CHAPTER 6 



PREPARATION FOR RESPONSE TO 

 PHOTOPERIOD 



The higher plants will not flower, nor in many cases respond to the 

 environmental stimulus which leads to flowering, until they have 

 reached some minimal age; that is until they have reached "ripeness 

 to flower". The flowering machine must be constructed. Further- 

 more, experiments showing the necessity for high intensity light 

 (probably photosynthesis) preceding the inductive dark period seem 

 to indicate that an energy source is essential before the machinery of 

 flowering will begin to function. Ripeness to flower and the high 

 intensity light process, then, will be discussed in this short chapter. 



Juvenility and Ripeness to Flower 



(3, 13, 20, 24, 31, 37, 72, 76) 



Probably all of the common species used in studies on photo- 

 periodism and vernalization have been investigated as to the minimal 

 age at which they will respond to the environmental stimulus. As 

 mentioned in Chapter 4, the vernalization requiring cereals will 

 respond even before the embryo has reached maturity in the develop- 

 ing seed on the mother plant. The later requirement for long days, 

 however, can be satisfied only after they have produced a few leaves 

 which will respond to the long-day stimulus. The biennial strain of 

 Hyoscyamus niger must be 10 to 30 days old before it becomes 

 sensitive to vernalization. Chenopodium rubrum will respond to 

 short days and flower as a seedling on moist filter paper in a petri 

 dish. The cotyledons of Japanese morning glory respond to short 

 days, but the cocklebur is probably more typical, in that the cotyle- 

 dons and very young leaves (less than a centimeter in length) will not 

 respond, but true leaves longer than about 2 cm are sensitive (see 

 Fig. 5-5). On the other end of the scale are many trees which will 

 not flower until they are 5 to 40 years old, and certain bamboo 



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