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PLANT GROWTH AND PLANT COMMUNITIES 



Figure 12. Petasites hyhridus 

 (L.) Gaertn. May and Sherb. 

 A young inflorescence bud 

 with the vegetative leaves 

 and the outer bracts (Br) dis- 

 sected off. The younger inner 

 bracts, normally non-lami- 

 nate, have developed laminas 

 (L). This is a diagrammatic 

 tracing from a photograph. 



fore be determined at a very early stage; i.e., with the onset of flower- 

 ing some growth centers become loci for the accumulation of the 

 metabolic substances required in flower-primordium formation, as dis- 

 tinct from leaf-primordium formation. This conclusion seems inescap- 

 able, but how adjacent growth centers become different metabolically 

 is a problem in developmental physiology for which no adequate ex- 

 planation has yet been advanced. In Victoria sp. ( see Figure 9 ) alterna- 

 tive spirals of leaf and flower primordia can be traced from older 

 regions of the rhizome upwards into the apical meristem (Cutter, 

 1960 ) . The position of a young flower primordium is not axillary in the 

 usually accepted meaning; it may be described as originating on the 

 apical meristem above the anodic flange of an older tangentially ex- 

 tended leaf primordium, or as being interfoliar. Indeed, it is not unlike 

 the interfoliar loci of bud rudiments in leptosporangiate ferns. In 

 Victoria, as in Nymphaea, the phenomena of flower disposition and 

 inception must also be sought in the apical meristem— a relatively small 

 mass of embryonic tissue in which these definite but subtly distinctive 

 patternized distributions of metabolites will have to be investigated. 

 ( For a full account of inflorescences, see Rickett, 1944. ) 



Effects of external factors on floral morphogenesis. Some interest- 

 ing and important results of the effects of external factors in floral mor- 

 phogenesis are being published at the present time. While there can 



