500 PLANT GROWTH AND PLANT COMMUNITIES 



Y. Heslop-Harrison's (1956-59) studies of the morphogenetic effects 

 of temperature, light, and applied auxin on the development of the 

 androecial and gynoecial regions of the floral meristem illustrate this 

 point. 



The action of specific genes in flower formation is essentially serial 

 in character. As the florigenic stimulus begins to affect the apical re- 

 action system, the specific action of certain genes, hitherto inert or non- 

 specific in their effects, is induced. The changes thereby effected in the 

 system lead to further specific genie action, and so on in a chain or 

 serial fashion until flower' formation teiTninates with the utilization of 

 all or practically all of the residual distal meristem. It is to the action of 

 particular genes, as components of the reaction system, that the clear 

 delimitation of the several organogenic phases (i.e., the formation of 

 calyx, corolla, androecium, and gynoecium ) must be attributed. 



The foregoing conceptions afford a basis for explaining some of 

 the main features of floral ontogenesis, such as (1) that the several 

 floral organs are homologous with leaves, (2) that they originate in a 

 characteristic pattern on the meristem, and (3) that the successive 

 groups of growth centers, formed during the course of floral ontogene- 

 sis, have distinctive quantitative and qualitative growth properties, and 

 accordingly give rise to groups of organs differing in form and struc- 

 ture. The theory has the merits of treating floral ontogenesis in terms 

 of physiology, genetics, and the dymamic geometry of the embryonic 

 apical region. Moreover, several formerly important and highly con- 

 troversial hypotheses (for example, relating to homologous and non- 

 homologous floral organs, to organs sui generis, to morphological 

 "transition," etc. ) are seen in quite a different light as consideration of 

 the relevant phenomena is transferred from the morphological to the 

 physiological plane. Not least, the theory suggests opportunities for 

 new observational and experimental work on floral ontogenesis, in both 

 its general and its specific aspects. 



To summarize: A basis for explaining the major features in floral 

 ontogenesis is afforded by the following concepts: (1) the several 

 floral organs are homologous with leaves; (2) they originate from 

 growth centers which have their inception in a characteristic pattern 

 on the floral meristem; (3) in relation to the serial evocation of genes, 

 the successive groups of growth centers have distinctive physiological 

 properties and accordingly give rise to organs differing in form, struc- 

 ture, and function; (4) factors affecting correlative developments and 

 other factors are also important in determining the unified and har- 

 monious floral morphogenesis. 



Evidence relating to factors in floral morphogenesis 



The theoretical considerations outlined above indicate that the 



