266 CONTROL OF REPRODUCTION 



ance; the same stage was reached at the end of two photoperiods, 

 with no subsequent wait in the greenhouse. 



Subsequent photoperiods gave evidence of incipient changes already 

 present in the apex when removed from the light room. Figure 15 

 shows a transitional stage not pictured in Figs. 7-14, for it is often 

 missed in collections made every second day. This apex had had three 

 photoperiods and 2 days in the greenhouse. Subapical multiplication 

 of cells had taken place as usual, as had the acropetal growth of in- 

 cipient pith. The flanks of the central zone had extended and preco- 

 cious enlargement of the peripheral cells had occurred just adaxial to 

 each leaf primordium, in the region of the leaf gaps. Lateral exten- 

 sion of the pithlike parenchyma is at once obvious above and near 

 each leaf primordium as soon as the extension of the pith has oc- 

 curred acropetally after induction. Moreover, not infrequently did 

 these enlarged cells show mitoses, indicating that both cell division and 

 cell enlargement may take part in this parenchymatization of the 

 peripheral region. In all species studied, this seems to be present as a 

 step in sequential events following the initiation of reproduction. The 

 same picture can be readily recognized in Fig. 18, a section of the 

 apex of a bud 2 days after removal from the light room in which it 

 had been exposed to five photoperiods; this stage was already present 

 in buds of plants at the end of 6 days in the light room (Fig. 19). 

 Buds from similar plants which had had 2 days in the greenhouse in 

 addition show a later stage in the apex (Fig. 20) than found following 

 two photoperiods and 6 days in the greenhouse (Fig. 13) or than 

 after 10 days in the greenhouse subsequent to a single photoperiod 



(Fig. 10). 



When one examines such an apex as is shown in Fig. 14, it seems 

 clear that floral bracts and axillary florets are borne all around the axis. 

 If one considers that bracts are leaves borne at nodes, in the ordinary 

 concept of an inflorescence, then the stem bearing these can be 

 followed, as such, very close to the tip. In other words, almost no 

 apex is left. Across this small portion can be seen no large central 

 cells of the central zone, rather the characteristic small cells of the 

 subdivided central cells which follow induction. It would seem 

 difficult to consider this inflorescence axis or head as other than a 

 short stem, or short shoot, bearing bracts, with axillary florets. As is 



