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TEXTBOOK OF BOTANY 



summer and to short days in late summer and autumn without being 

 transferred. What is the explanation of the differences in the bloom- 

 ing of the cocklebur plants mentioned in the third paragraph of this 

 chapter? 



Finally, there is a third group of plants, in which the initiation of 

 flowers occurs during either long or short photoperiods, although it may 

 occur more readily in one type of photoperiod than in the other. These 

 plants are called day-neutral plants. They may bloom any time during 



Fig. 144. Photographs of a cabbage plant kept in a greenhouse at about 67° F. 

 At the end of two years plant (A) was more than 6 feet tall and had borne 4 

 compact heads of leaves at the points indicated by the figures 1, 2, 3, 4. At the 

 end^of 2.5 years it had borne 6 heads (B). When it was transferred to a cool 

 (55° F.) greenhouse, flower primordia developed and the plant bloomed within 

 a few months (B). Photo from H. C. Thompson. 



