32 THE FLOWERING PROCESS 



horizon. Approximate times are shown for 40° latitude. A typical 

 curve for a cloudy day is also indicated. 



As further background material for the discussion of flowering 

 response to twilight, we need some more of the information which 

 is to be discussed in Chapter 7. Plants respond to light or its absence 

 through a reversible pigment system called phytochrome. The 

 situation may be simplified for this discussion as follows : when the 

 pigment system is illuminated with natural light, it is driven in one 

 direction; at the same time metabolic processes are tending to 

 convert it back to the first condition. We can enter light in the 

 following equation by thinking of it in a quantitative sense, a given 

 number of light quanta2 reacting with a given number of pigment 

 molecules : 



n. Einstein's red light + 



m.moles R-phytochrome — >-m.moles^F-phytochrome 



Metabolically 



Thus in a given interval of time, illumination with a given intensity 

 will produce a certain amount of F-phytochrome. In the same interval 

 of time another amount of F-phytochrome will be reconverted to 

 R-phytochrome by metabolic processes (we will see in Chapter 7 that 

 far-red light also converts F-phytochrome to R-phytochrome, but 

 this can be ignored for the moment). Thus the amount of F-phyto- 

 chrome present at any time will reflect the rates of the conversions 

 brought about in one direction by light and in the other direction by 

 metabolism. When the amount of F-phytochrome is great the plant 

 "knows" it is in the light (its biochemistry is adjusted to the light 

 status by the presence of F-phytochrome); when the amount of 

 F-phytochrome is small the plant "knows" it is in the dark. 



In the evening, then, as light intensity decreases, a point will be 

 reached at which metabolic removal of F-phytochrome begins to 

 exceed its production by light, after which F-phytochrome will 

 gradually decrease. The rate of decrease after this point will depend 

 upon how fast light intensity is decreasing: if the lights are turned off" 



2 Avogadro's number (the number of molecules in a gram molecular weight 

 of an element or compound; 6.023 x lO^^ molecules per mole) of light quanta 

 or photons is called an Einstein. 



