468 DECAY. 



5. Decay. There is a time when all vegetables 

 must decay ; but this is often very much hastened 

 by constitutional or accidental disease. The tree 

 in a healthy state grows in circumference, and 

 throws off the old bark, replacing it by a layer of 

 new beneath it : a similar process goes on in the 

 wood. This is annually increased in thickness by 

 layers upon its outside. The sap ascends from the 

 roots to the foliage through the wood of the last 

 two or three years, the latest being the most active ; 

 and the sap, as it returns elaborated by the leaves, 

 forms these new layers of wood and bark. 



In process of time the ducts of the new wood 

 receive such a coating of woody fibre on their 

 interior surface as to fill up the orifice through 

 which the sap passed, and thus become useless. 

 This old wood is now as dead as the old bark ; but, 

 not being exposed to the air, it remains sound as 

 long as it is not so exposed, and is useful, mechani- 

 cally, in strengthening the structure of the tree. 

 But if an accident breaks this outer covering of new 

 wood, the air will be admitted, and the limb decay, 

 spreading the disease to the very heart of the tree. 

 Although this does not essentially weaken the nu- 

 tritive power, yet the tree will not have proper 

 mechanical support. 



The proper remedy for a tree thus diseased is to 

 cut out carefully all the decayed parts, and fill the 

 cavity with a cement made of sand or gravel, lime, 



