GROWTH AND DEVELOPMENT OF THE INFLORESCEXCE AND FLOWER 



513 



very young capitula, umbels, and the corymb of Iheris spp., in which 

 the outer florets or flowers show characteristic zygomorphic develop- 

 ments. 



Earlier, in a somewhat different type of surgical experiment, Mur- 

 neek (1927) had shown that in the spider flower (Cleome spinosa), 

 in which a phase of pistil formation is normally followed by a phase 

 of stamen production, the excision of the very young pistils results in 

 the formation of new ones for an abnormally long time. 



Zygomorphij. Only the simplest and commonest case of median 

 zygomorphy— i.e., where the plane of dorsiventrality is that of the bract 

 and shoot axis— can be considered here, though the other types also 

 suggest interesting opportunities for new work. Goebel ( 1900, 1913 ) 

 has given an excellent survey of this phenomenon, especially from the 

 standpoint of the causal factors that may be involved. 



In some species with a racemose inflorescence and median zy- 

 gomorphy, Goebel noted that if, as a result of some disturbance or 

 injury, a lateral flower primordium develops in a terminal position, it 

 tends to be actinomorphic. Such evidence— from one of "nature's ex- 

 periments"— suggests interesting possibilities for the further exploration 

 of the zygomorphic condition by surgical, chemical, and other treat- 

 ments. These investigations might be of special interest in families such 

 as Orchidaceae and Zingiberaceae, which show somewhat parallel, but 

 not identical, evolutionary trends in their reduced, dorsiventral flowers. 

 Figure 15. In passing, the possibility that an incipient dorsiventrality 



Figure 15. Floral diagrams of Orchidaceae (left) and Zingiberaceae (right), showing 

 parallelism, but not identity, in the marked reduction in the number of stamens. (After 

 Eichler, Bliithendiagramme.) 



