The Biclogy of Polyporus Pargamenus Fries 105 
which has been described in full detail so that it may serve 
as a type. The decay of sugar maple differs from that of 
yellow birch only in ‘the followi ing respects. The pockets, 
which are highly irregular in outline, are much broader as 
viewed radially than in the tangential section. On the radial 
surface of the decayed wood examined, the largest pockets 
were one cm. long and five mm. wide; on the tangential sur- 
face, however, the largest pockets were not over two mm. 
wide. The tendency for the pockets or decayed areas to 
enlarge mostly in a radial direction is explained by the pecu- 
lar structure of the wood. The wood is of homogeneous 
structure except for the numerous broad pith-rays which 
transverse the growth rings in a radial direction. Examina- 
tion of a transverse section of sige wood reveals the fact 
that the broad medullary rays, the outer margin of the 
growth ring, and the vessels are the last elements of the 
zrowth ring to decay. The broad medullary rays offer excep- 
tional resistance to the spread of the decay in other than 
a radial direction. The vessels remain behind until long 
after practically all the other elements in the growth ring 
between the broad medullary rays have been dissolved out. 
At this stage of the decay the vessels appear as isolated capil- 
lary tubes loosely held between the medullary rays by par- 
tially disintegrated tissue. The numerous broad medullary 
rays which transverse the annual rings of growth in a radial 
direction serve to bind together the layers of growth so that 
there is no separation of the decayed wood along the line of 
the annual rings. The numerous broad medullary rays act 
like so many reinforcing structures so that the decayed wood 
responds far less to tangential compression than to radial 
compression. They likewise materially increase the trans- 
verse breaking strength of the decayed wood. As a result 
of the strengthening effect afforded by the medullary rays of 
this wood, a trunk of sugar maple rotted by Polyporus par- 
gamenus would be less subject to breakage than a similar 
trunk of yellow birch rotted to the same extent by the same 
species of fungus. The difference in the appearance of the 
decay by this fungus of two woods so similar in their general 
