THE OAK. 85 



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 simultaneously re- 

 newed, 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 of wave-circles is that of the annual wood- 

 circles of a tree, only that on the water we have only 

 an evanescent effect, while in the tree there is new 

 substance and solidity. The mode of gowth and the 

 phenomena referred to are denoted by this word " ex- 

 ogenous," which is literally no more than " expansion 

 outwards." Very different are the mode of growth 

 and the internal condition of the trees called " endog- 

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