THE LEAF AS A BUILDER. 15 



our health ; and it modifies heat which would other- 

 wise be overpowering. Step into the thick woods 

 from an open space on a very hot day, and imme- 

 diate relief is experienced from the intense heat. 

 This is not wholly the result of shade furnished 

 by the trees ; much of it proceeds from the modifi- 

 cation of the air through the breathing of the tree 

 leaves. These leaves not only absorb heat and sun- 

 light, but also carbonic-acid gas, and through tiny 

 channels transmit them to the growing wood fiber 

 of the tree. 



The fact is, a tree is built up far more by the sun 

 and the atmosphere than it is by the soil from which 

 it grows. In the delicate structure of the leaf, which, 

 upon close examination, we will see is composed of a 

 complicated net work of nervelike " veins," carbonic- 

 acid gas is broken up into carbon, which is retained 

 by the tree to form its woody structure, and into 

 oxygen, which is liberated and passes into the atmos- 

 phere. Each leaf, therefore, is a builder and an air- 

 regulator of a nature which is beneficial to us. Its 

 capacity for heat and sunshine is something astonish- 

 ing. I have estimated that a certain sugar maple of 

 large proportions, which grows near my cottage, puts 

 forth in one season about four hundred and thirty- 

 two thousand leaves ; these leaves combined present 

 a surface to sunlight of about twenty-one thousand 



