268 CONTROL OF REPRODUCTION 



well known, the terminal bud gives a head of male florets with the 

 basal ones sometimes female. Lower heads tend to be female. 



In another short-day plant, Chenopodium album, chosen as pos- 

 sessing a branched inflorescence rather than a head or capitulum, 

 transition to flowering has been followed through the same routine 

 treatment in the same detail as was done in Xanthium. A vegetative 

 terminal apex is pictured in Fig. 4. Though not too clear, it can be 

 noted that the central zone embodies not only the first and second 

 layers but also a few cells in the third and fourth layers until the 

 whole resembles almost a letter V in section, or actually something of 

 an inverted cone, the apex of the cone being essentially against the top 

 of the pith rib meristem. The first signs of initiation of flowering, 

 seen histologically, again appear just above the pith rib meristem, 

 involving the bottom of the central zone; these signs take the form of 

 cell multiplication which show up on the second day in the greenhouse 

 under long days after two short-day (8 hr in light and 16 hr in dark- 

 ness) photoperiods. By the sixth day in the greenhouse, similar plants 

 with two photoperiods possessed essentially the same basic organiza- 

 tion as Figs. 14 and 17 indicate for Xanthium. Instead of forming a 

 capitulum or head of flowers as does Xanthium, in Chenopodium the 

 inflorescence axis is more elongate as is characteristic of a developing 

 spiked panicle. The successive changes involve the enlargement of the 

 newly divided cells above the rib meristem extending the pith well up 

 into more mounded apex. The cells of the central zone undergo 

 division, mostly anticlinal, and the regular, even, outer rows can be 

 seen extended well down the flanks of the developing axis of the 

 inflorescence. Bracts and axillary buds, which produce the axes of 

 the branched inflorescence, develop rapidly so that plants subjected to 

 two photoperiods have an entire inflorescence recognizable in bud form 

 by the eighteenth day in the greenhouse, involving even the last of the 

 apical meristem in a terminal flower. When such inflorescences were 

 removed on the sixth day from like plants, and planted in vitro on a 

 medium which permits the growth of angiosperm apices, the in- 

 florescences produced flowers that appeared normal in all respects for 

 Chenopodium album, shedding pollen and showing a receptive stigma. 

 No attempt was made to pollinate these flowers and no seeds were set. 



As with Xanthium, so with Chenopodium, it would be difficult to 



