80 THE HEALING OF WOUNDS 



now see it. As soon as it died, it became a men- 

 ace to the tree, for the rot in its tissues might ex- 

 tend into the trunk. The tree made an effort to 

 cover it up. The tissue piled higher and higher 

 about its base, reaching for the end of the 

 wound. The limb was eaten away by decay, and 

 became smaller and smaller in diameter, leaving 

 a cup -like ring about its base. Finally it broke 

 off, and a knot-hole was left. Such a knot-hole 

 is seen in Fig. 71. Knot-holes on the bodies 

 of trees, then, are the cavities left by dead and 

 decaying limbs. 



A hemlock log, split lengthwise, is drawn in 

 Fig. 72. A knot extends to the center. This 

 knot is the remains of a limb, and is nearly as 

 old as the trunk, because it starts from the very 

 center ; that is, the limb sprung off when the 

 tree was a mere sapling. The probability is 

 that it is just one year younger than the trunk, 

 for branches usually start only on the second 

 year's wood, unless some stress of circumstances 

 starts out the older and dormant buds. The 

 limb finally died and broke off, and the stub 

 was buried. The tissue has now grown out to 

 the end of the stub, and nothing remains but 

 to close over the hole. If the limb had rotted 

 away, a squirrel or a woodpecker might have 

 taken up his quarters in the cavity. The wood- 

 chopper, however, found only a knot ; and a 

 board sawed from the log would have had a 



