188 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



the tube, 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. 



(e) 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 com- 

 parison. 



248. Conditions essential for Formation of Chloro- 

 phyll. We have seen that seedlings grown in darkness 

 have no chlorophyll ; their leaves are yellow, owing to the 

 presence of etiolin in the plastids. That iron is essential 

 is shown by means of water cultures ( 221). 



(a) Grow seedlings, e.g. Cress or Mustard, in darkness, 

 then place some of them in a good light, close to a window, 

 and note the time required for the production of a dis- 

 tinct green colour. Place the others in a dark part of the 

 room, and when they have become green test the leaves 

 for starch. These observations will show that (a) a green 

 tinge, due to formation of chlorophyll, may be developed 

 in an hour, or less, in good light ; (6) light too weak for 

 photosynthesis is strong enough for the production of 

 chlorophyll. 



(6) Sow in the same pot or box some seeds of Pine and 

 of Bean or Pea, keeping them in darkness, and compare 

 the colour of the Pine- seedlings with that of the others. 



(c) Place some etiolated seedlings (Cress, Mustard, 

 Beau, etc.) in a bottle or small glass jar, cover with a glass 

 plate, and set it in a larger jar half filled with water. 

 Keep the water at 30 C. In a similar apparatus keep 

 some of the seedlings in cool water, or water kept at 10 C., 

 by adding bits of ice from time to time. Compare the 

 depth of the green colour developed in the two sets of 

 seedlings after an hour or two of exposure to light. 



