314 CONTROL OF REPRODUCTION 



Fig. 2. The induction of flowering in Xanthium with a cold treatment. 

 Both plants were grown under long days of 16 hr of light. The plant at 

 the left was maintained at 23 °C. The plant at the right received 8 hr of 

 4°C temperature during the first 8 hr of the light period; temperature 

 during the rest of the time was 23 °C. Photograph taken 15 days after the 

 beginning of the treatments. 



show that factors other than photoperiod alone are able to regulate 

 flowering even in a species which has been regarded up to now as most 

 strictly photoperiodic. This is not an isolated case. The photoperiodic 

 behavior of many plants is greatly changed by temperature. This is true 

 not only of the flowering response, but also of dormancy (Downs and 

 Borthwick, 1956; Waxman, 1957), winter hardiness (Moshkov, 

 1935), and seed germination (Vaartaja, 1956). Thus, as has been 

 demonstrated in the case of Hydrocharis morsus ranae by Vegis 

 (1955), the photoperiodic mechanism may be operative inside certain 

 temperature limits only. 



REFERENCES 



De Zeeuw, D. 1957. Flowering of Xanthium under long-day conditions. Nature, 

 180, 588. 



Downs, R. J., and H. A. Borthwick. 1956. Effect of photoperiod on growth of 

 trees. Botan. Gaz., 117, 310-26. 



Moshkov, B. S. 1935. Photoperiodismus and Frostharte ausdauernder Gewachse. 

 Planta, 23, 774-803. 



Vaartaja, O. 1956. Photoperiodic response of germination of seed of certain 

 trees. Can. J. Botany, 34, 377-88. 



Vegis, A. 1955. Uber den Einfluss der Temperatur und der taglichen Licht- 

 Dunkel-Periode auf die Bildung der Ruheknospen zugleich ein Beitrag zur 

 Entstehung des Ruhezustandes. Symbolae Botan. Upsalienses, 14, 1-175. 



Waxman, S. 1957. The development of woody plants as affected by photo- 

 periodic treatments. Ph.D. thesis, Cornell University, Ithaca, N. Y. 



