12 THE FLOWERING PROCESS 



when the day-length is exactly right, flowering ensues. Examples of 

 the converse situation are also known. 



Another complication was discovered in the early part of the 

 1930's, but it has been largely ignored until recent years. Thus some 

 plants require a combination of day-lengths: either short-days 

 followed by long-days or long-days followed by short-days. Again 

 it appears that there is an absolute and a quantitative response; some 

 plants require this combination, and others are only promoted by it. 



Even before the discovery of the day-length response it was known 

 that some plants were promoted in their flowering by exposure to 

 low temperature. Work since that time has revealed a rather bewilder- 

 ing series of responses of this type. Some plants have an absolute 

 requirement for a low-temperature treatment. Others may flower 

 without the low-temperature treatment but are quantitatively pro- 

 moted by it. Some respond to lower temperatures by immediately 

 developing flower buds; others produce flowers only after they are 

 returned to warm temperatures. Still others are promoted in their 

 flowering by exposure to warm temperatures or to alternating 

 temperatures. 



In addition to the flowering response to a specific day-length or to 

 a specific temperature treatment, there are a number of interactions 

 between day-length and temperature. As one might expect, the 

 measurement of the day-length or the dark period by the plant is 

 influenced by the temperature (but much less than one might expect). 

 In addition, there are some rather striking effects, such as replace- 

 ment of a day-length requirement by a given temperature treatment, 

 or an absolute day-length requirement at one temperature and no 

 day-length requirement at another. Furthermore, any given plant 

 may have virtually any combination of day-length or temperature 

 requirements along with any of the interactions between day-length 

 and temperature. 



The Dimensions of Response Type 



Thus any classification of the response types becomes a formidable 

 task. A preliminary classification recently appeared in a paper by 

 P. Chouard (13) who is at the Sorbonne in Paris. He considered an 

 absolute requirement for low temperature, a partial requirement 

 (which he divided into "great" and "small"), and no requirement. 



