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SUMMARY The sequence of ultrastructural changes which take place during chloroplast development in leaves of plants of Phaseolus vulgaris grown in the light is compared with that of plants grown in the dark. Further comparisons are made with the developmental sequence found in the hypocotyls of Phaseolus and in the leaves of Zea mays. It is concluded that there is a single basic pathway of chloroplast development. Variations in this pathway are related to blocks in the sequence resulting in the accumulation of storage materials such as phytoferritin and prolamellar bodies, characteristic of the species or tissue concerned. An attempt has been made to assess the ubiquity and duration of an association between plastids and the E.R. in lower plant species compared with the angiosperms. A plastid-E.R. association is common and persistent in lower plant species whereas in angiosperms it is apparently transitory and confined to immature or specialized cells.

Summary When seeds of Phaseolus vulgaris are allowed to develop normally in fruits on the mature plant, the plastids in the primary leaves of the embryo within the seed do not develop into mature chloroplasts. However, when the developing seeds are removed from the parent plants, the cotyledons detached, and the embryonic axes grown on agar, primary leaf plastids differentiate into mature chloroplasts within 7 days. In embryos with attached cotyledons, and similarly grown for a week on agar, plastids fail to differentiate. In primary leaves of dormant seeds allowed to germinate on agar, lamellar development begins earlier in seedlings from which the cotyledons have been removed than in those with attached cotyledons, although there is no significant difference between the two groups of seedlings in the time required for the chloroplasts to become fully mature. Plastid differentiation in primary leaves of developing seeds of Phaseolus therefore appears to be inhibited initially by the presence of the cotyledons. This inhibitory effect apparently declines during seed ripening and dormancy.

SUMMARY The sizes and numbers of plastids were determined in the cells of the upper epidermal and upper palisade layers during development of the primary leaf of Phaseolus vulgaris. Plastids of both epidermal and palisade cells divide at all stages of plastid development, but division ceases soon after the plastids become mature. The process of plastid division involves both extension growth and fission. During growth of the primary leaf within a seed developing on the parent plant, plastids in both the epidermal and the palisade cells show little overall growth in size. During germination epidermal cell plastids remain small but plastids in palisade cells increase considerably in size as the leaf expands. Populations of potentially dividing plastids can be identified by the characteristic distribution of their size classes which, when set out graphically, give curves of distinctive shape. Cell division takes place during development of the primary leaf both prior to and following dormancy. Plastid division also occurs during these two periods. However, within each cell the plastids do not divide in synchrony. Nor are the periods of cell and plastid division synchronized, for the plastids continue to divide during the phase of maximum leaf expansion, when cell division has ceased but while cell extension continues. In the primary leaf of Phaseolus the overall number of cycles of plastid division exceeds that of cell division by between two and three cycles.

SUMMARY Differentially thickened wall bars are formed in cells of the anther endothecium as well as in the xylem. In Phaseolus vulgaris both types of wall bar appear to be lignified. As thickening takes place, Golgi bodies become numerous, endoplasmic reticulum proliferates and both types of organelle occupy characteristic sites within the cell. Thickening begins at an early stage of xylem development but in the endothecium it is long delayed. Subsequent loss of cytoplasm is associated with increasing vacuolation of xylem elements, but may be associated with plasmolysis in the endothecium.

Summary During the period of cellular differentiation, plastids in all cells throughout the primary leaf of Phaseolus vulgaris undergo the same sequence of structural changes. However plastids in different types of cell can, at an early stage, be distinguished from each other quantitatively, i.e. in size, in the amount of starch accumulated, and, later, in the extent of their thylakoid systems. These quantitative differences depend in part on the position of the cell within the lamina and in part on the type of cell involved, and they persist throughout development. Further modifications in plastid structure affect only a few specialized types of cell. These later modifications in plastid structure take place only as the cells become fully differentiated. Plastid division occurs in two separate phases, during seed development on the parent plant and during germination. It is probable that differing rates of plastid division and cell division in epidermal, palisade and mesophyll cells during early stages of germination are responsible for the characteristically different numbers of plastids present in each type of cell in the mature leaf.

Summary Plants of Phaseolus vulgaris were grown for a period of 4 weeks in nutrient solutions deficient in S, Mg, K, N, P and Fe, and compared with controls growing in complete nutrient solutions. At weekly intervals, tissue from all leaf groups fully expanded at the time of sampling was prepared for examination under the electron microscope. It was possible to examine chloroplast ultrastructure, firstly, during leaf development under each of the nutrient deficient conditions and, secondly, to compare the chloroplasts of similar leaf groups produced under different deficiency conditions. Although distinctive chloroplast types could be associated with conditions of Mg, K, N and Fe deficiency, chloroplasts from S and P deficient plants were variable in appearance. The chloroplast patterns produced were found to be consistent in palisade parenchyma tissue but in spongy mesophyll tissue cells frequently appeared plasmolysed and chloroplasts varied considerably in appearance. Particularly in the spongy mesophyll distinctive crystalline configurations were found within the chloroplasts.

Summary The ultrastructure of mature guard cells of Phaseolus vulgaris L. grown under continuous light regime showed remarkable stability, remaining constant in appearance under a wide range of conditions of nutrient deficiency. The ultrastructure was quite unlike that of leaf parenchyma cells of normal plants, but had certain characteristics similar to those found in palisade parenchyma cells suffering from magnesium and potassium deficiency.