The Structure of a Tree n 



reached a large size before Julius Caesar or Hannibal 

 was born. One way in which a tree differs from a person 

 is that some of its parts are always young; for unlike 

 ourselves, trees grow as long as they live. 



Only part of a living tree alive. The outer bark of 

 an old tree is dead and a great part of the wood is dead. 

 But just as the dead mineral material that is found in 

 the bones of men and animals is useful in supporting 

 them, so the dead wood of a tree forms a skeleton and 

 holds the tree in place ; and as the dead outer layers of 

 our skin protect the living tissues beneath, so the dead 

 outer bark of a tree protects the living parts within. 

 The dead parts, of course, do not grow, but they form a 

 skeleton that is useful in supporting and protecting the 

 tree. Each year a new set of leaves, twigs, and small 

 roots are produced, and the part that has changed to 

 wood and died is buried within the tree to aid in its 

 support. You have seen trees with hollow trunks 

 whose crowns remained thrifty. In such trees it is not 

 the living part but the dead wood in the heart that has 

 decayed ; but a tree in this condition is more likely to 

 be blown down than one whose trunk is sound through- 

 out. 



The living material of a tree. The living parts of a 

 tree, like the living parts of every other plant and 

 animal, are composed of cells. In the young growing 

 parts the cells are shaped like little boxes fitted together 

 in an irregular manner. Each cell has a wall about it, 

 and the inside of the cell is composed of living material 

 called protoplasm, which in appearance is something 

 like raw white of egg. Besides the protoplasm there is 



