56 



COMPANION TO TROPICAL READERS 



Expt. 79. Plants take in Carbon Dioxide, split it 

 up, retain the Carbon, and give off the Oxygen. 



(a) Carbon Dioxide is present in the Air. Expose some 

 freshly prepared lime-water in a saucer and observe 

 the thin white film formed on the surface owing to the 

 action of carbon dioxide. 



(b) Carbon Dioxide gets into the Air by the Breathing of 

 Animals. Breathe into lime-water and the same preci- 

 pitate as found above will be formed, though in larger 



quantities. 



(c) Plants take in Carbon 

 Dioxide, forming Starch. 



1. Assimilation. Pick 

 a green leaf in the after- 

 noon of a day that has 

 been sunny, and test with 

 iodine. 1 Starch will be 

 found. 



2. Assimilation will not 

 take place- in the Dark. 

 Put a pot plant in a dark 

 cupboard for twenty-four 

 hours. Test with iodine; 

 no starch will be found. 



3. If Stomata are closed, no Assimilation will take place. 

 In the morning vaseline both sides of a leaf; pick it 

 in the afternoon; test with iodine, and observe that no 

 starch will be found. 



(d) Plants give off Oxygen from the Carbon Dioxide taken 

 in. Put some watercress or duckweed in a vessel of 



^Iodine Test for Starch in Leaves. After the leaf has been picked, it should 

 be put in boiling water for two or three minutes. It should then be bleached 

 by soaking in alcohol. Put the leaf, when more or less colourless, in a solu- 

 tion of iodine, prepared by dissolving a few crystals of iodine in alcohol or in 

 a solution of potassium iodide. If starch is present the leaf will be coloured 

 dark purplish. 



Fig. 25. Plant giving off Oxygen 



