402 Groom .—‘ Brown Oak ' and its Origin. 
rays at successive levels. Such a mode of infection was particularly clear in 
the case of multiseriate rays, which crossed infected strands of vessels and 
showed hyphae and brown substance merely in their sides towards the 
infected vessels. 
Occasional hyphae were seen running tangentially in tracheides to or 
from other tracheides and a medullary ray. 
(c) Production of the brown substance. 
The following stages were observed in the hydrofluoric-acid material: 
1. Hyphae entering colourless parenchyma cells or vessels were studded 
with glistening globules. And in the Edinburgh material such was the case 
with parts of the hyphae freely traversing the lumen of the cell, and not in 
contact with the cell-wall. 
2. Cells showed granular colourless, or very faintly yellow, contents in 
proximity to the active intracellular young hyphae. 
3. The contents were definitely yellow, in larger quantity, and formed 
a homogeneous hyaline mass surrounding the hyphae. 
4. In the final stage, as seen in completely ‘ brown oak ’, the substance 
is definitely brown, present in still greater quantity, and may fill the 
parenchyma-cell, except where it is permeated by dead, often almost un¬ 
recognizable, hyphae, which are separated from it by a clear space. 
These stages indicate that the brown substance is deposited outside the 
hyphae as colourless globules, which later increase in number and undergo 
some change so as to give rise to a yellow mass of granules or globules; 
these in turn remain in or assume a colloid condition to form eventually 
a homogeneous solid mass of constantly deepening colour. The appearances 
presented suggest that the substance is excreted by the fungus, but an 
alternative suggestion is given in the sequel where the food of the fungus 
is discussed. 
(d) An additional plugging substance. 
Mention must be made of an entirely different solid substance of 
unknown nature present even in the material, subject to treatment with 
hydrofluoric acid and so forth. This substance, granular in nature, when 
seen in thin sections often showed a yellowish tinge, but when seen in 
thicker masses was dark and opaque. Its occurrence and distribution were 
independent of those of the hyphae. It occurred in normal heart-wood as 
abundantly as in brown heart-wood. In distribution it was localized, often 
being found in longitudinal strips, especially in wood-parenchyma and 
ordinary tracheides, but also in the adjoining thick-walled fibro-tracheides. 
In such cases it was present in the uniseriate rays traversing the strip solely 
where the former crossed the latter. 
