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 con- 

 structed on the principle of a cone ; for the main axis or stem 

 of the tree is broadest at its base, and gradually decreases 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 regarded 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 concentrical 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 section, 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. 

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