FORM AND STRUCTURE 



of narrow plates which make the " silver grain " of the 

 wood. 



In the transverse section these appear as lines but when 

 the wood is cut lengthwise parallel to them, "quartered," 

 their faces show as glimmering plates which give a peculiar 

 and beautiful appearance to the wood. Trees differ in the 

 size and number of their medullary rays. 



Each of the rings is supposed to mark a year's growth of 

 the tree ; as a matter of fact it may or may not do so, but 

 the number of concentric rings will give the approximate 

 age of the tree. 



The heartwood is the more valuable part of the trunk for 

 timber. It is drier, harder, and more solid than the sapwood. 

 The cells have been so filled by the deposition of hard mat- 

 ter that they are no longer able to take any part in the cir- 

 culation of the tree ; the protoplasm has receded from them 

 and they are virtually dead. 



The zone of sapwood is a zone of living tissue. But the 

 impulse of life is ever leaving the old and entering the new, 

 and the cells of its inner circumference are continually being 

 transformed into heartwood, and those of its outer circum- 

 ference increased by new growth. 



Between the sapwood and the bark, united to each, is a 

 zone of growth called the Cambium Layer. This is a tissue 

 of young and growing cells and it is here that the tree in- 

 creases in diameter. Here is the newest wood and the new- 

 est bark, here new cells are formed, the inner ones adding 

 to the wood, the outer to the bark, producing the annual, 

 layers of the two which are ever renewing and continuing the 

 life of the tree. 



The Bark is the outer covering of the trunk. At the sur- 

 face it is made up of dead and dying tissue which is stretched 

 and torn and shed in plates or scales as the wood beneath it 

 increases in size and requires room to expand. The inner 

 bark consists essentially of sieve-tissue or bast and forms a 

 zone capable of rapidly conducting the fluids of the tree. 



In all young bark is found a peculiar group of cells, called 

 Lenticels, which protrude through the skin or epidermis. In 

 some trees these lenticels disappear when the bark becomes 



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