Abscisic acid maintains embryos in a state of dormancy and inhibits the incorporation of H(3)uridine and H(3)thymidine but not the incorporation of H(3)leucine. Ribosomes present in imbibed but dormant embryos do not become associated into polysomes until actual germination of the embryos. Protein synthesis still occurs in embryos when RNA synthesis is inhibited and therefore stable m-RNA must be present in dormant embryos. It is concluded that abscisic acid maintains dormancy by inhibiting the production of specific types of m-RNA, and therefore the formation of specific proteins. The activity of abscisic acid is antagonistic to the effect of gibberellic acid in dormancy.

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MATERIALS AND METHODS"Seeds" (fruits) of Lactuca sativa L. vars. Arctic King and Grand Rapids were obtained from Suttons Seeds Ltd., Reading, U. K. Batches of these seeds were placed in closed containers with separate vessels of sulfuric acid, water, or diluted sulphuric acid giving relative humidities of 0, 25, 50, 75, and 100% at 20 C in the container atmospheres (9). After 2 weeks, the moisture contents of the seed batches were determined, and the seeds were sealed into separate glass bottles. Other large batches of seeds from the same provenances were imbibed on moist filter papers in Petri dishes. These Petri dishes, together with the bottles of air-dry seeds, were placed in dark incubators maintained continuously at 30 C (Arctic King), and 25 C (Grand Rapids), in order to prevent germination of the imbibed batches of seeds.Fruits of Fraxinus americana L. were purchased from Vilmorin-Andrieux of Paris. Samples were brought to a range of moisture contents as described for the lettuce seeds. They were then sealed into bottles, and stored in the dark at 22 C. A large sample of fruits was imbibed by mixing with moist sand and stored at 22 C in the dark in order to prevent germination of the imbibed fruits (unpublished results).At the times stated in the results, seeds from these treatments were tested for germination under 40-w fluorescent daylight lamps at 22 C. Petri dishes of seeds which had been maintained continuously imbibed were simply brought out from the dark incubators, while samples of seeds from air-dry store were imbibed on moist filter papers in Petri dishes at the beginning of each germination test. Germination was scored as the emergence and geotropic curvature of the radicle.Chromosomes in 1-mm radicle tips of lettuce were ex-875 www.plantphysiol.org on May 12, 2018 -Published by Downloaded from

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SUMMARYThe rate of ageing of a batch of caryopses of Zea mays was accelerated by placing in a controlled environment of high temperature and humidity. Samples were withdrawn representing a sequence of increasing physiological ageing, and selected stages were subjected to ultrastructural and cytochemical investigation. !\Iembrane aberrations increased with increasing age of the embryos. The damage was seen immediately upon imbibition, hut there was much evidence of repair of this damage at later stages of germination. The existence and operation of mechanisms for the repair of such damage are discussed.