24 THE LEAF 



thus treated, in a weak solution of iodine for half an hour ; 



then wash them and hold them up to the light. Iodine 

 turns starch blue; hence if there 

 are any blue spots on the leaves, 

 what are you to conclude ? Other 

 food substances can be detected 

 by proper tests, but none of them 

 so readily as starch. 



27. Necessity of Light and Air. 

 Exclude the light from parts of 

 healthy leaves on a growing plant 

 of tropaeolum, bean, etc., by plac- 



9 ~-Leaf arranged with a 8 bands r P atcheS f . *** f U 



piece of tin foil to exclude light over them. Leave in a bright win- 



:ace - dow, or preferably out of doors, for 



twenty-four to forty-eight hours, and then test for starch as 



in the last experiment ; do you find any in the shaded spots ? 



Cover the lower side of several leaves with vaseline or 

 other oily substance so as to exclude the air, and after a 

 day or two test as before. 



From these experiments we learn that leaves can not do 

 their work without light and air. The particular element 

 of the atmosphere used by them in the process of food 

 making is carbon dioxide, a poisonous gas that is being 

 constantly produced by the decay of vegetable and animal 

 matter, by the respiration of animals, and by combustion of 

 all sorts. It constitutes about one fourth of one per cent 

 of our atmosphere, and when the proportion rises much 

 above this, the air becomes unfit to breathe, so that the 

 work of plants in eliminating it is a very important 

 one. 



28. Respiration. The leaf is also an organ of respira- 

 tion ; that is, it is always taking in oxygen and giving off 

 carbon dioxide, just as animals do, but in such small quan- 

 tities that the process is entirely obscured during the day 

 by the much more active function of photosynthesis, or 

 food making, which goes on at the same time. For this 



