GROWTH AND DEVELOPMENT OF THE INFLORESCENCE AND FLOWER 519 



ing ontogenesis, and whether the information so obtained accords or is 

 at variance with current explanations of these structures in terms of 

 their evolution and function. 



Discussion 



The morphological diversity of inflorescences and flowers is im- 

 pressive, but some simplification of our ideas concerning them begins 

 to seem possible. From the generally accepted prototypic flower, very 

 extensive diversification could come about as a result of genetical 

 changes determining modifications in the distribution of growth in the 

 floral meristem, or receptacle, these modifications being efiFected by 

 direct gene action and by correlative developments. In the search for 

 general factors in floral morphogenesis, som.e of the many instances of 

 parallel structural development in different, or seemingly different, 

 phyletic lines should be fully investigated. While each of the develop- 

 ments in which we can perceive homology of organization is ultimately 

 related to, and limited by, the underlying genetical constitution, some 

 of them do not seem to be closely or directly determined by genetical 

 factors. Thus parallelisms in floral construction may be indicative of 

 (1) common ancestry and parallel changes in the genetical constitu- 

 tions, (2) dissimilar genetical constitutions which nevertheless have 

 comparable morphogenetic manifestations, (3) obscured but latent 

 homology, or (4) the effect of extrinsic factors. Other possibilities also 

 suggest themselves. 



In support of the view that certain major developments are ap- 

 parently not closely and specifically gene-determined, it may be noted 

 that ( 1 ) a flower typically consists of an axis and lateral members, 

 develops as a unified structure, and in the growth of its component 

 organs affords evidence of regulation and correlation; (2) the same 

 assemblage of amino acids and other general metabolic substances that 

 is essential for the growth of embryonic tissues has been detected in 

 the apical meristems of taxonomically unrelated species; (3) the 

 "florigenic substance" that determines the onset of the reproductive 

 phase in apical and lateral meristems is probably the same in all flow- 

 ering plants; and (4) certain environmental factors may determine 

 closely comparable morphogenetic developments, some of critical im- 

 portance, in unrelated species. On some of these aspects contemporary 

 geneticists have so far had little to say, at least as judged by the con- 

 tents of standard works. 



Although there is great variety in the minor details of floral struc- 

 ture within a species, genus, or family, the main trends in floral evolu- 

 tion can probably be referred to a few major kinds of change, starting 

 from a central prototype. As parallel developments have taken place 



