THE GROWTH OF THE TREES. 27 



bium layer which unites the two. {Plate III.) It is composed 

 of young and delicate cells. In the spring, a rich sap, some- 

 thing like mucilage in appearance, begins to flow freely and 

 to supply to them abundant nourishment. As they then begin 

 to increase in a manner that has been already mentioned, the 

 inner ones attach themselves to the wood, while the outer ones 

 are added to the liber ; and it is in this way that the two an- 

 nual layers which really renew the life of the trunk are formed. 



With the bark it is different : the green layer seldom increases 

 much after the first year of its growth ; and although the 

 corky layer often makes from year to year new growth inside 

 of the old, after a time it all dies. It has to contend with the 

 roughness of the elements, and it is especially hurt by being 

 stretched beyond its endurance by the growing wood and liber 

 within. Finally it cracks apart and the rift is patched by the 

 formation of new corky layers. As the outer bark vanishes, 

 the enlarged sheath of bark is thus torn and patched each suc- 

 ceeding year. The outer and older layers of the much mended 

 garment of the tree are constantly falling off and decaying. In 

 old trees the cambium Jayer and the cells recently formed from 

 it only are alive. Furthermore it is only in the younger wood 

 that sap ascends. As the wood in each annual ring grows 

 older the walls of its cells harden and thicken, and it is no 

 longer regarded as a living part of the tree. It is the heart- 

 wood and, owing to its dryness and hardness, is chosen in 

 preference to the living sap-wood for timber. In different 

 species of trees a colouring matter peculiar to each is deposited 

 in the cells of the heart-wood and it is therefore of various 

 shades. Black in ebony may be cited as an example. 



As we have now thought somewhat about the growth of the 

 tree in height and in diameter, we may begin to concern our- 

 selves about its branching ; for we shall have little to do with 

 simple-stemmed plants, or those which are known as monocoty- 

 Iedonous, their embryos having but one seed leaf. Our path 

 leads us rather among dicotyledonous trees, which are so called 



