316 CONTROL OF REPRODUCTION 



vernalization requirement, it has sometimes been referred to as "SD- 

 vernalization" (Lang, 1952). The data of Gott et al. indicate that SD 

 induction is possible only when vernalization is incomplete, that is, the 

 two treatments are interchangeable, and perhaps the term "SD-vernali- 

 zation" is acceptable here (although Gott et al. believe that vernaliza- 

 tion and SD induction act upon different steps in the synthesis of the 

 floral stimulus); however, McKinney and Sando (1935) have clearly 

 demonstrated that vernalization and SD induction were not additive in 

 winter wheat, even after 54 days of vernalization, and in this case "SD- 

 vernalization" does not properly describe the action of SD induction or 

 of the cold treatment. 



Resende (1952) recently reported that floral initiation in several 

 species of the Lihaceae and Crassulaceae required a sequence of LD 

 followed by SD induction, and he suggested that these plants be classi- 

 fied as long-short day plants (LSDP). Since then, Cestrum noctumum, 

 a member of the Solanaceae, has also been described as a LSDP 

 (Sachs, 1956a). Cestrum will remain vegetative if grown continuously 

 in LD or SD conditions, the sequence of LD followed by SD cannot 

 be reversed, and there is no morphological change at the terminal or 

 axillary buds until sometime after SD induction is completed. 



To the author's knowledge these are the only descriptions of plants 

 with a dual day length requirement for floral initiation, although it has 

 been suggested that the intermediate-day plants described by Allard 

 (1938)"may be specal cases of either SLDP or LSDP (Sachs, 1956a). 



One of the most important criteria used in recent years to classify 

 plants as either LDP or SDP has been the light interruption of the 

 dark period associated with short photoperiods (on a 24-hr cycle) ; the 

 light break inhibits floral initiation in SDP and promotes it in LDP 

 (Lang, 1952). In the few cases in which this criterion has been applied 

 to SLDP and LSDP, it has been shown that both the SD and LD induc- 

 tion requirements are comparable with those in SDP and LDP, re- 

 spectively (Resende, 1953; Gott et al, 1955; Sachs, 1956a). Un- 

 fortunately action spectra for the light-interruption phenomena in the 

 dual day length-requiring plants are not available to compare with 

 those for barley, Hyoscyamus and Xanthium (Borthwick et al, 1948b; 

 Parker et al, 1950; Parker et al, 1946), and it cannot be said with 

 certainty that the photoreceptor is the same in all cases. In two respects 



