WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 187 



yello wish- green leaves. Leathery leaves and those of 

 Grasses should be chopped up, being boiled in water and 

 steeped in alcohol. The extraction is best carried on in 

 darkness ; the leaves may be left in a covered dish of 

 alcohol overnight ; filter, and place the filtered extract in 

 corked bottles. A wedge-shaped bottle (" indigo prism ") 

 should be used ; by its means one can examine different 

 thicknesses of the solution. 



(&) Note the colour of the solution on holding the 

 bottle up to the light, and on holding it against a black 

 surface : it is green by transmitted light, red by reflected 

 light. If a spectroscope is not available, obtain a con- 

 tinuous 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 re- 

 placed by dark bands. The most prominent dark band 

 appears in the red part, but if the solution is strong bands 

 will also be seen in other regions of the spectrum. Hence 

 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. 



Our experiments in 245 show, roughly, the relation 

 between the dark bands of the chlorophyll spectrum and 

 the light energy used in photosynthesis. 



(c) 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, dis- 

 solves out a bluish-green colouring- matter, leaving a yellow 

 substance dissolved in the alcohol. These two pigments 

 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. 



(d) 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), cork 



