HEART-ROT 91 



(iii) Insoluble gum. As can be seen in figs. 32 and 36, 

 the larch is generally rotted in a very irregular manner. 

 A transverse section of the trunk has often a curious figured 

 appearance, the truly rotted wood being in more or less 

 isolated patches, or the rot may be annular as in fig. 31, 

 with a peg of sound wood in the middle. In every case the 

 rotted wood is surrounded by a layer of very dark hard 

 wood which appears to prevent the further progress of the 

 rot. The colour and hardness of this layer are due to the 

 precipitation of an insoluble gum which fills all the tracheides 

 and medullary rays and forms a prominent object in all 

 sections of the wood. 



Though it imparts to the wood a red-brown colour, in 

 microscopic sections it is yellowish, so that the red tinge is 

 supplied by the wood itself. It is only found in close 

 proximity with the fungus, and apparently only where air 

 is present, for it occurs especially near the outside of infected 

 blocks, and often appears surrounding a bubble in a tracheide. 



The nature of this substance has not been ascertained. 

 It is insoluble, but swells slightly in water, is insoluble in 

 alcohol, ether, chloroform, acetic acid, HN0 3 ; in 10 per 

 cent. HC1 or H2S04 it is not dissolved after five days, though 

 it then appears slightly corroded. It contracts markedly 

 in a 5 per cent, oxalic acid solution in spirit, and appears 

 somewhat discoloured. It dissolves entirely in 10 per cent. 

 KOH solution after three days' immersion. 



It thus differs from the ' wound gum ' of Dicotyledons, 

 which, according to Temme (1885), is insoluble in KOH, 

 but is soluble in warm HN0 3 . Also Temme states that 

 if thin sections containing ' wound gum ' be placed for 

 a quarter of an hour in dilute HC1 and potassium chloride, 

 the gum, though still insoluble in water and ether, becomes 

 soluble in alcohol. With the gummy substance in the 

 larch this was not the case. This gummy substance has 

 considerable pathological importance, for it is impermeable 

 to hyphae of the fungus and acts as a screen, preventing 

 the unlimited growth of the mycelium. It is deposited as 

 a layer up to twenty tracheides thick, which entirely sur- 



