THE MANY RESPONSE TYPES 19 



Q, B. Increasing temperature during the day-length treat- 

 ment increases the length of the critical dark period. 

 The critical dark period may be defined as the mini- 

 mum time length in darkness which will promote the 

 flowering of short-day plants or inhibit the flowering 

 of long-day plants. 



O* C. Increase in temperature decreases the length of the 

 critical dark period. Probably the majority of day- 

 length responding plants would fit in this category if 

 sufficient work had been done to permit their classifi- 

 cation. Placement in the last category (III. LB) is very 

 rare, and on theoretical grounds it is not to be 

 expected. 



2. Temperature influences day-length response type when 

 applied during the day-length treatment. 



A. Most qualitative or absolute requirement at low tempera- 

 ture. 



^(gf (a) High temperature: quantitative. This means 

 QT) that a plant has an absolute requirement for 



either short- or long-days at relatively low 

 temperatures, but at high temperatures short- or 

 long-days only promote flowering but are not 

 essential. 

 dfi^ (b) High temperature : day-neutral. The plant may 

 #♦ either require or be promoted by either short- 



C"* days or long-days at low temperatures, but at 



O* high temperature it is day-neutral. 



#Xr) (c) High temperature: response changes. Such a 

 #^0 plant might require short-days at low tempera- 



•O ture and require or be promoted by long-days at 



CX# high temperature. This category could, of 



CyW course, be easily subdivided on the basis of 



Ow quantitative and qualitative response. It is 



assumed that the plant might be day-neutral at 

 the intermediate temperature, although an inter- 

 mediate day requirement cannot be eUminated. 

 Of course this category is very interesting, with 

 only limited examples. 



