THE CONICAL GEOWTH OF TREES. 57 



CHAPTER IV. 



THE TREE IS CONSTRUCTED ON THE PRINCIPLE OF A CONE. ITS 

 LEAVES ARE THE SOURCES WHENCE PROCEED THE FORMATIVE 

 MATERIAL USED IN THE BUILDING UP OF ITS STEM AND 

 BRANCHES, WHICH IS DISTRIBUTED AMONGST THEM AFTER A 

 COMMON LAW. 



If we look at the stem and branches of a tree in winter, 

 when it is deprived of its leaves, we shall see at once that it 

 is constructed on the principle of a cone; for the main axis 

 or stem of the tree is broadest at its base, and gradually de- 

 creases in thickness toward the extremities of its branches. 

 Any branch is, in the place where a side branch originates, 

 stronger than the last at its base, so also this side branch is 

 stronger than the branchlet which it produces, and in this 

 manner the thickness of the stem or principal axis steps, as 

 it were, away by degrees from branch to branch, until at 

 length it loses itself in the fine branches of the youngest 

 generation of shoots. It is well known that the cone is 

 the stablest structure in nature, and the tree may be re- 

 garded as an arborescent cone. 



It was shown on page 38, that the stem of a young beech 

 tree exhibits, on the cross-section, a number of concentri- 

 cal and almost circular beds or layers of wood ensheathing 

 one another about a common centre, which is occupied by 

 a canal of medulla or pith, the whole being covered by the 

 bark formed on the exterior of the stem. The longitudinal 

 sectiop, on the contrary, shows that the stem is composed of 

 a series of superposed, and hollow, elongated cones, the old 

 conical layers or growths of the last and previous seasons 

 constituting a firm foundation for the new conical layers 

 of the next and succeeding years. Through the whole of 

 these cones the pith penetrates as a continuous cylinder. 



The conical growth of the tree is the result of the conical 

 formation of the first year's shoot, which is the foundation 



