NATURE STUDY MADE EASY 



65 



able. The teacher showed the place where the pith had been. 

 The boys quickly guessed that the wood near the pith was the 

 hardest, because it was the oldest and driest. She told them 

 timber men call it the "heartwood." 



Then the teacher pointed out the bark, or outside layer of the 

 tree. Inside the bark she showed an inner skin which is called 

 bast, and is made of stringy fibers of 

 wood. Gardeners use this for tying up 

 plants. Between the bark and the white 

 wood she pointed out a sticky layer. 

 This, she told the boys, was the impor- 

 tant part of the stem, for in that layer all 

 the new wood was formed. This is the 

 growing part of the stem. Each year it 

 builds up a ring of wood. This wood is lighter than last year's 

 ring, so that, by counting the rings, one can guess rightly just 

 how many years the tree has been growing. 



"Then a ring is formed each year," said Tom. 



" Right," replied his teacher. "Now, you can imagine that as 

 the growth goes on all the time, the stem must expand and grow 

 bigger and bigger, until the bark can no longer bear the strain. 

 It cracks in all directions, and sometimes peels off." 



"I have often noticed how rough is the bark of most trees." 



"New bark grows, however," said the teacher. "It is formed 

 by the living layer called the cambium layer. If this layer be 

 destroyed, the tree will surely die." 



1, BARK; 2, RING; 3, PITH 



