406 Groom.— 1 Brown Oak ’ and its Origin. 
the cavities of the wood-constituents. The change of tint of the heart-wood 
as a whole and the production of the brown substance in the individual cells 
definitely succeed the entry of the hyphae. Artificial infections of fragments 
of normal heart-wood caused this to assume colours approximating to or 
agreeing with those of true £ brown oak \ 
The fungus (and colour change) advances most rapidly in a longitudinal 
direction along the lines of vessels and drcumvasal tissue, and in a transverse 
direction along the medullary rays: the advance in a tangential direction is 
comparatively slow. These facts find at least partial explanation, first, in 
the extremely limited power of the fungus to attack and delignify lignified 
walls, and in the consequent advance from constituent to constituent 
exclusively through pits or pores; secondly, in the circumstance that the 
fungus thrives particularly in parenchyma (wood and ray) in which it runs 
mainly in the direction of the long axes of the cells, passing out through 
the numerous pits in the terminal walls. 
Among the consequences of the mode of advance and limited power of 
dealing with lignified walls are the following: 
(a) ‘ Brown oak ’ can remain firm and hard in the tree for a long time. 
And since the mycelium of the fungus concerned in mature * brown oak * is 
largely, if not entirely, dead (even though conidia may occur in it) there is 
no reason to believe that the obvious decay of £ brown oak ’ occurring in some 
cases is due to this fungus. Such decay may be induced by other wood- 
destroying fungi that attack normal heart-wood of the oak. 
(i b ) In early stages of conversion of heart-wood into ‘ brown oak ’ the 
latter is seen in its incipient condition as longitudinal darker bands traversing 
normal coloured wood. This condition is reflected in and explains the 
tortoiseshell variety of mature ‘ brown oak 
(f) The advance of the process of browning is arrested or obstructed by 
large knots, though burr-wood with numerous small knots may be com¬ 
pletely brown. 
(d) Associated, at least partly, with the limited power possessed by the 
hyphae of advancing in a transverse and above all in a tangential direction, 
are the cases where brown oak becomes limited to one side of a stem, or to 
one or two among several ‘ leaders ’ into which the infected trunk divides. 
The source of the food of the fungus constitutes a complex chemical 
problem at present insoluble by microchemical methods. That tannin is 
one of the sources is suggested, first, by the development of the fungus 
particularly in tannin-containing constituents of the heart-wood ; secondly, 
by the power of the fungus to obtain all its essential organic food from 
commercial tannin; and, thirdly, by the smaller quantity of tannin in £ brown 
oak’ than in the adjoining normal heart-wood (of the one specimen 
investigated). 
The fungus responsible has conidiophores closely resembling those of 
