92 PHOTOSYNTHESIS AND RESPIRATION. 



126. Chlorophyll. Since only the green parts of plants 

 can carry on photosynthesis and the green colour of these 

 parts is due to chlorophyll, it is necessary to study the 

 properties of this substance. 



Chlorophyll is a colouring substance, or pigment, of a 

 complex nature; its composition is not yet fully known. 

 It contains magnesium and phosphorus in addition to carbon, 

 hydrogen, oxygen, and nitrogen. It is probably a mixture 

 of several pigments. 



In nature, chlorophyll is readily decomposed by bright 

 light, and it probably undergoes continuous changes, being 

 built up as rapidly, in ordinary conditions, as it is decom- 

 posed. It is a fluid or semi-fluid substance, produced in, 

 and held by, the special masses of protoplasm called chloro- 

 plasts. The chlorophyll present in a chloroplast only forms 

 about O'l per cent, of the latter's substance. 



Chlorophyll can be extracted by means of alcohol, ether, 

 chloroform, etc. A solution of chlorophyll is fluorescent it is 

 green by transmitted light, dark red by reflected light. When 

 the solution is held against the light and examined with a 

 spectroscope, or placed in the path of a beam of light, which 

 is then passed through a prism, the spectrum shows dark 

 bands, in the red, blue, and violet regions especially ; the band 

 in the red is very marked, appearing even if a weak solution 

 is used. These dark bands (also seen on examining a thin 

 green leaf with a spectroscope) are of course due to the 

 absorption by the chlorophyll of these rays of light, the other 

 rays being allowed to pass through the leaf. 



* (a) Extract chlorophyll from green leaves (e.g. Bean, Grass ; almost 

 any leaves will do, but leathery ones should be chopped up) by boiling 

 them in water, draining off the water, and covering the leaves with 

 alcohol. Then place the dish containing the leaves and alcohol in the 

 dark ; light destroys the colouring matter in the solution. Filter the 

 solution, and place it in a corked bottle. 



* (6) Notice the colour of the filtered extract by holding the bottle up 

 to the light, and by holding it against a black surface ; it is green by 

 transmitted light, red by reflected light. Obtain a continuous spectrum 

 on a screen by fastening on the lens of an optical lantern a card with a 

 vertical slit, and holding a prism in the path of the light. Hold a test- 

 tube of alcoholic chlorophyll-solution against the slit, and notice that 

 the colours in several parts of the spectrum are replaced by dark bands. 

 The most prominent dark band appears in the red part, but if the solu- 

 tion is strong bands will also be seen in other regions of the spectrum. 



