94 PHOTOSYNTHESIS AJSTD RESPIRATION. 



turn yellow. The yellow colour of the leaves is due to the pre- 

 sence of a pigment called etiolin, developed in the chloroplasts 

 instead of chlorophyll, and the plant is said to be etiolated. 



It is easy to prove that chlorophyll is formed in light too 

 weak to allow of photosynthesis. It is produced in shaded 

 places with light-intensity of less than O'OOl that of full sun- 

 light, though very few flowering plants can thrive in such 

 feeble light, e.g. in very dense Beech woods. 



What other conditions, if any, are essential (in addition 

 to light) for the formation of chlorophyll ? It is easy to 

 discover by experiment whether warmth, oxygen, and carbon 

 dioxide, for instance, are required. The presence of iron is 

 essential, but it is not so easy to prove this ; sometimes one 

 comes upon a case in which the leaves are pale and sickly 

 (chlorotic) owing to lack of iron, and can cause the production 

 of chlorophyll by adding iron to the plant's food (e.g. in 

 water-cultures, Art. 143) or by applying a weak solution of 

 an iron salt to the leaves with a camel-hair brush. 



* (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 distinct 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, Bean, 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 

 10C., 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. 



* (d) To show that oxygen is necessary for the formation of chlorophyll, 

 fill a test-tube with water, invert it in water, and pass under its rim 

 some etiolated Mustard seedlings. Though exposed to light, the seed- 

 lings do not become green, owing to lack of oxygen. Another method 

 is to place heavier seedlings e.g. Bean, Pea in a glass jar and cover 

 it with water. In each case similar etiolated seedlings should be 

 placed on wet blotting-paper at the bottom of a jar, whose mouth must 

 of course be left open. 



