CONTROL OF DECAY IN PULP AND PULP WOOD 5 
and other easily digested organic compounds. It may also pass 
through the pits in the walls of the wood fibers, but rarely bores 
through the solid wood substance. For this reason, molds on timber 
do not affect its pulping quality. The mycelium of wood destroyers, 
though often quite similar in appearance to that of molds, can 
usually be distinguished by its disintegrating action on the wood fiber. 
(See PL I, fig. 2.) 
In order to propagate itself successfully, every fungus must have a 
fruiting or spore-bearing stage. The spores of molds are usually 
borne directly on somewhat specialized superficial mycelium. The 
spores of wood-destroying fungi are, with the exception of secondary 
ones produced in certain cases, borne on or within definite fruit 
bodies (conchs, brackets, toadstools, mushrooms, leathery incrusta- 
tions, etc.), whose shape, color, and texture are quite characteristic 
for the different fungi. (See Pis. I, II, and XVIII.) 
Spores, in function, are entirely comparable to seeds. They are 
microscopic in size and extremely light, and appear (PL I, fig. 3) 
as a very fine powder, which is very often white, though the color 
varies for different fungi. Frequently the spores from a single 
fruit body will number into the billions, most of them being capable 
of producing a new plant. With so many spores blowing about in 
the air and settling on new timber, it is evident that the chances for 
infection are very great, provided the conditions for germination 
are favorable. The most active period for the casting of spores from 
the fruit bodies is during moist weather, which in turn is most favor- 
able for germination and subsequent infection. 
CONDITIONS NECESSARY FOR GROWTH OF FUNGI 
The conditions necessary for the growth of fungi are (1) the presence 
of an adequate food supply, (2) sufficient moisture, (3) at least a 
small amount of air, and (4) a suitable temperature. 
Food is supplied by the wood tissues, and the more easily these 
tissues are attacked the more readily will the wood disintegrate. 
The sapwood of practically all species of American timber is non- 
resistant to fungous attack, and the heartwood of most of the prin- 
cipal pulp-wood species is hardly more durable than the sapwood. 
Where there is a suitable food supply for the fungi, moisture is, 
without doubt, the most important factor in decay. The different 
fungi, however, appear to vary somewhat as to their water require- 
ments. For infection and the early stages of decay a comparatively 
high moisture content of the wood and the surrounding air is highly 
favorable for all fungi. In discussing the effect of atmospheric 
moisture on the germination of spores of Lenzites sepiaria on shavings 
of shortleaf pine sapwood, Zeller {28) shows that 85 to 100 per cent 
of them germinate only at the high relative humidities of 98 to 99 
per cent. This writer considers that fiber saturation 2 of the wood 
is attained at 95 to 96 per cent relative humidity, and concludes that 
even at this point germination is considerably retarded, and that 
below fiber saturation it falls off with extreme rapidity. A slight 
2 Green wood usually contains water within the cell walls and "free" water in the pores. In drying, 
the water in the pores is the first to be evaporated. The fiber saturation point is that point at which no 
water exists in the pores of the timber, but at which the cell walls are still saturated with moisture. The 
fiber saturation point varies with the species. The ordinary proportion of moisture, based on the dry 
weight of the wood, at the fiber saturation point is from 20 to 30 per cent. 
