PHOTOPERIODISM 485 



Shiill (1929), working with the potato aphid, Macrosiphum solanifolii, 

 established that a cycle of 6 hr of light and 10 hr of darkness led to the 

 same effect as a cycle of 14 hr of light and 10 hr of darkness. Under 

 these conditions and at temperatures below 20°C, wingless forms pro- 

 duced wingless offspring, but on cycles of 12 hr of hght and 12 hr of 

 darkness almost all the progenies were winged. Thus dark periods 

 shorter than 12 hr, rather than long light periods, were required for 

 parthenogenetic reproduction as wingless forms. 



Effectiveness of the dark period is best shown by its interruption with 

 short light periods. The short-day plant Biloxi soybean, grown with 

 16-hr light periods, develops vegetatively but flowers when given cycles 

 of 8 hr of light and 16 hr of darkness. If the 16-hr dark periods are 

 interrupted after 8 hr for as short a time as 1 min with 160 ft-c of illumi- 

 nation, the plants remain vegetative (Emsweller et al., 1941). In the 

 case of Xanthium pensylvaniGum, flowering is induced on a 9-hr dark 

 period and a 15-hr light period, but if light of moderate intensity is given 

 for 1 min at the middle of the dark period, floral primordia do not develop 

 (Hamner and Bonner, 1938). Razumov (1941), working both with short- 

 day plants such as Perilla, Chrysanthemum, and various kinds of millet 

 and with long-day plants such as Avena saliva, found that the effective- 

 ness of the dark period was nullified by a short interruption. 



Hamner and Bonner (1938) found that a single long dark period leads 

 to floral induction of X. pensylvanicum, but that a single short light 

 period is without effect. In this species the dark period has to be greater 

 than 8.5 hr for flower formation irrespective of the length of the light 

 period. Thus plants on 16-hr light periods and 8-hr dark periods remain 

 vegetative, whereas those on 16-hr light periods and 32-hr dark periods 

 flower. 



Interruption of dark periods by short periods of light has also been 

 shown to be effective in controlling the photoperiodic response of some 

 animals. Thus Jenner (1951) found that a snail, Lymnaea palustris, 

 reacted as a long-day animal, producing eggs on 13.5-hr light and 10.5-hr 

 dark periods but faihng to do so on 11-hr light and 13-hr dark periods. 

 It produced eggs freely on 9-hr photoperiods when the accompanying 

 15-hr dark periods were interrupted near the middle for 1 hr with 60 ft-c 

 of hght. D. S. Hart found, according to Yeates (1949), that ferrets sub- 

 jected to interruption of long dark periods went into oestrus, again empha- 

 sizing the importance of the dark period. 



Intermediate plants such as Mikania scandens (AUard, 1938), and 

 Saccharum spontaneum (Sartoris, 1939) apparently require a dark period 

 of very precise duration. Parthenogenetic production of summer migrant 

 aphids by wingless forms might also be a response to dark periods of 

 intermediate duration. 



