DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 25 
large and conspicuous, appear on the surface after the hyphe 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 hyphe develop. __ 
Fungi growing on wood may be roughly divided into two groups, 
depending on the habit. of growth of hyphe. In the first group are 
placed thosé fungi whose hyphe live on the substances contained in 
the various cells of the wood, while to the second group belong those 
whose hyphe 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 hyphe through the cell walls of the 
wood or by an actual color solution excreted by the hyphe, 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 (4/,; 56, p. 13-14; 
Pe npe Lh): 
aah 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- 
SB 
