132 HEART-ROT CAUSED BY OTHER FUNGI 
of a state of tension can be observed in the cell walls them- 
selves. The general form of the tracheides is unaffected, 
but cracks appear in the walls, always rising from right to 
left, as shown in fig. 54, and usually in tiers, one above 
another. ‘These cracks appear first in the summer wood, 
and in all stages are more conspicuous in the summer wood 
than the spring wood. They do not rupture the complete 
wall, but the thickening on each 
side of the middle lamella cracks 
independently, and whereas on 
one side the cracks run from right 
to left, on the other side they 
appear (as seen through the wall) 
to run from left to right, whilst 
the middle lamella itself remains 
intact. Naturally the tension 
breaks away weak places in the 
walls, and bordered pits andhyphal 
bore-holes are commonly centres 
for cracking, and, where both sides 
of the wall come within focus, the 
cracks appear as a cross as shown 
Fic. 54.—Tangential longi- ee 
tudinal section of larch wood Later on, larger cleavages occur 
after attack by Polyporus in the wood in the transverse, 
Schweinitzii, showing cracks . : 
in the walls: 6.h., hyphal bore- radial, and tangential planes, 
hole; b.p., bordered pit; h.c., and it is these cleavages which 
pase crack; m.r.,medul- jater become filled with the cheesy 
mycelium. Also if a section of 
a trunk with a rotten core is allowed to dry, further 
irregular cracks occur in all directions in the wood, so that 
in such wood the microscopic details are apt to become 
obscured. 
General remarks on Polyporus Schweinitzii. We know 
very little indeed about the mode of infection of this fungus. 
In fact the text-books give all the information which is 
available on the subject when they say that the fungus 
first attacks the roots of a tree and grows up from the roots 
he. 
bho dl} 
