PRESERVATION OF WOOD 
353 
The part of a tree nearest the center is called the heart- 
wood, and that outside of it the sap-wood. Heart-wood is 
often of a different color from sap-wood, due to substances 
deposited in the cell-walls when they become old. For many 
purposes, heart-wood lasts longer than sap-wood, especially 
in posts or timbers used under ground. Charring the ends 
of posts put into the ground increases their durability. 
301. Preservation of Wood. — There are great differences 
in woods as to their ability to last in the soil or under water. 
Figure 327. — Nursery Where Young Trees Are Started. 
Some, like cypress, cedar, and locust, have the ability to 
withstand decay on account of substances contained in the 
wood, such as resins. All wood lasts longer if it is seasoned, 
that is, allowed to dry out in the air before being used. 
Wood that is to be used in damp places, however, usually 
needs treatment to prevent or at least to retard the process 
of decay. As decay is caused largely by the work of bacteria 
or of fungi, both of which depend upon moisture as one of 
their chief vital conditions, the use of timber thoroughly 
