DEVELOPMENT OF VEGETATIVE AND FLORAL BUDS 261 



cept and have given it substance by suggesting that it is the central 

 zone, essentially inactive during the vegetative life of the plant, which 

 gives rise to the reproductive axis, receptacle, and flowers. In their 

 thesis, the peripheral zone plays no persisting part in this rebuilding. 

 In the studies reported here, a comparative histological examina- 

 tion was made of the apical regions of three short-day and two long- 

 day plants. Collections were made of apices in the vegetative period 

 and at regular intervals throughout the period of transition to flower- 

 ing during and after photoinduction. We are convinced that it is not 

 necessary to disregard the vegetative apex and invoke a new flower- 

 ing apex to envision the changes which take place. 



METHODS 



In our studies, we have utilized the short-day plants Xanthium 

 pensylvanicum Wallr., Chenopodium album L., and Glycine max 

 (L.) Mer., and the long-day plants Hyoscyamus niger L., annual 

 form, and Papaver somnijerum L. For the supply of seeds of all these 

 we are grateful to Dr. Harry Borthwick and the United States Depart- 

 ment of Agriculture. 



The procedure employed will be described in some detail for 

 Xamhium; for the other species, variations in procedure were planned 

 to fit the individual needs in keeping with their long- or short-day 

 habits. 



Plants of Xanthium were grown in the greenhouse under long-day 

 conditions. Before induction was initiated, vegetative apices were 

 killed and fixed for the histological preparation of microscopic slides. 

 A large number of plants were then placed in a light room on an 

 8-hr day, 16-hr night schedule. After one photocycle, apices were 

 killed and fixed from two plants, and a number of plants were returned 

 to the greenhouse with long-day illumination. Collections for histo- 

 logical study were made from apices of these plants every other day. 

 After two photocycles in the light room, more plants were moved to 

 the same greenhouse conditions, two terminal regions collected at once 

 for histological study, and two every other day thereafter until the 

 plants were recognizably flowering. On each succeeding day, the same 

 procedure was followed until the seventh day, when even casual inspec- 



