18 • Photoperiodism: An Outline 



to Xanthium; it has been reported also in the Japanese morning 

 glory, Phcrbitis (or Ipomoea) nil (Imamura and Takimoto, 1955a), 

 a duckweed, Lemna perpusilla (Hillman, 1959a), and pigweeds, 

 Chenopodium (Gumming, 1959), all SDP. Many other SDP also 

 can be induced by 2 to 10 days of the appropriate photoperiodic 

 treatment. Induction by a very few cycles is perhaps less common 

 among LDP, although at least dill, Anetham graveolens (A. W. 

 Naylor, 1941), and mature plants of the grass Lolium temulentum 

 (Evans, 1960) are both inducible by one long-day cycle. 



THE CENTRAL ROLE OF THE DARK PERIOD 



While the terms "short-day" and "long-day" plant have been 

 maintained by constant usage, probably the most important single 

 difference between these two response classes is in their reactions 

 to the nightlength, or dark period. In general, flowering in SDP 

 is promoted by certain reactions taking place during the dark 

 periods, and the "critical daylength" actually represents the maxi- 

 mum daylength that will allow a dark period of sufficient length 

 in a normal 24-hour cycle. In LDP, on the other hand, dark periods 

 have an inhibitory effect on flower initiation, and the critical 

 daylength is thus the minimum which in a 24-hour cycle will keep 

 the dark period short enough to allow flowering. These generaliza- 

 tions are supported by the fact that LDP usually flower best on 

 continuous light, so that apparently the entire role of the dark 

 period is inhibitory (A. W. Naylor, 1941; see Lang, 1952). Several 

 SDP, on the contrary, flower in continuous darkness if they are 

 given sucrose (see Doorenbos and Wellensiek, 1959; Hillman, 

 1959a), suggesting that light is unnecessary if its photosynthetic 

 function is replaced by another source of carbohydrate. However, 

 at least one LDP, spinach, Spiiuicia oleracea, also flowers in total 

 darkness when supplied with sucrose (GentschefT and Gustaffson, 

 1940) so that reliance on this sort of evidence alone is undesirable. 



Hamner and Bonner (1938) were able to show that in 

 Xanthium the critical time for an appropriate photoperiodic 

 treatment lay in the dark period length. When 24-hour cycles of 

 light and darkness were used, these plants flowered with dark 

 periods of 8% hours or longer. Thus the critical daylength was 

 15% hours. No flowering occurred on schedules of 16 hours light- 



