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SUMMARYThe sequence of ultrastructural changes which take place during chloroplast development in leaves of plants of P/ia^eo/u^ vw/^arw 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.

SUMMARYChanges in plastid ultrastructure in the primary leaves of Phaseolus vulgaris were followed during the period of seed development and, following a period of dormancy, throughout the life span of the leaf from germination to senescence. The total life span of the leaf was about 14 weeks. During this time the plastid remained as a proplastid for about 7 weeks. Following germination, synchronous developmental changes associated with lamellar production, which lasted for a period of i week, resulted in the change of the proplastid into a mature chloroplast. This mature chloroplast was characteristic of the subsequent period of 2 weeks, then changes in plastid shape and in the lamellar alignment led eventually to senescence some 3-4 weeks later. The period of rapid synchronous development during the week following germination was observed in detail and a comparison made with plastid development in plants grown in continuous darkness. Three transitory stages were consistently observed between other more persistent and generally recognized plastid developmental stages. The first occurs when the E.R. partially surrounds the proplastid during early germination; the second when the proplastid assumes an amoeboid configuration which is postulated as a precursor to various divergent plastid types; the third occurs between the developing plastid with perforated stroma lamellae and incipient grana and the mature chloroplast with continuous aligned lamellae and stacked grana.

SUMMARYIt is suggested that developmental changes in plastid structure follow a cyclic pathway which may involve the dedifferentiation of mature chloroplasts to proplastids and the subsequent redifferentiation of proplastids to mature chloroplasts. The proposal of a cyclic plastid relationship revives the earlier concept of Schimper. This has generally been discarded in favour of a unidirectional concept which discounts dedifferentiation as a normal process. Justification of the cyclic concept depends largely on the verification that plastid dedifferentiation and redifferentiation do indeed take place. Evidence for this comes not only from the literature, but also from personal observations.

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