148 BOTANY FOR YOUNG PEOPLE. 



E. They are generally very easily distin- 

 guishable. Here is a piece of fire wood ; how 

 old is it ? 



L. There are twenty-two rings, it is twenty- 

 two years old. 



E. Examine the other end ; how many rings 

 in that? 



L. Twelve. 



7! E. Twelve from twenty-two, leaves ten ; it 

 was, consequently, ten years growing the length 

 of this piece. 



L. But does it ever stop growing ? 



E. Yes ; for at last the inside layers become 

 so extremely hard as to be no longer capable of 

 yielding to pressure, it is then perfect wood, be- 

 fore which it was alburnum or white wood, so 

 called from its color. 



L. But that could not apply to the Mahogany 

 tree whose wood is not white. 



L. It does, however, apply in every case ; not 

 until it becomes perfect wood, and ceases to be 

 alburnum, does the deposit of coloring matter 

 take place in it which gives each kind its pecu- 

 liar character ; even ebony when young is per- 

 fectly white. 



L. But how does the bark grow all this time ? 



