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Two basic kinds of dormancy have been distinguished by BüNNING (1947): ectogenous, influenced by external factors such as light, temperature, water, etc.; and endogenous, conditioned by the internal constitution of the seed. Whether seeds possess either one or both of these types of rest depends upon two main factors, environmental influences and heredity. It should, however, be realized that the two influences are mutually dependent and sometimes cannot be separated.I. Embryo dormancy.The most genuine case of seed dormancy is when the embryo of the mature seedisnot capable of development even under optimal moisture and temperature conditions because of the presence of some physiological "block". In very many cases, however, the isolated embryo germinates quite readily and produces a normal seedling. In such cases one might conclude that the dormancy of the seed is determined exclusively by the seed coat. This, however, is not always correct. When the dormant seed is subjected to the action of some specific environmental factor, e.g. light, which takes its effect within the embryo, the latter becomes capable of rupturing the coats and germinates. Thus, we must assume that while the resistance to germination may be localized in the coat, the embryo of the dormant seed is at an actively level insufficient for overcoming this resistance.The purpose of this discussion is to provide a brief, comparative survey of the various cases of embryo dormancy; for physiological details the reader is referred to other chapters of this volume 1 • 1. Seeds requiring low-temperature after-ripening. a) Seeds with a single cold-requirement.Different types of embryo dormancy exhibited by varied and numerous plants have been reviewed previously CROCKER 1948, ÜROCKER andBARTON 1953). One of the best-known types is that where germination A. Lang (ed.), Differentiation and Development

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