Morphology of Flowering • 3 



ontogeny. The evidence suggests that the various flower parts, from 

 sepals to carpels, are homologous with ordinary foliage leaves. 

 That is, they bear essentially the same anatomical and morpho- 

 logical relation to the axes on which they are borne as do the 

 leaves. This does not necessarily mean that the flower parts have 

 been derived from foliage leaves, even though the flower parts of 

 many plants, particularly those considered more primitive, may 

 show distinctly leaflike characteristics. Probably the most widely 

 accepted view is that both leaves and flower parts were evolu- 

 tionarily derived from similar structures. These may have been 

 fused branch systems, some of them entirely sterile and represented 

 in our present leaves, some of them sporogenous and represented 

 in modern carpels and anthers, and still others with functions 

 accessory to the sporophylls and represented in modern sepals and 

 petals. While the details of such questions remain speculative for 

 the present since the ancestry of the angiosperms is not really 

 known, the homology between leaves and flower parts is generally 

 accepted and may be of some importance physiologically; it is at 

 least implicitly challenged, however, by some of the work on 

 flower ontogeny to be considered next. 



The flower, like the leaves and the shoot itself, is derived 

 from the apical meristem. This is a region of relatively small, 

 undifferentiated, more or less actively dividing cells located at the 

 very apex of the shoot. Meristems in general are the sources of new 

 growth in all higher plants, and this has given rise to the concept 

 that plants, unlike animals, show a "continuing embryogeny." 

 Relatively little is known about the mechanism of the formation 

 of new organs by such embryonic, seemingly slightly organized 

 groups of cells. The central problem of the physiology of flowering 

 might be stated as the question of how various factors affecting 

 flowering, be they environmental or genetic, are translated by the 

 plant into physico-chemical "signals" to the meristem, and how 

 these determine whether the meristem will produce flowers. The 

 major morphological question on which there is disagreement is 

 whether the meristematic activity that produces flower primordia— 

 recognizably distinct structures that will develop into flowers under 

 favorable conditions— is qualitatively different from that which 

 produces leaf initials, which develop into leaves. 



According to the majority of recent workers there is no essen- 



