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 HC1 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. 



