ECOLOGY AND THE FLOWERING PROCESS 25 



is SO variable at a given location from year to year. Many plants do 

 require the low temperatures of a cold winter for flower initiation. 

 Thus response occurs if there is a cold winter as contrasted with a 

 winter in which temperatures never even drop to the freezing point. 

 If a plant will not flower unless it has experienced a number of days 

 (a few weeks to a few months) of near freezing temperatures, then 

 it is protected from blooming in the fall, when sudden frosts might 

 stop growth of the flower. 



It is clear that response to the cold temperatures of winter might 

 be of strong ecological advantage, but flowering in response to 18° 

 instead of 22°C would seem to be of httle advantage to most plants. 

 Yet when our researches become sufliciently detailed we may discover 

 examples in which response to such slight changes is of ecological 

 significance. This could be the case with certain alpine plants, where 

 maximum day temperatures are commonly below 20°C and seldom 

 above. Most of our present knowledge, however, is concerned with 

 a response to near freezing temperatures extended over a considerable 

 period of time. We will consider this in the next chapter and reserve 

 the rest of this chapter for discussion of light and the flowering 

 process from the ecological standpoint. 



Flowering in Response to Natural Light (36) 



1 . Light as a Natural Variable 



Light varies in three ways : quahty (color or wavelength), intensity, 

 and duration. We might also think of light in a quantity or total 

 energy sense, considering both intensity and duration. High intensity 

 Hght applied for a short interval of time supplies a quantity of energy 

 which is only equalled at lower intensity by a long period of time. 

 All of these factors vary more or less continuously in nature and all 

 are important to the photoperiodism aspect of the flowering process. 



2. Natural Light Qualities and Flowering 



Perhaps quality of light varies the least in nature. Figure 3-1 

 illustrates the quality of sunUght, showing the intensity (relative 

 energy) at any given wavelength or color twice during the day. 

 Note that about half of the area under the curve falls within the 

 region to which our eyes are sensitive — the so-called visible light. 

 Nearly half, however, consists of longer wavelengths, the infra-red. 



