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Much more striking, however, is the fact that the initiation of flowers 

 may occur much sooner in some plants if the germinating seeds or the 

 young seedhngs are exposed to a low temperature for a short time. For 

 example, wheat plants bloom sooner if the germinating seeds are ex- 

 posed to a low temperature. Celery and beets, which are regularly 

 biennials in the latitude of New York, grow as annuals if the seedlings 



Fig. 146. Preconditioning effects of temperature on the development of flower 

 primordia in cabbage plants. The four plants pictured above grew in the field 

 during Summer. The two on the right were transferred directly from the field to a 

 warm greenhouse, while the two on the left were brought from the field and kept 

 at 40° F. for 2 months before they were placed in the warm greenhouse. Photo 

 from H. C. Thompson. 



are exposed to a temperature of 40° F. for about one month. Some of the 

 effects of temperature at these early stages of development persist in 

 the plant as it continues to grow and finally result in an earlier initiation 

 of flowers (Figs. 146 and 147). This phenomenon may be referred to as 

 preconditioning. That is, the plants bloomed sooner because they were 

 preconditioned by a low temperature during some earlier stage of their 

 development. Likewise, a similar preconditioning occurs when short-day 

 plants are exposed to a long dark period. Our present knowledge of 



