Kinds of Photoperiodic Flowering Responses ■ 15 



periodically controlled. Flowering in such plants, which may 

 constitute the majority, has been relatively little studied. Com- 

 mon examples are tomato, Lycopersicon esculentum, and many 

 varieties of peas, Pisum sativum. 



Note that in this classification the distinction between SDP 

 and LDP is based not on the absolute values of the critical day- 

 lengths (which may range from four to over 18 hours for LDP, 

 for example); the distinction is whether flowering is promoted 

 by photoperiods shorter or longer than the critical. The critical 

 daylength for Xanthium, for example, is about \o l / 2 hours, and 

 that for Hyoscyamus about 11 hours. Yet the former is properly 

 classified as an SDP since it flowers on photoperiods shorter than 

 its critical value, whereas the latter is an LDP, requiring photo- 

 periods longer than its critical. It is necessary to belabor this 

 distinction since it is possible to find textbooks that should know 

 better implying that LDP flower with more hours of light per day 

 than SDP. Such statements miss the point. Both Xanthium and 

 Hyoscyamus flower with 14 hours of light per day. The daylength 

 in which a plant flowers is no indication of its response class in 

 the absence of further information. 



In addition to the classes of response described, the following 

 considerations should be recognized before proceeding further. 

 There are plants in which the appropriate photoperiodic treatment 

 is an absolute requirement for flowering under all naturally 

 occurring conditions. Neither Xanthium nor Hyoscyamus, for 

 example, ever flowers unless exposed to the proper photoperiodic 

 conditions. Such plants are referred to as having a qualitative 

 photoperiodic response, or requirement. In other plants, differing 

 photoperiodic conditions merely hasten or delay but do not abso- 

 lutely determine flower initiation. Such plants have a quantitative 

 response to photoperiod. There are also plants in which qualitative 

 or quantitative photoperiodic responses are observed only under 

 particular conditions of temperature or some other environmental 

 factors; these would be conditional photoperiodic responses. Still 

 other plants may require one photoperiodic condition for flower 

 initiation but a markedly different one for flower development. 

 Finally, there are many species in which the photoperiodic response 

 may change with age; such changes are usually in the direction 



