GROWTH AND DEVELOPMENT OF THE INFLORESCENCE AND FLOWER 495 



interest in the present context. Thus, when various physiologically 

 active substances were applied directly to the apical meristem of Dry- 

 opteris austriaca, the novel eflFects included ( 1 ) the failure of growth 

 centers to develop into primordia, (2) the inception of double pri- 

 mordia, and (3) the formation of vegetative buds in leaf sites. To 

 summarize: During the vegetative phase, growth centers in the apical 

 meristem usually give rise to leaf primordia, but the activity of a 

 growth center is subject to modification by changes in its metabolic 

 components, and these may have significant eflEects on the ensuing 

 morphological development. 



Lastly, the actual pattern of growth centers at any particular time 

 is determined by the organization of the apical meristem, the stage 

 reached in ontogenesis being important. The genetical constitution is, 

 of course, fundamentally involved. As to the proximate cause, how- 

 ever, a physico-chemical theory along the lines proposed by Turing 

 (1952) seems to be relevant (Wardlaw, 1953, 1955, 1957b). In this 

 theory the apical meristem is regarded as a complex reaction system 

 capable of yielding a patternized distribution of specific metabolic 

 substances ( Wardlaw, 1957b ) , these b'eing accumulated in evenly dis- 

 tributed loci which we describe as growth centers. 



The transition apex. With the onset of the reproductive phase, the 

 shoot apical meristem may, in different species, be transformed into a 

 single flower or into an inflorescence. These new morphogenetic activi- 

 ties are usually marked by more or less conspicuous changes in the 

 distribution and kind of growth in the apex. Moreover, as judged by 

 external appearances, the showy, often brightly colored inflorescence 

 or flower seems to bear little relation to the vegetative leafy shoot. 

 Hence some botanists regard the inflorescence and flowers as organs of 

 a completely different category from those formed during the vege- 

 tative phase. This, of course, is in marked contrast to the classical 

 concept of the flower as an axis of limited growth with variously modi- 

 fied lateral members which are homologous with leaves. In fact, both 

 in the transition apex and the floral apex we may recognize that we are 

 still dealing with an apical meristem yielding a pattern of growth 

 centers; and while on the one hand certain far-reaching morphogenetic 

 changes do take place in the further development of some of the 

 growth centers, on the other hand certain aspects of apical activity 

 remain singularly unchanged in any fundamental sense. After all, the 

 same genetical constitution underlies all development in both the vege- 

 tative and the reproductive phases. 



General aspects of organogenesis in a "simple" flower. In a "simple' 

 or prototypic flower {e.g., Ranunculus sp.) all the organs can be re- 

 ferred to the development of growth centers, these usually constituting 

 a pattern of considerable regularity. As Tepfer (1953) has so clearly 



