382 GROWTH FACTORS AND FLOWERING 



time interval required for this spontaneous conversion in the dark must 

 be short (in the neighborhood of 2 to 3 hr). 



2. Preparatory Reaction(s) . It is known that the minimum period 

 of darkness required to produce the first perceptible signs of flowering 

 in cocklebur always exceeds approximately 8.5 hr. This period so de- 

 fined has been referred to as the critical night. Since pigment conver- 

 sion accounts for only some 2 to 3 hr of this period, another time- 

 measuring reaction has been postulated and referred to as preparatory 

 reaction(s) (Salisbury and Bonner, 1956). Nothing is known about 

 the biochemical nature of this reaction except that it is quite likely 

 different from the reaction which follows (Sahsbury, 1957b; Salisbury 

 and Bonner, 1956). 



3. Hormone Synthesis. After the completion of the critical night, 

 flowering hormone appears to be synthesized in the leaf, and the 

 amount of hormone synthesized increases during the 4 to 8 hr which 

 follow the critical night until a saturation point is reached. 



An evidence in favor of this quantitative picture of hormone 

 synthesis is that the rate of floral bud development, as measured by a 

 series of floral stages observed a few days (usually 9) after induction, 

 is proportional to the number of hours by which a single dark period 

 exceeds the critical night (Sahsbury, 1955). Many systems of floral 

 stages have been proposed (Downs, 1956; Khadairi and Hamner, 

 1954; Mann, 1940), and one which has been used by the author 

 (Salisbury, 1955) appears to be particularly wefl suited, not only to 

 the study of the hormone synthesis phase of the dark period, but also 

 to many other steps in the flowering process. This particular system of 

 floral stages distinguishes smafl increments of change in rate of floral 

 bud development. Furthermore, the numbers assigned to the stages are 

 such that increase in floral stage is linear with time starting approxi- 

 mately 2.5 days after the beginning of induction (providing en- 

 vironmental conditions are optimal). 



The conclusion drawn from the night length and floral stage experi- 

 ment is based upon two assumptions. First, it is assumed that increas- 

 ing amounts of flowering hormone cause an increased rate of bud 

 development. Secondly, it is assumed that an increasing length of the 

 dark period results in an increase in amount of flowering hormone pro- 

 duced. The truth of either assumption, in light of this experiment, im- 



