156 ARM ILL ARIA MELLEA, 



the branches and some of tlie 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 collajDse, and their contents fill 

 the interstices between the other bladders and stain the 

 walls of the tracheides. This stage is seen in tracheide 

 h of fig. 67 and also in fig. GS. 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 cluingcs occur. Whole tracheide walls 



