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TEXTBOOK OF BOTANY 



Within the visible spectrum the radiant energy that we call red, orange, 

 yellow, and blue light is the most effective in photosynthesis. The color 

 of leaves is evidence that relatively more of the radiant energy we see as 

 green is reflected by chlorophyll. In land plants most of the photosyn- 

 thesis is brought about by the radiant energ\' of the red end of the 

 spectrum, but the radiant energy that penetrates the water to the depth 

 of deep-sea algae is mainly the blue end. The effectiveness of the blue 

 and the red rays is about equal when their intensities are equal ( Fig. 54 ) . 



4O0 ASO 500 5SO 600 650 700 TSO 



WAVE LENGTHS IN MILLIMICRONS 



Fig. 54. Relative rates of photosynthesis in different rays of Hght of equal intensity. 

 After B. S. Meyer and D. B. Anderson. 



Apparently only the radiant energy that is absorbed (transformed) 

 by chlorophyll is directly effective in photosynthesis. Any artificial light 

 that emits these particular rays may induce photosynthesis. Some plants 

 may start from seeds, grow to maturity, and produce flowers, fruits, and 

 seeds when exposed only to electric light at an intensity about one-tenth 

 that of full sunlight. Other plants will grow in this light intensity, but 

 without producing flowers and fruits. 



The lowest intensity of light at which photosynthesis will occur has 

 not been discovered. It has been reported to occur in deep-sea algae at 

 light intensities lower than that of full moonlight. 



A much more important fact to know is the light intensity from da>' 

 to day in which the plant can survive on the sugar made. For conven- 

 ience of reference we may call this the critical light intensity. This critical 

 light intensity varies when other external factors, such as temperature, 

 available moisture, and the concentration of carbon dioxide, are altered. 

 This fact will be demonstrated later. Among different plants there is a 



