DEVELOPMENT OF VEGETATIVE AND FLORAL BUDS 265 



periods, mitotic activity, as well as the enlargement of some of the 

 recently divided cells, can be seen below the mother cell zone. Also 

 certain cells of the mother cell zone itself show signs of divisions, one 

 cell on the left dividing and others having divided so that the zone 

 appears to be two cells thick. It has not yet shown signs of extension 

 down the flanks of the already somewhat mounded apex. Figure 12, 

 with 2 days in the greenhouse, and Fig. 13 with 6 days, are somewhat 

 comparable to Figs. 9 and 10, though somewhat more mounded. The 

 apex of Fig. 1 2 was slightly more advanced toward flowering after 2 

 days in the greenhouse with two photoperiods than the apex of Fig. 

 9 was after 8 days on one photoperiod. Figure 13 already gives 

 evidence of growth of many of the proliferated subjacent cells with 

 vacuolation causing much mounding of the whole apex. The cap of 

 central zone cells has extended down the flanks, and the larger cells 

 of the zone itself have been cut up to smaller units. 



Figure 14 represents a stage of a two-photoperiod plant 8 days 

 after removal from the light room. Here clearly is a young head or 

 inflorescence elaborated, with bracts and axillary flower primordia 

 indicated. Presumably the top florets would have been male; that 

 present on the low left would probably have produced a female 

 flower. As indicated above, histological evidence of floral induction 

 follows after one photoperiod, but it takes 6 to 8 days for its appear- 



FlGS. 7-14. Apices of Xanthium pensyhanicwn during induction. 



7. Terminal apex 4 days after one photoperiod. Xl75. 



8. Apex of lateral bud 6 days after one photoperiod. Note increased 

 number of cells below prominent central zone cap in first and second 

 layers. Xl75. 



9. Terminal apex 8 days after one photoperiod. Even more prominent 

 multiplication of cells between central zone and pith rib meristem. Xl55. 



10. Terminal apex 10 days after one photoperiod. Note enlargement of 

 cells of region of recent cell multiplication, referred to in 8 and 9, and 

 cutting up by anticlinal divisions of cells of central zone. Xl60. 



11. Apex of lateral bud at end of two photoperiods. Some mitotic 

 activity below and in central zone. Xl60. 



12. Terminal apex 2 days after two photoperiods. xlOO. 



13. Apex of lateral bud 6 days after two photoperiods. Note apex in 

 about same state as 10, 10 days after one photoperiod. XlOO. 



14. Terminal apex 8 days after two photoperiods. Note flower buds in 

 axils of bracts. Xl25. 



