Groom.—*Brown Oak 9 and its Origin. 4 01 
that the hyphae advance most rapidly in a longitudinal direction in the 
vessels and circumvasal tissue, which form radial series. In a transverse 
plane advance of the hyphae is most rapid in a radial direction by means 
of the medullary rays. 
These facts, including the slow advance of the hyphae in a tangential 
direction, help to explain the cases where the ‘ brown oak * advances and 
tapers from the base of the trunk upwards, or where it becomes restricted 
to one side or one of two stems in a double-stemmed oak, though in the 
first case the taper of the heart-wood itself may intervene. Further, the 
mode of advance of the fungus (coupled with the very feeble power that 
the fungus has of attacking lignified walls) at least partially accounts for the 
obstructive influence of large knots. 
(b) Brown heart-wood mainly in the form of longitudinal strands. 
In this stage and the preceding one in which the hyphae and brown 
substance are in strands, though not clearly marked to the naked eye, all 
steps in the advance of the hyphae and in the manufacture of the brown 
substance are to be seen. Both stages will therefore be described together. 
The advance of the hyphae along the medullary rays was revealed 
especially clearly in the normal-coloured heart-wood in places where hyphae 
and brown substance were abundant in the ray, but absent or scanty in the 
neighbouring vessels, tracheides and parenchyma. In the rays the hyphae 
run mainly in a transverse radial direction, passing through the copious pits 
of the terminal cell-walls. Yet here and there they emit branches to the 
tissue on their flanks; for often the wood-parenchyma in the immediate 
vicinity of infected ray cells also contained hyphae and brown substance, 
whereas wood-parenchyma tangentially more remote lacked these. 
In uniseriate and multiseriate rays alike the cells containing the brown 
substance always entertained hyphae, which could be traced farther out in 
the ray towards the normal wood than could the brown substance. With 
this exception very few of the colourless ray-cells in the infected region 
contained hyphae. These facts show that the brown substance is the effect , not 
the cause, of the presence of hyphae. In the medullary rays of the Edinburgh 
specimen the hyphae extended outwards approximately to the same distance 
as the first granular traces of the brown substance. 
In the vessels , tracheides , and wood-parenchyma the course of the hyphae 
is generally longitudinal. Consequently in transverse section there are 
isolated islands of vessels and surrounding tissue containing hyphae and 
brown substance. The actually contiguous uniseriate rays may be devoid 
of hyphae or contain these only where the ray actually crosses the infected 
island. Traced further outward such a ray is devoid of hyphae until 
another infected island is touched, when hyphae may reappear in it. Thus 
as they travel along the vessels and circumvasal tissue hyphae can infect 
