156 ARMILLARIA MELLEA, 
the branches and some of the original segments swell up 
into bladder-shaped bodies and their walls become tinted 
with a pale brown pigment. This stage is seen in tracheide 
a in fig. 67. The branching continues and more segments 
swell up into bladders, so that whole tracheides become 
tightly packed with them. At the same time their walls 
become thickened, often to such an extent that the lumen 
is nearly obliterated, and the pigmentation becomes much 
more marked. 
Some of the swollen cells collapse, and their contents fill 
the interstices between the other bladders and stain the 
walls of the tracheides. This stage is seen in tracheide 
6 of fig. 67 and also in fig. 68. Next the swollen hyphae 
become bleached and empty, their walls again become thin, 
and finally they disappear (fig. 67, c). In a radial section 
of the larch these three stages can usually be seen in three 
successive tracheides, and the black line itself generally 
covers only a single tracheide. It is clear that this series 
of changes will secure the forward movement of the line. 
When a section of a trunk containing a black line is looked 
at with the naked eye, the wood behind the line is found to 
be different in colour from that in front of it. The purplish 
red colour of the wood in the first stage of rot has given 
place to a dull yellowish-brown tinge, and the wood takes 
a polish with less brilliance. But when looked at with 
a microscope it is remarkable how little difference can be 
seen in the wood on the two sides of the black line. No 
more bore-holes are seen; the tracheide walls stain red 
with phloroglucol and hydrochloric acid and do not stain 
with the chlor-zinc-iodine reagent; the walls are not 
appreciably thinner, and even the tori of the bordered pits 
remain intact. (Hartig says that after the passage of the 
black line the walls stain blue with chlor-zinc-iodine, but 
in the larch this is certainly not the case.) Nevertheless, 
at some distance behind the black line marked delignifica- 
tion does take place. Comparatively few scattered fila- 
mentous hyphae are found in this third state of rot, but in 
it the most marked changes occur. Whole tracheide walls 
