*5 THE GROWTH OF THE TREES. 



it will be noticed how much it has strengthened the wall of the 

 cell. {Plate III.) 



Running vertically throughout the wood there is also a set 

 of thin plates of cellular tissue. They are the medullary rays ; 

 and it is to them that is owing the beautiful silver grain in 

 many varieties of wood. The feature is one that is easily 

 noticed. 



In the liber, the inner bark which covers the wood, the wood 

 cells grow longer and finer than they do in the wood proper. 

 They appear more like fibres and are extremely tough. Bast- 

 cells, or bast-fibres, are the names by which they are known. 

 {Plate III.) 



The outer bark is made up of soft cellular tissue. In its 

 green or inner layer the cells are soft and delicate and have 

 within them grains of green colouring matter similar to those 

 contained in the leaves. Early in the tree's growth its trunk 

 becomes covered with the outer, or corky layer, a substance 

 the same as our common cork. It is admirably adapted to pre- 

 vent the evaporation of the ascending fluids,and to it is due the 

 various colourings that we are familiar with in the twigs and 

 branches of different trees. This outer bark, it must be re- 

 membered, is finally covered with an epidermis which is also 

 a layer of cells. {Plate III.) 



Such is the order in which we should find arranged the stem 

 of a young exogenous tree in the first or second season of its 

 growth ; and it should now be of interest to us to see how it 

 increases year after year in diameter. 



The age of a tree is approximately known by counting its con- 

 centric rings of wood ; as every year it generally forms only 

 one new layer of wood outside of the old one. The liber also 

 makes an annual growth, but inside of that of the year before, 

 and next to the surface of the new forming wood. These ad- 

 joining parts of the stem are the only two that are annually 

 renewed. The process is most interesting. Between the wood 

 and the inner side of the liber there is a layer called the cam- 



