262 CONTROL OF REPRODUCTION 



tion indicated that all apices of plants in the light room were of flower- 

 ing nature. The experiment in the light room was then terminated. 

 Histological preparations representative of photoinduction with the 

 subsequent changes in the apices with the onset of flowering have been 

 followed in the species listed. Only the changes in Xanthium pensyl- 

 vanicwn will be considered in detail. Others will be dealt with in a 

 broad way, as differences reflect essentially only individual differences 

 in the organization of the vegetative apex and not in fundamental 

 changes during flowering. 



RESULTS 



In Fig. 3, a vegetative apex shows a section of a caplike zone of 

 central cells in the second row, the outer row of the corpus, usually 

 6 to 9 cells in diameter."* Overlying it, the tunica shows a section of 

 a cap, often fewer (5 to 7) cells in diameter. Because of cytological 

 similarities, these can, for the present, be considered as part of the 

 central zone. Flanking the central zone, and therefore completely 

 surrounding it circumferentially, is the small-celled peripheral zone. 

 Beneath it, and merging laterally with the peripheral zone, is the sub- 

 jacent rib meristem zone, below which lie the radiating rows of pith 

 cells. Between the central zone and the rib meristem zone is a flat zone, 

 only a few cells in thickness, in which, as will be pointed out, 

 demonstrable activity preceding flowering, after induction, is initiated. 

 Sections of leaf primordia are evident on the flanks of the apex. 



Apices of plants with one short day before being brought back to 

 long days in the greenhouse showed no recognizable histological 

 change until the fourth day. In Figs. 7 and 8, sections of a 4-day and 

 a 6-day apex, one can recognize considerable increase in the number 

 of cells below the central zone and above the rib meristem, a result 



^ The description that we give to the vegetative apex of Xanthium pensyl- 

 vanicum results from our own studies; it is, however, in accord with the 

 excellent, detailed and well-illustrated study of the vegetative apex of this 

 species given recently by Millington and Fisk (1956). The subsequent account 

 of the transition from the vegetative to the ilowering apex which the authors 

 present in this paper is also in agreement with the corresponding study by 

 Millington (1951). We are grateful for access to this information, some of it 

 unpublished. 



