INTRODUCTION 



the pith and first coat of wood bear that age ; the next coat 

 is only ninety-nine years old, and the outermost but one. More- 

 over the topmost branches are quite young, and their innermost 

 parts, scores of feet above the tip of the little seedling from which 

 they have grown, are scores of years younger, even to their pith. 

 Of an ancient hollow tree it may be said that perhaps Robin 

 Hood dined beneath its branches. The general impression is 

 correct, but the fact is not precise ; for the tree is no older than 

 its component parts, which, being entirely outer parts, are merely 

 the growth of the last fifty years or so, as a section of the wood 

 will prove. 



A tree, then, is a living organism whose component parts are 

 of various ages; and upon it arise successive generations of 

 leaves, much as a city rears its people or a coral reef its polyps. 

 The living portion is renewed from time to time, adds something 

 to the tree, and passes away. Time too brings other changes due 

 to other causes than the nourishment afforded by the yearly 

 labour of the leaves, and as year by year a new coat of wood is 

 added, so does an inner layer lose its vitality and become inert, 

 and so does a still deeper layer undergo still further changes which 

 produce the heart-wood. This heart-wood will not be found in 

 all trees, for some exhibit no distinction in this respect and are 

 the so-called Sap-wood trees, though even here changes take place 

 which bring the timber to maturity. In a living ' Heart-wood 

 tree,' there can be distinguished the pith, heart-wood, sap-wood, 

 the active living layer, or Cambium, the bast and the bark. The 

 Cambium layer is a very delicate sheath of thin tissue which is 

 the source of the new wood, and which, by the multiplication of 

 its elements (cells), adds layer upon layer to the wood already 

 formed. On its outer side it adds to the bast but in a much smaller 

 degree. 



The bark, enclosing all in an impervious clothing, serves the 

 purpose of checking the evaporation which would endanger the 

 existence of the delicate Cambium layer within. 



The young wood lying nearest the outside partakes of the nature 

 of the Cambium at first but rapidly becomes altered. The 

 walls of its cells, originally thin, become thickened and woody, 

 but for some time they retain their capacity for absorbing water, 

 and form the channel by which the water from the roots ascends 

 to the leaves. The further from the bark, the less is this capacity 

 possessed by the wood, and though it is rarely if ever quite lost, 

 yet the difference in the rate of absorption of water by the sap- 

 wood and heart-wood is considerable, as can be seen by moisten- 

 ing the cut surface of a piece of wood where both are present. 

 While the water sinks rapidly into the sap-wood it sensibly 

 lingers on the surface of the riper part. 



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