158 THE FLOWERING PROCESS 



After a number of physiological experiments not unrelated to the 

 ones discussed in the above two sections, C. G. Gutteridge in 

 Scotland postulated that flowering in a strawberry variety occurred 

 in response to cessation of production of an inhibitor on long days. 

 Finally, along with P. A. Thompson, Gutteridge (75) tried the 

 critical experiment, successfully causing flowering by removal of the 

 leaves. It even appears possible that the long-day inhibitor in this 

 case might be related to the gibberellins. 



In conclusion, it appears that flower inhibiting substances are 

 clearly a reality, if only as a complicating factor. Of course, their 

 importance will depend strongly upon the species, but their presence 

 has been indicated by experiments with short-day, long-day and even 

 day-neutral (pea) plants. The cocklebur is one plant in which there 

 is no clear cut evidence for inhibitors in the flowering process, but 

 in this respect our "type" plant could easily be the exception rather 

 than the rule. 



Night Length and Flowering Hormone Synthesis 



One of the best ways to demonstrate the kinetics of flowering 

 hormone synthesis in a short-day plant is to treat diff'erent groups 

 with dark periods of different durations and then see how they flower 

 after a few days. Cocklebur plants which receive 8 hr of darkness 

 do not flower at all. Plants receiving only one 9-hr period of darkness 

 probably will flower, but the flower buds are very small (slow rate 

 of development) compared to plants which are given still longer dark 

 periods. Such an experiment has been discussed already in relation 

 to timing. Examples were shown in Figs. 3-9, 8-2, and 8-3. Two 

 complete curves obtained with a growth chamber are shown in 

 Fig. 9-2, and curves obtained at various temperatures are shown in 

 Figs. 9-5 and 9-7. The simple way to interpret this experiment is to 

 assume that virtually no flowering hormone is made until the end of 

 the critical dark period, and then the longer plants remain in the 

 dark, the more flowering hormone is synthesized. Under certain 

 conditions, as in the broken Une in Fig. 9-2, the first part of the curve 

 has a steep slope for about an hour (part A) and then a sharp break 

 to a much less steep slope (part B). Sometimes the curve is more 

 rounded as in the solid line of Fig. 9-2, but the two parts are still 

 clearly present. This is another instance where we reasonably assume 



