INITIATION OF FLOWERS 



345 



[Chap. XXXI 



the growing season. Hybrid roses, chickweed, dandelion in gardens and 

 lawns, and tomatoes and buckwheat in greenhouses are familiar examples 

 of day-neutral plants. 



Temperature. Both the degree and the duration of any particular de- 

 gree of temperature are important factors in the development of plants. 

 Under certain conditions the initiation of flowers may be influenced more 

 bv temperature than by the length of day. Some species, such as beets, 



Fig. 145. Effects of temperature on vegetative growth and the formation of 

 fiowers in celery. The four plants pictured on the left grew in a cool (60° F.) 

 greenhouse; at the same time those on the right grew in a warm (70° F.) green- 

 house. Photo from H. C. Thompson, Cornell University. 



stocks, and celery, do not bloom when kept continuously at a relatively 

 high temperature in greenhouses. Cabbage, a biennial, may grow vege- 

 tatively as a perennial if kept continuously at 70° F. ( Figs. 144 and 145 ) . 

 Some plants of temperate climates when transported to the tropics 

 similarly remain vegetative. Low temperature delays the time of flower 

 formation in some species and shortens it in others. In many species the 

 initiation of flowers occurs sooner at a moderate temperature than at 

 either a low or a high temperature. 



