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PLANT GROWTH AND PLANT COMMUNITIES 



Figure 4. This and the next figure are a diagrammatic illustration of a theory 

 of floral morphogenesis in a prototypic flower. The longitudinal section here 

 shows, in acropetal sequence, a bract (Br), young sepals (S), young petal 

 primordia (Pe) , stamen primordia (St) at their inception, the positions of 

 the next growth centers to become primordia (G), and the beginning of 

 growth centers still closer to the summit of the apex (A), tunica (T), corpus 

 (C), and pith (P). The discontinuous transverse lines are intended to indi- 

 cate a number of zones into which the organized apex is differentiated: the 

 distal zone (D); the sub-distal zone(s) (SD) , in which the reaction system 

 is giving rise to a pattern of growth centers; the organogenic zone (OR), in 

 which the active growth centers have given rise to very young primordia. 

 The transverse lines also indicate the successive phases, or stages, through 

 which the apex passes as floral morphogenesis progresses. 



lateral organs to which they give rise, are now formed in closely as- 

 sociated groups, either whorls or condensed helices, on an abbreviated 

 axis (see Figure 6 and 7). In normal floral ontogenesis the apical re- 

 action system passes through a sequence of distinctive phases, these 

 being determined and controlled by specific genes and also by physio- 

 logical correlations and environmental factors ( see Figure 4 ) . The fun- 

 damental pattern in flower inception, which usually exhibits remark- 

 able stability and constancy, is primarily due to the specific genetical 

 constitution, but during the elaboration of this pattern, extrinsic fac- 

 tors may sometimes exercise important morphogenetic efiFects. J. and 



