64 • Temperature and Flowering 



lower than those optimal during the light period (see Went, 1957). 

 Work on this question will not be dealt with here, since relatively 

 little of it directly concerns flower initiation. In addition, the 

 interactions of temperature changes with high-intensity light 

 periods of different lengths are extremely complex and have not 

 been carefully analyzed. Many of the data do suggest, however, 

 that "typical" vernalization, the effects of moderately low tempera- 

 tures, the effects of varying day and night temperatures, and the 

 interactions of temperature with photoperiod (Chapter Two) all 

 intergrade. 



Recall in this connection the observation of Schwabe (1955b, 

 1957) that discontinuous vernalizing cold treatments were more 

 effective on Chrysanthemum when given during each night rather 

 than in the day. This sounds very much like thermoperiodism. 

 Note also that the tomato, Lycopersicon esculentum, in which 

 major effects of temperature have been studied as thermoperiodism, 

 is quantitatively vernalizable; exposure of the seedlings to tempera- 

 tures near 10° C soon after cotyledon expansion significantly de- 

 creases the number of leaves formed before the first inflorescence 

 and increases the number of flowers in that inflorescence (Wittwer 

 and Teubner, 1956). Since one effect of low night temperatures is 

 also to increase the number of flowers per inflorescence (Went, 

 1957, Chap. 6), vernalization in the tomato, as in Chrysanthemum, 

 is perhaps not completely distinguishable from thermoperiodism. 



A further expansion of the phenomena that need to be con- 

 sidered in connection with vernalization is suggested by some 

 work of Guttridge (1958). By the definition previously given, 

 vernalization results in the promotion of flowering. However, a 

 cold treatment affects certain varieties of strawberry (Fragaria) in 

 the opposite fashion, inductively bringing about a condition in 

 which flower initiation is delayed and runner production promoted 

 when the plants are subsequently transferred to conditions that 

 would otherwise make for continued flowering and low vegetative 

 growth. This effect is certainly formally similar to vernalization, 

 though inverse in result. 



TEMPERATURE AND FLOWERING IN BULB PLANTS 



Among the most detailed studies yet done on temperature 

 and flowering are those of Blaauw, Hartsema, Luyten, and their 



