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 valuatite 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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