VEGETABLES. 9t' 



Througn these, the sap, which may be regarded as the 

 blood of the tree, flows upward into the leaves. The 

 leaves may be compared to the lungs of animals. The 

 office of the former is to bring the sap and the. air 

 into contact, as that of the latter is. to bring the air 

 into contact with the blood. As the blood is strikingly 

 modified and changed in the lungs, so is the sap in the 

 leaves. The lungs are a net- work of blood and air 

 vessels surrounded by a membranous tissue. So also 

 the^ leaves are a net- work of woody fibre, continued 

 from the leaf-stem, and covered above and below with 

 a spongy membrane. The upper side of the leaf 

 emits gases and vapor into the air ; the under side 

 gathers in from the air for the nourishment of the 

 plant 



161. When the sap has circulated through the leaf, 

 it commences a retrograde course towards the earth. 

 It is not always a downward course. That depends 

 upon the position of the limbs. Its return to the earth 

 is by the inner bark ; and its depositions by the way 

 form the annual layer of wood. 



DECAY AND PRODUCTS OF PLANTS. 



162. In the present order of things, whatever lives, 

 must die. Men, brutes, and plants, live on their pre- 

 decessors. The floating matter of the universe is un- 

 dergoing a succession of life and. death. Probably all 

 the dead matter around us, all that we can see, has 

 been alive some time, much of it a thousand times. 

 The succession of living beings, vegetable and animal, 



