4 • Background 



tial difference between the organization of a meristem producing 

 only leaves and one producing flowers. Gross differences of course 

 exist between floral and vegetative apices in a given plant. These 

 differences appear to be correlated with the vegetative and in- 

 florescence structures of the particular plant involved, and no 

 generalizations true for all plants can be made. But the question 

 of essential organization goes beyond this, which is largely a matter 

 of shape and size. 



The organization of many vegetative shoot apices can be ex- 

 pressed loosely in terms of the tunica, or outer layers of cells, and 

 the corpus, or inner core of cells, the developmental functions of 

 which may be somewhat different. Most recent investigators have 

 observed that where this organization is present it continues with 

 no sharp change into the floral meristems, which are thus not 

 qualitatively different from the vegetative. See, for examples, Wet- 

 more, Gifford, and Green (1959); Stein and Stein (1960); and 

 Tucker (1960). However, according to a minority of investigators 

 working chiefly in France, floral development is the exclusive 

 function of a "waiting meristem" (meristeme d'attente) that re- 

 mains inactive until the onset of flowering, whereas leaf production 

 and purely vegetative growth are carried on by an "initial ring" 

 (annean initial) surrounding it. This work is reviewed by Buvat 

 (1955). In this view, then, reproductive and vegetative development 

 are quite different, originating in different meristem regions, 

 whereas the majority view is that there are not two sorts of develop- 

 ment but merely a continuum with extremes. 



The view of no essential difference seems to be supported by 

 experimental work, to be described later, showing that certain 

 plants (Cosmos, Kalanchoe), given a treatment insufficient to in- 

 duce flowering but having some effect in that direction, may re- 

 spond by producing a series of structures intermediate between 

 normal inflorescences and leafy shoots (see Fig. 1-1). Although 

 one can interpret such "vegetative flowering" as the interaction of 

 two fairly distinct meristematic activities, the majority view appears 

 to involve less difficulty. 



Descriptive morphology of the meristem has little more to tell 

 the student of flowering physiology, although experimental (oper- 

 ative) morphological studies may well do so in the future. The reader 

 should bear in mind that, in general, experiments on the physiol- 



