THE SEQUOIA 
than during the fall and winter, when the tree receives little 
nutrition. These periods of interrupted growth are represented 
in trees of several seasons’ growth by distinct lines separating the 
rings of wood. In the trees of cold-temperate climates, where 
the contrast of seasons is great, the rings of wood are very dis- 
tinct, as, for instance, in the Big Tree. In many trees the increase 
in the wood forces the bark asunder, which, through the action 
of the weather, becomes rough and rugged. If it were not for 
the constant renewal of the inner layers by the Cambium cells, 
the bark might entirely disappear. 
In the case of the wood it is quite different. The inner rings, 
which are the older, are entirely surrounded by the outer rings of 
fresh young sap-wood, by which they are protected from climatic 
changes. Every new circle of wood moves the zone of growth 
farther from the center. The central wood undergoes a change, 
its cell walls becoming thicker and the calibre of the ducts or 
vessels smaller. It usually takes on a different color from the 
sap-wood. This is now called heart-wood, although it performs 
no vital function in the life of the tree and is practically dead. 
In some of the Big Trees much of the heart-wood has decayed 
and disintegrated, with no more injurious effect upon the tree 
than a weakening of the trunk. So much of the heart-wood had 
decayed in one specimen, which was blown down by the wind, 
that men on horseback were able to ride into the trunk a dis- 
tance of seventy feet, and pass out through a hole in the side. 
Injuries to the wood are sometimes repaired by the deposit of 
new layers of cells. It is even possible to determine the year 
when such injuries occur by merely counting the rings of repair. 
On the extreme right of the specimen in the Museum are two 
such wounds. In each of the two places marked with a cross a 
bullet was found, but the wounds had been covered by at least ~ 
five years’ growth of wood. That the bullets did permanent 
injury to the wood in the immediate vicinity is indicated by the 
change of sap-wood into heart-wood, which in these places ex- 
tends nearly to the edge of the bark. 
Since, therefore, the rings of wood correspond to periods of 
vegetable growth, which are seasonal, and the lines of separation 
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