88 THE VEGETATIVE ORGANS 



Chlorophyll, the green colouring matter of plants, is 

 a very complex nitrogenous substance. As we have seen 

 (p. 81) it may be extracted by means of alcohol, and if 

 sections of leaves so decolorized are examined under 

 the microscope the corpuscles will still be found in the 

 cells (Fig. 29, 5 cli). In the living plant the pigment is 

 probably dissolved in some oil, and this solution is enclosed 

 in the meshwork of the corpuscles. Each chlorophyll 

 corpuscle or chloroplast, therefore, consists of (a) a proto- 

 plasmic body or plastid and (6) a pigment, chlorophyll. 



Light rays absorbed by chlorophyll. An alcoholic solution 

 of chlorophyll is fluorescent : if it is held up to the light 

 and examined it is green, but if examined against a dark 

 background it is blood-red. Examine a beam of light 

 by means of a spectroscope and note the band of colours 

 red, orange, yellow, green, blue, indigo, and violet. This 

 band is called the spectrum of white light. Place an 

 alcoholic solution of chlorophyll in the path of a beam 

 of light before it reaches the slit of the spectroscope ; 

 note the spectrum which results, and compare it with 

 that of white light. Observe the dark bands produced 

 and note carefully their position in the spectrum. Seven 

 vertical bands are produced, but some of them are difficult 

 to see. The darkest is at the red end of the spectrum ; 

 three fainter, but broader, bands occur at the blue end ; 

 the remaining three bands are much paler and occur in 

 the yellow and green. We thus see that the rays of light 

 falling on a green leaf do not all pass through it. Chloro- 

 phyll has the power of absorbing most of the red rays, 

 many of the blue and violet ones, and, to a much less extent, 

 some of the yellow and green. It is the energy thus 

 absorbed from the sun's rays that enables the chloroplasts 

 to carry on the constructive work of photosynthesis. We 

 are now able to understand why starch is not formed in the 

 white parts of leaves or in green leaves kept in darkness. 



