24 MISSOURI BOTANICAL GARDEN, 
cambium layer and the living bark, killing them both. A 
large mass of hyphae forming a compact red brown felt 
then develops in the space left when the live surrounding 
wood increases outward. Such a space, filled with the 
brown felt, can be seen near the upper end of the figure on 
Plate 1, immediately under the bark. : 
On a tangential section of a diseased trunk the decayed — 
sheets stand out as lens-shaped masses, the round core in 
the centre surrounded by yellow wood gradually merging 
into the yellow-brown of the heartwood. The lens-shaped 
areas are of all sizes from 2 inches high and 4 inch broad 
to the size of a pinhead, which can be recognized only by 
their differing in color from the surrounding wood. These 
sheets of decayed wood grow in height, and in the course 
of time join in a vertical direction; new sheets form be- 
tween the primary ones, and ultimately a tangential union 
takes place. The whole wood is then completely decayed. 
The sheets do not begin to form until a part of the 
heartwood is decayed. In an endeavor to explain this 
rather singular method of spread through the wood, one 
would at first sight suppose that the core of each sheet had 
taken the place of the remnant of a small branch, which 
had been healed over as the tree grew in diameter. This 
seems very plausible from the fact that the decay extends 
so rapidly in what appears to be a predetermined path, 7. e., 
a path extending through the sound wood on either side, 
through which the fungus hyphae can grow more readily 
than through the sound wood. It was found that. this 
could not be the case, because there are not enough small 
branches in the sound wood to account for the many radial 
lines along which the fungus traveled. In seeking for 
another explanation it was found that the medullary rays 
are probably the paths selected in this case, as in so many 
other trees. 
The manner in which fungus hyphae spread through a 
piece of timber is determined to some extent by the struc- 
