THE ANGIOSPERMAE 1123 



FLORAL ONTOGENY 



The ontogeny of the flower begins with the initiation of a floral growing 

 point at the apex of the branch which is destined to become a floral pedicel. 

 Over this development considerable controversy has centred, because ithas 

 an important bearing on the theory of floral morphology. The question at 

 issue is whether the development of the floral axis is comparable with that 

 of a vegetative axis or whether it is fundamentally diflferent. In other 

 words, whether the history of floral development supports the classical view 

 that the flower is essentially a contracted shoot, or not. 



In 1938 Gregoire published an extensive series of researches which 

 showed, as he claimed, that the organization of a flowering apex was quite 

 distinct from that of a vegetative apex. According to his views there are 

 only two zones in the floral meristem; a superficial meristematic mantle 

 and an internal parenchymatous mass. Mitosis is almost confined to the 

 first zone, while the second has highly vacuolated cells. This growing 

 region has no apex, and shows no regular plastochrones. The outer surface 

 is a dome, expanding only in area. There is no group of initial cells, the 

 central mass being added to all round by periclinal divisions in the mantle 

 layer and adapting itself to the extending area of the latter. Furthermore 

 he claimed that the vascular traces arise in the central core and extend 

 towards the rudimentary organs of the young flower, whereas in the vegeta- 

 tive apex the vascular traces first appear at the bases of the leaf rudiments 

 and diflFerentiate both downwards and upwards. Such a structure, Gregoire 

 maintained, is unique ; it is in a category by itself. If homology is considered 

 to imply ontogenetic identity, and if there is no such identity, then the 

 floral organs cannot be homologous with any vegetative organs of the plant, 

 but are sui generis or of their own kind. 



Ideas of so novel a character have provoked contradiction at the hands 

 of those who support the classical view of the flower and a number of obser- 

 vations which are incompatible with Gregoire's views serve to show that 

 his theory is, at least, not of universal application. The fundamental unity 

 of floral plan makes it improbable that structures so characteristic as flowers 

 can differ entirely from each other in their morphological nature and con- 

 sequently a true morphological explanation should either be universally 

 valid or not at all. 



One fairly obvious difficulty in the general application of Gregoire's 

 theory is that in many species a vegetative growing point may be directly 

 converted into a floral rudiment, in which case we cannot be dealing with 

 two fundamentally diflterent structures but rather with a change of one into 

 another. For example, in Riibus (Fig. 1098), as shown by Engard, the 

 alteration takes place as follows. The first step is the cessation of vertical 

 growth, accompanied by a globose expansion of the apex. Then come a 

 series of very short plastochrones which produce, nearly simultaneously, a 

 group of closely related primordia, the sepals, which expand laterally into a 

 continuous rim around the growing point. They originate from the second 



