LIFE-HISTORY OF A TREE 73 



wood. The former is more durable when exposed, is stronger, 

 shrinks less in drying, and is heavier and more compact, 

 the latter feature arising, no doubt, from the pores and 

 ducts being filled with gums or^other solid matter. There 

 appears to be some irregularity in the change from sapwood 

 to heartwood, but just why is not known. Practically all 

 species are subject to it. Usually as a new ring of sapwood 

 is laid on there is an inner one changed to heartwood ; but 

 this does not invariably occur, as some trees of the same 

 species may and do have more sapwood than others, and 

 more towards the top than at the butt, or the reverse, and 

 even more on one side than on the other. With some spe- 

 cies of trees this change to heartwood does not occur until 

 the tree reaches thirty or even seventy-five years of age, 

 while in others there are seldom more than six or seven 

 annual rings of that kind of wood. They generally lessen 

 in number proportionally in all species as the tree reaches 

 maturity and old age. The whole matter appears to be in- 

 volved in more or less mystery. It appears to be an effort 

 of Nature to discard that for which there is no further use, 

 just as she discards the dead outer scales of bark. 



Pith and Medullary Rays. Another interesting feature 

 in tree-growth is the pith in the centre of the stem and all 

 the limbs, and the thin sheets or rays radiating therefrom. 

 The pith is a small, porous, and somewhat spongy cylinder 

 of cellular tissue, and what purpose it serves in the economy 

 of the tree's life is not fully known, if at all. The glassy 

 sheets radiating from the pith are technically known as the 

 Medullary Rays. They are harder than the pith and some- 

 times harder than the surrounding wood. They can be 

 found in all of our timber trees, but are more conspicu- 

 ous in some than in others, and where plainly visible are 

 deemed to add beauty to the wood when it is used for inter- 

 ior finish and furniture. They do not run uninterruptedly 

 the entire length of the tree, but are broken up into short, 

 irregular patches and are seldom more than six or seven 

 inches long. The major portion of them radiate from and 





