HEART-ROT 95 
The centre of a fully formed patch is so deeply coloured 
that even in a thin section no details can be made out. But, 
as will be shown later, treatment with dilute HCl bleaches 
the whole patch, so that subsequent to this reaction the 
structure can be investigated. 
The mode of development of these patches has been 
observed in detail in artificial infections on sterilized larch 
wood blocks. They originate a few weeks after infection 
at points where a large amount of soluble gum has collected, 
and the first sign of their formation is seen in one or two 
broad hyphae becoming light brown in colour. These 
hyphae are sometimes situated in the tracheides, but more 
generally in the medullary ray parenchyma. Thus Hartig’s 
statement that the black specks originate in the medullary 
rays is in general right, though his contention that the 
blackness is due to substance diffusing from the medullary 
rays into the tracheides is certainly wrong. From the original 
brown hyphae other hyphae arise by branching, and they 
all become much darker in colour. The branch hyphae 
enter neighbouring tracheides and medullary rays so that 
the black patch increases in size. 
The hyphae penetrate everywhere, making very numerous 
bore-holes and entering and filling the cavities of the bordered 
pits. Where three or four tracheides abut on to each other 
the middle lamella is sometimes digested, and an inter- 
cellular space is formed which becomes filled with hyphae. 
In the black state the hyphae delignify all the walls they 
touch, so that these walls cease to stain red with phloroglucol 
and hydrochloric acid, but give a blue reaction with chlor- 
zinc-iodine. This shows that the lignone is absorbed and 
the cellulose substance left. Further, the fungus may destroy 
this cellulose substratum and whole walls may disappear, or 
half a wall may be absorbed, i.e. the part on one side of 
the middle lamella, and the other half remain. Thus it will 
be seen that the digestion of the cellulose is somewhat 
irregular. Probably some of the cellulose is digested from 
every wall, for the walls are always thinner after this kind 
of attack than before. 
