GROWTH AND DEVELOPMENT OF THE INFLORESCENCE AND FLOWER 497 



species from groups not closely related. For example, zygomorphic 

 flowers are usually lateral, the plane of zygomorphy being very com- 

 monly that of the axis and the subtending bract ( Goebel, 1900, 1913 ) . 

 This suggests that certain genetically determined adaxial/abaxial 

 growth relationships, established at an early stage in floral ontogenesis, 

 are of general occurrence; and similarly for other major features. 



The floral meristem as a serial reaction system. The shoot apex, as 

 we have seen, may be regarded as a complex, gene-determined, physi- 

 co-chemical reaction system, with the property of giving rise to a pat- 

 tern of growth centers which typically become dorsiventral foliar mem- 

 bers, leaves, scales, etc. In the transition meristem, growth centers 

 continue to be formed. The first of these may develop as evident foliar 

 organs, often exhibiting the phenomenon of heteroblastic development. 

 The next group of growth centers gives rise to the perianth members, 

 and centers formed later produce the stamens and carpels ( see Figures 

 4 to 7). Even in highly modified, condensed, epigynous flowers, this 

 conception of the basic nature of floral organogenesis is probably 

 still applicable, attention being paid also to the distribution of growth 

 in the receptacle and to correlative 'developments, especially during 

 the earliest stages. The ontogenetic approach has, of course, already 

 been eflFectively used by many workers, mostly in comparative studies, 

 but it is no less essential in causal investigations. What is now required 

 is the investigation of characteristic floral developments in terms of the 

 underlying physiological-genetical factors and others. To this end the 

 writer (Wardlaw, 1957c) has proposed a theory of floral morpho- 

 genesis which may now be briefly outlined. 



The theory is based on the view that, at the onset of flowering, the 

 shoot apex continues to give rise to a succession of regularly spaced 

 growth centers, the further development of which as primordia, with 

 the distinctive characters of sepals, petals, stamens, and carpels, is the 

 result of the serial evocation and action of particular genes ( or groups 

 of genes ) together with the other factors at work in the meristem. The 

 existence of metabolic differences between different growth centers, al- 

 ready mentioned, is an important part of the theory. As Engard ( 1944 ) 

 noted, there are no real transitions at the morphological level between 

 homologous organs. But, as we now see, there can be important 

 changes and differences in the metabolism of growth centers. The in- 

 ception of the flower is marked by the entry of new metabolic sub- 

 stance(s), not yet specified or isolated, into the reaction system of the 

 apical meristem, with consequent effects on the metabolism of its 

 growth centers. Other important, usually irreversible, changes also en- 

 sue. The allometric growth pattern of the apex is modified to a more or 

 less marked extent, the elongation of the axis being usually conspicu- 

 ously dimished or arrested. As a result, the growth centers, and the 



