Shoot Histogenesis and the Subapical Meristem 573 



Within 4 days after the application of Amo-1618 the number of 

 mitotic figures and the length of the zone of subapical activity were 

 substantially reduced (Figure 5); as the inhibition progressed mitotic 

 activity practically disappeared in the subapical meristem (Figure 5, 

 Amo-1618, 14 days) whereas the apical meristem remained relatively 

 unaffected. If GA was added simultaneously with Amo-1618, there 

 was no inhibition of cell division and, perhaps more significant, GA 

 applied some time after Amo-1618 almost completely reversed the 

 inhibition within a period of 24 to 96 hrs. (Figure 5, Amo-1618, 14 

 days -^ GA, 4 days). In every case, the activity of the subapical meri- 

 stem could be correlated with stem elongation: The plants that 

 received Amo-1618 alone assumed a rosette habit of growth; i.e., leaf 

 initiation was normal, or almost so, but stem elongation ceased; plants 

 receiving Amo-1618 and GA at the same time grew normally; plants 

 receiving GA after Amo-1618 reverted from a rosette to a caulescent 

 habit of growth. These results thus strongly support the contention 

 that the subapical meristem is responsible for shoot histogenesis in 

 caulescent plants no less than in rosette species. GA or GA-like sub- 

 stances appear to play an important part in the regulation of the 

 subapical meristem. In rosette plants they seem to be the factors 

 limiting its activity; in caulescent plants they are apparently pres- 

 ent at levels insuring optimum mitotic activity, as application of 

 GA to caulescent plants — at least those with which we have worked 

 — does not markedly increase subapical cell division. By use of Amo- 

 1618 to reduce subapical meristematic activity in these plants, we 

 have been able to demonstrate that GA participates in the regula- 

 tion of this activity and, thus, of shoot histogenesis, in caulescent as 

 well as rosette plants (21). 



MALEIC HYDRAZIDE INHIBITION OF SHOOT GROWTH 



There have been several studies on the inhibition of stem growth 

 by maleic hydrazide (MH) (7, 22, 25); one report on tomatoes (8) 

 cited evidence for a reduction in cell number in the treated plants 

 and suggested that this was the main reason for the inhibition of stem 

 elongation by MH. In view of these experiments, we studied the 

 effect of MH upon the subapical meristem in Xanthium. Our re- 

 sults show that MH, in appropriate doses (0.4 mg. in aqueous solu- 

 tion) completely prevents cell division not only in the subapical re- 

 gions (Figure 6) but also in the apical meristem. 



MH-treated plants, though no longer capable of stem elongation, 

 do not assume a rosette habit of growth because leaf initiation is also 

 prevented; hence, its action as a regulator of cell division is quite 

 different from that of Amo-1618 or GA, lacking the selectivity of the 

 latter two substances. Furthermore, GA (0.4 mg. in aqueous solution). 



