form of microfilaments thought to be involved in cytoplasmic streaming.