The Structure of a Tree 19 



The bark of a tree. The inner cells of the bark of a 

 tree are alive, and as the tree increases in size the bark 

 grows by the multiplication of these cells. Thus the 

 bark increases in thickness year by year, unless at the 

 same time the outer layers are scaling off and falling 

 away. Why are the outer layers of bark on an old oak 

 or elm furrowed and broken? Why is the bark of an 

 oak or elm rougher than the bark of a sycamore or 

 birch of the same size? 



Cork and how it is formed. In the bark of trees the 

 outer cells become changed into cork. This is done by 

 thickening the cell walls with a waxlike material, after 

 which the protoplasm in the cells dies. The corky 

 layer of cells thus formed protects the tree from drying 

 and also keeps out fungi that would otherwise attack 

 the tree and cause decay. An idea of the great useful- 

 ness of the cork may be gained by noting what happens 

 to an Irish potato when the thin, corky layer on its 

 surface is broken or removed. When this is injured, 

 decay very readily sets in, and a peeled potato loses 

 water sixty times as fast as one that has its protective 

 coat. In the cork oak the corky layer is of great thick- 

 ness and is removed from the tree in large slabs. Exam- 

 ine the stoppers of bottles and see if you can find 

 evidences of annual rings in them. Can you explain 

 why a cork does not become water-soaked as a piece of 

 wood does ? 



Vessels and fibers in the bark. The inner part of 

 the bark is a layer of tissues consisting of vessels for 

 conducting food, cells in which food is stored, and 

 long, slender, and strong fibers. The latter are called 



