576 R. M. Sachs and A. Lang 



CONCLUSION 



From examination of the data in Table 2, which is a summary 

 of our investigations on mitotic activity in the subapical and apical 

 regions of various plants, there can be little doubt that subapical 

 meristematic activity is widespread and may far surpass the apical 

 meristem in the nimiber of cells contributed to stem elongation. The 

 mitotic counts in Table 2 are for the pith tissue alone, and since the 

 subapical meristem embraces the epidermis, cortex, and vascular tis- 

 sues as well, they represent no more than 50 per cent of the total 

 subapical cell divisions. Gerbera is a particularly interesting case be- 

 cause the flower is initiated while the shoot is still rudimentary, i.e., 

 before stem elongation occurs. Since the apical meristem is com- 

 pletely occupied with the processes of flower differentiation, the cells 

 for the flower stalk (or shoot) are generated in their entirety by the 

 subapical meristem. 



Thus, it appears that while shoot organization is mainly deter- 

 mined in the apical meristem (23), shoot histogenesis, i.e., the actual 

 formation of the cells and tissues which constitute the mature stem, 

 takes place in the subapical region which, for all purposes, may be 

 considered as an intercalary meristem. Furthermore, the action of 

 two growth regulators, GA and Amo-1618, is of fundamental im- 

 portance in controlling cell division apparently specifically in this 

 region. 



SUMMARY 



Gibberellic acid (GA) caused stem elongation in rosette plants by 

 stimulating mitotic activity in the regions immediately below the 

 apical (pro- or eu-) meristem. The cells produced in this zone, after 

 elongation, constitute the tissues of the mature, elongate stem. In 

 caulescent plants a subapical zone of mitotic activity was observed 

 as much as 2 cm. in length, similar in activity to that in rosette plants 

 which received prolonged treatment with GA. By treatment with 

 Amo-1618 subapical (but not apical) cell divisions were considerably 

 reduced and soon thereafter shoot elongation ceased. Since the apical 

 meristem functioned normally or almost so, and leaf initiation was 

 continuing, the treated plants assumed a dwarf or rosette habit of 

 growth. GA prevents and reverses inhibition of subapical cell divi- 

 sion induced by Amo-1618. Thus, in caulescent as well as in rosette 

 plants, GA plays an important role in regulating subapical mitotic 

 activity, thereby controlling shoot elojigation. Maleic hydrazide is 

 a powerful inhibitor of cell division in the subapical regions; how- 

 ever, in contrast to Amo-1618, it inhibited apical meristematic activity 

 as well and GA did not counteract the inhibition. 



