42
THE ESSEX NATURALIST.
Another species of this genus, D. borealis (Wahlenb.) Quel.,
is described by Hartig [17], under the name of Polyporus borealis,
as causing a white rot in spruce. The wood becomes yellowish
brown and numerous small pockets of mycelium appear in the
spring wood: Hartig notes the peculiarity of the hyphae to grow
horizontally, i.e., across the tracheids and not along them.
Fistulina hepatica (Huds.) Fr. Among wood-rotting fungi
the beef-steak fungus holds the almost unique distinction of
enhancing the value of wood which it attacks, and it is not
unlikely that before long steps will be taken to infect suitable
trees with this fungus, if this has not already been attempted,
for although in advanced decay F. hepatica produces a cubical
brown rot in the heartwood of oak and chestnut and renders
the wood worthless, it is responsible in the incipient stages for
the rich brown timber which we call brown oak, and which is
highly prized for ornamental purposes. The association of
Fistulina with brown oak has been due to the investigations of
Cartwright [6], although Groom [14] had concluded in 1915 that
the cause of brown oak was fungal. According to Groom, trees
of 12 inches diameter (computed to be about 20 years old) may
show the brown colour and trees with the brown heart may be
partly hollow ; but he also mentions that such trees may have
the butt converted into white decay, and suggests that the
hollowness is due to a causal organism other than that producing
the brown colour. Fistulina produces, in advanced decay, a
deep brown cubical rot, but one in which the wood is not specially
soft or crumbly. In the early stages of decay streaks of heart-
wood become coloured, producing the tortoiseshell type of
brown oak, but later the brown colour spreads and tints all the
heartwood. It has already been mentioned that the fungus,
in culture, grows better if tannin is present, and Groom found
that there was less tannin in brown oak than in the normal
wood. The fungus is a wound parasite, but works slowly, and.
according to Cartwright and Findlay [8], only over mature trees
are likely to be seriously damaged. When the timber is felled
the growth of the fungus is stopped. Latham and Armstrong
[23] have demonstrated that the mechanical strength of brown
oak is well below that of the normal wood, although, thanks to
this, it is mild working and consequently easier to finish, while
the decrease in mechanical strength is unimportant in the
ornamental purposes for which the wood is used.