ROY M. SACHS 



and 

 ANTON LANG^ 



University of California, Los Angeles 



Skoot Histogenesis and the Subapical Meristem: 

 the Action of Gibberellic Acid, Amo-1618, 



and Maleic Hydrazide 



Until the late 1950's most discussions of shoot growth were concerned 

 primarily with cell division at the apical meristem and cell elonga- 

 tion in the more distal regions o£ the stem. The few notable excep- 

 tions in which evidence was presented for cell division below the 

 apical meristem and its contribution to shoot development were ig- 

 nored (1,9, 17). With the discovery of the action of the gibberellins, 

 the quaternary ammonium carbamates and maleic hydrazide on 

 stem growth, subapical meristematic activity assumed new import- 

 ance. Thus, the experiments relating to the re-evaluation of sub- 

 apical cell division represent, at the same time, an account of one 

 mode of action of three plant growth regulators which have been 

 the subject of intensive study. 



GIBBERELLIN-INDUCED STEM ELONGATION IN ROSETTE 



PLANTS 



Cytological examination of Samolus parvifiorus, a long-day rosette 

 plant, revealed a great increase in mitotic figures in the regions im- 

 mediately below the apex within 24 hrs. after the application of gib- 

 berellic acid (gibberellin A3, abbrev. GA) (1 1). Similar evidence is 

 available for other rosette plants [ Table 2 in (18) ], but only the data 

 for Samolus, which has been studied in greater detail, will be pre- 

 sented. As the influence of GA continues, the zone of cell division 

 increases in length, exactly equalling the growth in length of the 

 stem (Figure 1). This new zone of division, comprising the cortical, 

 vascular, and pith tissues, can be considered as a virtual subapical 

 meristem. Another important feature of GA action is illustrated in 

 Table 1. There is no increase in cell length for 72 hrs. following GA 



* Subsequently: California Institute of Technology, Pasadena, California. 



[567] 



