the low temperature promotion of flowering 67 



Direct Effects of Temperature on Flowering 



In a broad sense, vernalization could be defined as any promotion 

 of flowering by low temperature, and such a definition would be as 

 good as any if it were accepted by workers in the field. There is a 

 current tendency, however, to limit use of the term to delayed or 

 inductive effects (see above and Chapter 2), although it is certainly 

 not easy to draw the line between direct and delayed effects in all 

 cases. Following current opinion, the two cases of flowering response 

 to temperature discussed in this last section are not to be considered 

 as vernalization. As a matter of fact, flower formation in the bulbs 

 often occurs in response to relatively high temperatures (and in one 

 case at least, this is inductive!) and thus cannot be considered vernali- 

 zation under any presently acceptable definition. 



1. Direct Flower Formation in Adult Plants 



There are a few instances in which mature plants must be exposed 

 to low temperatures if flowering is to occur, and the formation of 

 flower buds can be observed to occur during the cold treatment 

 instead of later. Brussels sprouts (Brassica oleracea gemmifera) were 

 studied in some detail by Pearl Stokes and K. Yerkerk (72) in 

 Wageningen, Holland, and according to these workers other species 

 responding this way include cabbage, sweet turnip, foxglove 

 (Digitalis purpurea), stocks (Matthiola), and Sweet William (Dianthus 

 barbatus). 



To begin with, response to the cold treatment is dependent upon 

 the Brussels sprouts plants first reaching ripeness to flower. The 

 presence of a juvenile phase is not unusual (see Chapter 6), but in this 

 case Stokes and Verkerk found that they could recognize ripeness to 

 flower by morphological changes in the bud. Following this stage, 

 the plants are then ready to respond to 3 weeks or more of low 

 temperature (3°C), after which flower primordia begin to develop. 

 If no primordia are evident when plants are returned to higher 

 temperatures, flowering will not occur. The extent of flowering is 

 dependent upon the length of the cold period (6 to 9 weeks produce 

 full bloom), and there seems to be no interaction with day-length so 

 far as flowering is concerned. 



Of course, the extent of flowering will be modified by the rate of 

 elongation (rate of bolting) of the stems after plants are returned to 



