CHAPTER IV 

 THE DETERMINING FACTORS IN PHOTOSYNTHESIS 



Forever . . . shines that high light whereby the world is saved. 



— John Hay. 



Light. — From what has been said in the previous chapter, 

 it is not surprising to learn that light plays a highly important 

 part in photosynthesis. The energy stored up in foods comes 

 from the energy of light. As a dynamo is the machine through 

 which the energy of falling water is transformed into that of 

 electricity, so the green plant is a machine which transforms the 

 energy of light into the energy of carbon compounds. 



Light Quality. — In any discussion of light, both the intensity 

 (brightness) and quality or wave length (color) must be considered. 

 If pure white light is passed through a prism, it is split up into 

 the seven colors of the visible spectrum with violet at one end and 

 red at the other. If this same sunlight is passed through the prism 

 of a spectroscope, about 750 dark bands or lines are found in 

 the spectrum obtained. The most conspicuous of these were 

 designated by Frauenhofer (after whom they were named) with 

 the letters A-H and are used as convenient reference points in 

 the spectrum. The wave lengths of the visible spectrum rays 

 vary from 0.00076 mm. in the red end to 0.00039 mm. in the 

 violet. Rays of corresponding greater and shorter wave lengths 

 have been found in the invisible infra-red and ultra-violet portions 

 of the spectrum. By quality is hence meant the wave length, of 

 which the color is a function. 



Now if a thermometer is placed in the various regions of the 

 visible spectrum, the highest temperature is obtained at the red 

 end between lines A and B. From here the temperature gradually 

 diminishes towards the blue end, whereas it increases to a maxi- 

 mum in the invisible infra-red portion beyond line A. Chlorophyll 

 is most rapidly decomposed in the orange-red region between 

 lines B and C, and more light is also absorbed by chlorophyll in 

 this region than in any other. Although this part of the spectrum 

 is most important as far as chlorophyll is concerned, not all re« 



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