WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 185 



with a spectroscope (an excellent direct-vision spectroscope can be 

 had for 25s. ), or with a lantern and prism. 



(b) A better method is to use coloured solutions (which should be 

 examined with the spectroscope to see which rays they absorb and 

 transmit) in flat bottles, instead of using coloured glass, in the pre- 

 ceding experiment. " Aniline scarlet " dye may be used for the red ; 

 a solution of potassium dichromate for orange-yellow ; a mixture of 

 ammoniacal copper sulphate and potassium dichromate for green ; 

 and ammoniacal copper sulphate solution for blue. 



(c) In Fig. 42 the light-screen is shown fitted with a series of five 

 corked glass vials, four of them containing the red, orange, green, 

 and blue solutions just mentioned, and the fifth filled with water as 

 a control. Between the vials tinfoil is placed, so that on five strips 

 of the starch-freed leaf there fall rays of red, orange-yellow, green, 

 blue, and white light respectively. 



(d) Another method is to use a pair of double-walled bell-jars. 

 One is filled with watery solution of potassium dichromate, which 

 allows the red, orange, and yellow rays to pass but absorbs the rest. 

 The other is filled with watery solution of copper sulphate to which 

 ammonia has been added ; this solution transmits blue and violet 

 but absorbs the rays of the red end of the spectrum. In this way 

 we can at any rate divide the spectrum into a red-end half and a 

 blue -end half. A starch-freed leaf or leafy shoot in a bottle of 

 water, or a pot plant, is set below each bell-jar, the edges of which 

 should rest on a damp cloth or on sawdust, so as to exclude white 

 light. Set both jars in diffuse light, and after several hours test 

 each leaf for starch. Put a thermometer under each jar, and note 

 the temperature registered in each case. 



(e) A makeshift double-walled jar can befitted up as follows. Get 

 two large wide-mouthed jars, and two narrower jars each of which 

 can be placed inside one of the large jars. Partly fill one large jar 

 with potassium dichromate solution, the other with ammoniacal 

 copper sulphate solution ; into each lower the smaller jar, placing 

 in the latter some shot or stones to sink it in the solution. Then 

 place in each of the small jars a starch-freed Tropaeolum leaf in a 

 test-tube of water, and securely seal the necks of the jars. Home- 

 made double- walled jars of this kind answer almost as well as those 

 sold by dealers in glass apparatus. 



246. Chlorophyll essential for Photosynthesis. 

 Some of our experiments have suggested that only green 

 (chlorophyll-containing) tissues, organs, or plants are able 

 to use the carbon dioxide of the air and to manufacture 

 carbohydrates. 



(a) Keep in darkness, until the leaves are starch-free, 

 some plant, or a cut shoot, which has variegated leaves 



