THE LEAF 97 



water. Leave them there for several hours, and then 

 test leaves from both for starch. It should be found 

 that those exposed to the air have formed starch, whilst 

 those under the water and consequently deprived of air 

 have formed none. 



Another way of carrying out the above experiment 

 is to take a plant the leaves of which have been found to 

 have the pores only on the underneath surface. Place 

 it in the dark until free from starch. Then smear the 

 under sides of some leaves with vaseline. This fills up 

 the pores and prevents air entering the leaf. Expose 

 the plant to sunlight, and after some hours test for starch 

 both coated and uncoated leaves. The latter should be 

 found to contain starch, but the former to contain none. 



We can now pursue our inquiry one step further, and 

 endeavour to ascertain what constituent of the air is 

 actually necessary for the formation of starch. There 

 are two chemical substances, soda lime and caustic 

 potash, which have the power of absorbing carbon 

 dioxide, and by using these we can obtain an atmosphere 

 in which oxygen and nitrogen are both present, but 

 carbon dioxide is not. Let us do so, and see if under 

 these circumstances a plant can form starch. This 

 experiment can be arranged as follows : 



Take two bottles of clear glass (not tinted) with wide 

 mouths ; in each put a tightly-fitting cork with a bent 

 glass tube, about half an inch in diameter, passing 

 through it. In one of the tubes put some lumps of soda 

 lime, but leave the other empty. In the bottom of the 

 bottle fitted with the tube containing soda lime, place a 

 little dish with some pieces of caustic potash in it. This 



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