PHOTOSYNTHESIS AND RESPIRATION. 93 



Try the effect of interposing pieces of glass of various colours, or 

 bottles containing solutions of dichromate of potash and of copper sul- 

 phate. In each case certain rays of light are stopped, that is, absorbed, 

 and the places of these rays in the spectrum are occupied by dark bands, 

 that is, by darkness. 



We see now that chlorophyll absorbs certain light-rays, allowing 

 the rest to pass through it, and we may conclude that these absorbed 

 rays in some way supply the energy which is needed in carrying 

 on the work of photosynthesis. A direct-vision spectroscope will 

 show the absorption-bands, especially that in the red part of the 

 spectrum. A very useful additional piece of apparatus is a wedge- 

 shaped bottle ("indigo prism") by means of which one can examine 

 different thicknesses of the solution. 



(c) It will be noticed that the extreme end of the red region (that 

 nearest the invisible infra-red) is not absorbed by the chlorophyll 

 solution. If you hold up to the light a thick layer of strong leaf- 

 extract (using the thick end of an "indigo-prism" or a flat-sided glass 

 vessel) you will find it appears red instead of green. 



(d) Place some leaf-extract in a test-tube, dilute with a few drops of 

 water, then add benzol, shake, and allow to settle. The benzol, which 

 floats above the alcohol, dissolves out a bluish-green colouring-matter, 

 leaving a yellow substance dissolved in the alcohol. These two pig- 

 ments present in the extract can also be separated by using ether or 

 olive oil instead of benzol. Find out, by using the spectroscope, or 

 lantern and prism, which light-rays each of these substances absorbs. 



* (e) Fill three test-tubes with leaf-extract, cork them, and place A in 

 sunlight, B in diffused light, C in darkness. Carefully boil some extract 

 in a fourth test-tube (D) , and place it with A in sunlight. Notice, after a 

 day's exposure, that A becomes brown, C is unchanged, while B and D 

 are only slightly changed ; the absence of oxygen in D hinders the 

 destructive effect of light. 



(/) Add some 10 per cent, solution of copper sulphate to some leaf- 

 extract in two test-tubes ; a copper compound is produced which is not 

 red by reflected light, and which is not destroyed by light. Verify the 

 latter point by placing one tube in sunlight, the other in darkness, in 

 each case with a tube of ordinary leaf-extract for comparison 



127. Under what conditions is Chlorophyll pro- 

 duced? We have seen that seedlings grown in darkness 

 differ from those grown in the light in several respects. In 

 most Flowering Plants chlorophyll is not formed in dark- 

 ness ; the chloroplasts are present in the cells of the stem and 

 leaves of darkened seedlings (or of the shoots produced, say, 

 by a potato- tuber kept in darkness), but in the stems the 

 chloroplasts remain colourless, while in the leaves they be- 

 come yellow. Hence the stems remain white and the leaves 



