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 otten 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, HNO;; in 10 per 
cent. HCl or H2SO, 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 HNO;. Also Temme states that 
if thin sections containing ‘wound gum’ be placed for 
a quarter of an hour in dilute HCl and potassium chloride, 
the gum, though still insoluble in water and ether, beeomes 
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- 
