A. lntroduction. I. Objectives of the chapter. Earlier reviews.Flower initiation marks the transition from vegetative to reproductive growth in seed plants. It is thus a crucial event in the life of these plants, particularly so because of the peculiar relation of vegetative and reproductive development in seed plants which is in turn an outcome of the morphological nature of the flower. Flowers are modified shoots which are produced by modified shoot meristems, the flower primordia. However, once a meristem has been determined as a flower primordium, it is usually unable-except perhaps at the very earliest stages -of reverting to vegetative growth. Vegetative growth and reproductive development in seed plants are thus in a certain sense mutually exclusive; as far as a particular meristem is concerned, flower initiation means the end of its life. The central problern of the physiology of flower initiation is to understand which factors cause a shoot meristem to become a flower primordium, and how they consummate their action.The objective of this chapter is to summarize our present information on this subject. However, certain aspects of the physiology of flower initiation, particularly those dealing with the control of this process by certain environmental factors, are treated in detail in volume XVI of this encyclopedia 1 ; some of these, particularly the more descriptive ones, will be reviewed here relatively briefly. There is also a considerable number of reviews and books dealing either with the entire physiology of flowering, or with certain phases of it. Among the reviews, we may name those by MELCHERS and LANG (1948b), , DoORENBOS and WELLENSIEK (1959), ÜHOUARD (1960), SALISBURY (1961) and ZEEVAART (1962); among the books, the recent one by HILLMAN (1962b) in which one author has summarized the entire field from his point of view-however, with exemplary objectivity -, and several, somewhat older collective volumes in which, or parts of which, several authors discuss individual aspects of flowering physiology (MuRNEEK and WmTE 1948, R. B. WITHROW 1959, Anon. 1959.
II. Endogenous and exogenous control of ßower initiation. lnductive effects.Like any other physiological process, flower initiation is ultimately determined by the genotype of the plant. In a considerable number of plants, the
