4 STBUCTURE OF TREES. 



held between the eye and the light, are found to be marked in every 

 portion by a delicate net-work of green lines, technically called the 

 "veins"; and upon the outside of the trunk there is bark, which can 

 be removed like the peel of an orange. When one of these exogenous 

 or branching trees is cut down, or if a branch be lopped off, the exposed 

 surface, on being polished horizontally, shows elegant concentric circles, 

 surrounding a central point, which in young parts of the tree indicates 

 a column of living pith. The concentric circles announce the age of 

 the tree or branch, which usually is just as many years old in that part 

 as there are rings. In its earliest stage, or while only in its first season 

 of growth, the stem of the seedling tree consists only of pith and an 

 enclosing skin. Woody matter is gradually prepared, and this becomes 

 deposited in a layer between the pith and the skin, which latter now 

 assumes the solidity of bark ; and should the stem be cut through at 

 Christmas, or at the end of its first year, the first of these annual 



Section of Exogenous Stem. 



rings will be plainly visible. Every successive year this process is 

 repeated. With the opening of the leaves in spring (for it is the leaves 

 that really effect the work) the preparation and deposit of a new layer 

 of wood is commenced, so that by the close of the second season there 

 are two layers; by the close of the third season, three layers; and so on 

 as long as the vital lease of the tree endures. The bark is simul- 

 taneously renewed, enclosing a larger mass every year. This mode of 

 growth is prettily illustrated in the spreading of the little wave-circles 

 upon the surface of still water. Standing on the margin of some lovely 

 lake or mere, and looking at the blue sky and the white clouds that are 

 reflected in its clear bosom, how often the fairy spectacle is broken in 

 an instant by the wing of some light bird that, skimming through the 

 air, just touches the surface and sweeps onwards. But the effect of 

 that touch is to cause circle after circle of tiny wavelet to move away 

 from the spot where the touch was given, and as far as the eye can 

 reach the beautiful phenomenon is continued. Just like this succession 



