﻿DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 25 



large and conspicuous, appear on the surface after the hyphas have 

 developed vigorously. The fruiting bodies bear the spores, which 

 are microscopically small reproductive bodies of relatively simple 

 structure. The spores, being very light, are borne about by air 

 currents. If they alight in a suitable place under proper conditions, 

 germination takes place and hyphse develop. 



Fungi growing on wood may be roughly divided into two groups, 

 depending on the habit of growth of hyphse. In the first group are 

 placed those fungi whose hyphse live on the substances contained in 

 the various cells of the wood, while to the second group belong those 

 whose hyphse attack the actual wood substance of the cell walls and 

 destroy it. The first group is principally represented by the sap- 

 staining or discoloring fungi, so called because they produce various 

 discolorations which are confined to the sapwood. To the second 

 group belong the wood-destroying fungi. 



SAP-STAIN. 



DESCRIPTION. 



Sap-stain, which has been extensively studied (23, 27, 38, 50, 51), 

 even though it may render wood very unsightly does not reduce 

 its strength for practical purposes. The discoloration is normally 

 limited to the green sapwood, because as a rule there is neither 

 sufficient food material nor moisture in the dry dead heartwood for 

 the development of the fungus. The discoloration is usually most 

 intense in the medullary rays, since in these tissues the bulk of the 

 food material is found. The stain is produced in two ways, either 

 by a reflection of the color of the hyphae through the cell walls of the 

 wood or by an actual color solution excreted by the hyphse, which 

 stains the wood itself. These stains vary in color from blue or 

 blackish to reddish, the former being the most common. Since these 

 fungi do not attack the cell walls in which the strength of the 

 wood reposes, except to a negligible extent, discolored wood is not 

 appreciably weakened. This has been determined by comparative 

 mechanical tests on stained and unstained wood (Ifl ; 56, p. 13-14; 

 72, p. 17). 



Although the strength of the wood fibers is not impaired by such 

 stains, the wood is objectionable in places where color is a factor. 

 In a highly varnished interplane strut, for example, a stained 

 streak is unpleasant to the eye. Furthermore, it may lead to a 

 strong prejudice against the airplane having such a member, be- 

 cause while by the uninitiated a dangerous defect not readily ap- 

 parent is passed unnoticed, an unsightly though harmless discolora- 

 tion is considered to indicate a serious weakness. Where the dis- 

 coloration is to be covered up or painted there is no reason to ex- 

 clude it. 



It must be remembered that the conditions which promote the 

 development of the fungus discoloration are highly favorable to 

 the development of true wood-destroying fungi. These conditions 

 are a comparatively high humidity and warm weather. Sap-stain 

 is at its worst during warm wet weather, when the humidity of the 

 air is relatively high and lumber dries slowly. It is at such periods 

 that the most severe staining may occur if the lumber is not properly 

 handled. The climate of the Pacific Northwest is usually exceed- 



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