60 ECONOMIC WOODS OF THE UNITED STATES 



siderable time. Furniture, wainscoting, interior finish, and cabinet 

 work may be badly damaged by prolonged drying, which opens 

 up joints, loosens tenons, and causes veneers to separate from 

 their backing. This property may be largely overcome by soaking 

 wood in oil or coating the surface with paint, oil, or varnish, 

 which excludes most of the air and moisture and keeps the con- 

 dition of the wood uniform. Light, porous woods "work" less 

 than dense woods. On account of their greater porosity and light- 

 ness, slowly grown ring-porous woods (Fig. 13) shrink and swell 

 less than specimens of the same species more rapidly grown 

 (Fig. 14). 



The presence of natural oils, gums, and pigments such as are 

 commonly found in the heartwood of many species usually reduces 

 the hj-groscopicity of woods. 



References 



Roth, F.: Timber, Bui. 10, U. S. Div. For., pp. 30-31. 



Exnbk, W. F.: Lorey's Handbuch der Forstwissenschaft, Vol. II, 1903, pp. 



128-129. 

 •Gayer, K.: Schlich's Manual of Forestry, Vol. V (1908), pp. 66-75. 



PERMEABILITY 



In all green wood the cells are separated from each other by a 

 thin membrane, the primary cell wall. The only important 

 exceptions are the vessels between whose segments there is free 

 communication vertically. Vessels, however, like other cells, are 

 separated from each other and from other elements by the primary 

 wall. This wall ordinarily persists intact unless ruptured by 

 parenchymatous outgrowths — tyloses. It is permeable by water 

 and certain dilute solutions which filter through slowly, but is 

 impervious to oils and resins. Gases can enter into living cells 

 only by going into solution, and in that condition diosmosing 

 through the cell wall. 



These facts have an important bearing on the process of 

 impregnating wood with preservatives to prevent decay. It is 

 not difficult to force gases or fluids through open vessels of green 

 wood, but it is impossible to do so if they are plugged with tyloses. 

 For example, it is very easy to blow through the vessels of green 

 wood of most red or black oaks, even in pieces of considerable 

 length. In green wood of the white oaks, on the other hand, 



