338 Morpho genetic Factors 



temperature for growth may therefore be different under different con- 

 ditions. This thermoperiodism affects growth in various ways (Went, 

 1944, 1945, 1948). Went found in tomato, for example, that if the green- 

 house temperature is held constant the optimum is about 26.5°C, at which 

 there is a steady growth in length of 23 mm. per day. At all other tem- 

 peratures, growth is less (Fig. 15-2). Plants kept warm (26.5°) during 

 the day, however, but cooler at night ( 17 to 20° ) grow still better, about 

 27 mm. per day. It is significant that this low temperature is effective only 

 if it is applied during the dark period of daily growth. The same thermo- 

 periodism is evident in fruit development, the best fruit set occurring 

 when the night temperatures are 15 to 20°. Evidently two different 



3-5P 



30 



X 



(II 



c 



01 



a 



<T3 



25 



-5 20 



1 5" 



J L 



-L 



a L 



Fig. 15-1. The effect of low and of 

 high night temperature on shape of 

 successive leaves in Ipomoea caerulea. 

 Shape index measures degree of lob- 

 ing. ( From Njoku. ) 



23456789 

 Leaves, numbered from base 



processes are involved, one going on in the light and the other in the 

 dark, and with different temperature optima. Plants differ considerably 

 in their response to thermoperiodism ( Knapp, 1956 ) . Sproston and Pease 

 ( 1957 ) have shown that thermoperiodism is related to the production of 

 the sexual stage in the fungus Sclerotinia. 



There is a close relation between temperature and photoperiodism, 

 for it has frequently been shown that a particular temperature can be 

 substituted for day-length in determining the balance between vegetative 

 growth and flowering. Thus in Rudbeckia, a long-day plant, flowers are 

 produced in shorter days if the temperature is kept high (Murneek, 

 1940). Flowering in beets can be controlled by manipulating the relation 



