198 THE FLOWERING PROCESS 



the first 8 hr or so of the dark period. Thus it appears that 5-FU 

 inhibits something which must be made in the bud even before 

 flowering hormone is made in the leaf. They have further shown that 

 RNA synthesis is inhibited. Thus the successful action of the flower- 

 ing hormone when it arrives at the bud two or three days after 

 induction may require conditions which are initiated by RNA 

 synthesis at the very beginning of induction (see Chapter 9). 

 Obviously, there is much to learn about all this. 



Zeevaart (79) further found that the DNA inhibitor 5-fluorode- 

 oxyuridine (5-FDU) would inhibit flowering of Japanese morning 

 glory when it was applied to the bud, and that this effect could be 

 fully reversed by application of DNA precursors such as thymidine. 

 Thus DNA multiplication in the bud appears to be essential to 

 flowering. The inhibitor is most eff"ective only when applied near to 

 the time of arrival of the hormone at the apex. If applied too early 

 (40 hr) it is dissipated before arrival of the hormone and hence 

 ineffective, and if it is applied after the bud has been transformed it 

 is also ineffective. Microscopic examination has shown that cell 

 division is inhibited by the 5-FDU. In this species 5-FU acts the 

 same way as 5-FDU and not as it does in cocklebur. 



Thus it appears that the flowering hormone must find dividing 

 cells (multiplying DNA) if it is to turn on the genes for flowering. 

 This agrees well with the observation in the last section that dormant 

 buds do not respond to the stimulus. Does it also mean that multi- 

 plication of the stimulus (in cocklebur) requires multiplication of 

 genetic material ? 



2. Vernalization, Juvenility^ and the Induced State 



The phenomenon of the induced state as described above seems 

 to have a great deal in common with the condition induced by ver- 

 naUzation (Chapter 4) or with the transformation of a plant from the 

 juvenile to the mature condition (Chapter 6). In all these cases two 

 conditions clearly identifiable at their extremes, are involved, and the 

 transformation is very stable and maintained in the plant through 

 subsequent cell divisions of the growing tissue (with the exception of 

 Perilla). Perhaps our best opportunity to study this phenomenon of 

 a condition transferred through cell divisions is off'ered by the 

 flowering process, in which the arrival of a hormone at the meriste- 

 matic sites triggers the transformation to the induced state. Since 



