CHAPTER XXXI 

 INITIATION OF FLOWERS 



Many plants bloom regularly only at certain seasons of the year. We all 

 know that some of them bloom much earlier during the growing season 

 than others. Without giving much thought to these familiar facts, we 

 might casually assume that a plant blooms when the temperature is just 

 right, or when it has reached the right age to bloom. But anyone who is 

 aware of the behavior of plants in greenhouses or in window boxes may 

 have noted that some of these plants likewise bloom only at certain sea- 

 sons of the year, or that they do not bloom at any season. 



Some of the plants that bloom regularly only during the summer may 

 bloom in winter if the room is lighted during the first half of each night. 

 Likewise, some that regularly bloom only in autumn, or in greenhouses 

 during the winter, will bloom in midsummer if they are placed in a dark 

 room from the middle of the afternoon until the next morning for several 

 successive days. 



Cocklebur plants that grow from seeds in late summer may be in 

 bloom two or three weeks later. Those that begin growth from seeds in 

 late spring may grow vegetatively for several months before the first 

 flowers appear. 



If the germinating seeds or seedlings of certain plants are exposed to 

 continuous low temperature for a few weeks, they may bloom sooner 

 than those kept warm at all times. In the eastern United States certain 

 varieties of apple and pear bloom abundantly in alternate years. When 

 facts such as those listed above are considered, one may feel that casual 

 explanations of why plants bloom when the>^ do may be inadequate or 

 even erroneous. 



All seed plants have a period of youth during which the growth of 

 roots, stems, and leaves precedes the fonnation of flowers and fruits. The 

 length of this purely vegetative period may vary from a few weeks to 

 many years, depending upon the species of plant and the habitat in 

 which it is growing. 



The occurrence of flower primordia in buds is the first visible evidence 



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