﻿DECAYS AND DISCOLOEATJONS IN AIRPLANE WOODS. 23 



are not subject to these pith-ray flecks, but a somewhat similar injury 

 in western hemlock known as black check results from the work of 

 a different insect {11). 



CHEMICAL DISCOLORATIONS. 



The sapwood of many species of wood is subject to discolorations, 

 varying widely in appearance but fundamentally the same, which are 

 the result of chemical action (3). Sapwood is rich in organic com- 

 pounds and also contains certain soluble ferments which facilitate 

 the oxidation of such compounds. Under favorable temperature con- 

 ditions, for example, when green sapwood is exposed to the oxygen 

 of the air, these ferments, known as oxidizing enzyms, act on the 

 organic compounds in the sapwood. The result of their action, which 

 is an oxidation process, is a discoloration of the sapwood, with the 

 colored substance most noticeable upon microscopic examination in 

 those cells mainly concerned in the storage and transportation of 

 food. 



Hot, humid weather is most favorable for this staining. Cool, dry 

 weather retards it or prevents it entirely. Logs immersed in water 

 are not affected. Light is not necessary for this reaction, as it takes 

 place just as readily in darkness. The stain is confined to the im- 

 mediate surface layer, and the wood is not weakened. The most 

 practical method of prevention, if this is considered necessary, is by 

 dipping the green sap boards into boiling water for a few minutes 

 as they come from the saw. 



HARDWOODS. 



Birch, maple, and cherry stain a reddish yellow or rusty color. 

 The wood of alder becomes very intensely red or red-brown on freshly 

 cut surfaces, often within an hour or so after the surface is exposed 

 (Ifi). In the case of red alder (Alnus oregona Nutt.), if the wood 

 dries and remains white, the red color will appear upon the addition 

 of water in the presence of air, provided the temperature is favor- 

 able. A bluish stain often occurs in red gum (Liquidcmiber styraci- 

 flua Linn.). 



The European linden {Tilia europaea, Linn.) is subject to a strik- 

 ing discoloration (39), which probably also occurs on basswood 

 in this country. When freshly sawed boards are so closely piled 

 that they dry slowly, a more or less apparent dirty green color ap- 

 pears in from 8 to 10 days. Under very favorable conditions the 

 color is exceedingly bright and intense. The color varies between 

 wide limits, from yellow-green or brown-green through all possible 

 gradations to the purest moss green. Only the outer layers of the 

 wood are colored. Usually the stain extends to a depth of one 

 thirty-second of an inch or rarely to a depth of one-eighth of an inch. 

 The staining, although it is the result of a chemical reaction (an iron- 

 tannin reaction), is not dependent on temperature, since it occurs 

 just as readily in winter as in summer. Too much moisture hinders 

 the reaction, but a certain degree of moisture is essential. If the 

 boards are dried quickly no staining results. 



SOFTWOODS. 



Coniferous woods are not so commonly subject to this type of dis- 

 coloration, but there are a few examples. The ends of incense-cedar 



