270 CONTROL OF REPRODUCTION 



from which the floral organs may develop, the meristem being carried 

 on a parenchymatous support." In his discussion, he notes "that the 

 zonation of the meristem in the inflorescence rudiment of Bellis and 

 Succisa is derived from that of the vegetative apex by suppression of 

 the central initial zone, its place being taken by an extension of the 

 peripheral zone." That this gradual and progressive change of the 

 vegetative apex to a flowering apex is a common phenomenon follow- 

 ing floral induction in various angiosperms (Popham and Chang in 

 Chrysanthemum, 1952; Boke in Vinca, 1947; Rauh and Reznik in 

 nine different species, 1951) proves to be true in the present study 

 for at least those short-day {Xanthhmi pensylvanicum, Chenopodium 

 album, and Glycine max) and long-day plants (Papaver somnijemm 

 and Hyoscyamus niger) studied. In these species, although individual 

 variations occur in the timing of response, the orderly sequence of 

 demonstrable histogenic changes are essentially the same. The initial 

 observable effect found in the five species studied proved to be mitotic 

 activity just below the central zone and above the rib meristem zone. 

 Gradually, this activity spread into the central cells, irrespective of the 

 pattern of the central zone, so that the peripheral zone of small, more 

 or less isodiametric cells now had added to it progressively the whole 

 region distal to the pith rib meristem zone. This set of descriptive 

 changes is followed by a cessation of mitotic activity and an increase 

 in size of all cells of the pith rib meristem, the newly formed paren- 

 chymatous region below the former central zone region, and laterally 

 into the regions above all newly forming primordia. Thereby seemingly 

 the apex becomes a parenchymatous core of pith covered with a thin 

 mantle of meristematic cells from which arise the bracts, the axillary 

 branches of the inflorescence, and the flowers as required genetically 

 for each of the several species. The apex has ceased extension in 

 species developing floral heads or single flowers and has slowed down 

 any increase in length of axis even in forms with limited inflorescences 

 such as Chenopodium and Hyoscyamus. What happens in rapidly 

 growing inflorescences such as Agave or the Talipot palm (Corypha 

 umbraculifera) is unknown. 



It is clear that the changes just dealt with are descriptive and 

 histological only; even so, they must represent fundamental morpho- 

 genetic modifications involving physiological and biochemical proc- 



