234 Transactions British Mycological Society. 
in the cortex. The wood finally became more and more diseased 
until the whole tissue was killed. Examining these sections 
microscopically fungal hyphae were not found in those of the 
first row although the fourth and fifth showed the wound reac- 
tions of the cortex and dark portions in the wood due to wound 
gum. The remainder of the sections (rows 2, 3, and 4) represent 
the older diseased parts. All these were heavily infected by 
fungal hyphae and wound gum was formed in large quantities 
in the wood vessels and in the thick walled wood parenchyma 
but not in the medullary rays, the phloem, or the cortex. Thus 
it is evident that wound reactions of the cortex and the forma- 
tion of wound gum takes place much in advance of penetration 
by mycelium. 
3. Method of advance of the mycelium. 
The microscopic details of the way in which the mycelium 
advances in the tissues are interesting. As described above the 
sudden appearance of cracks in the bark denotes the extension 
of the scar and if a transverse section is taken at such a level, 
nothing abnormal can be seen in the cortex except the deep 
cracks, usually two, which mark the area to be infected. A slight 
tendency of the tissues limited by these cracks to redden more 
rapidly than the normal cortex is sometimes observed. The cracks 
are usually almost radial and extend somewhat deeply into the 
cortex, almost if not quite, reaching the wood. Further toward 
the old infection, long zones of browned tissue are sometimes 
formed in the phloem region. These zones may extend tan- 
gentially (i.e. parallel to the circumference and concentric) for 
considerable lengths, isolated portions of brown tissue joining 
up laterally but never radially. There may be one, two or even 
three of them running parallel to each other but quite dis- 
connected. They consist of dead tissue and may reach a thick- 
ness of 100-I140p separated by rings of normal phloem’ about 
150 in width. The cell walls of the normal phloem measure 
about 1:5 thick, but in the browned zones they are swollen 
to 3 to 4:5 in thickness. Occasionally the cell contents in the 
affected area are turned brown and coagulated, and the portion 
of a medullary ray which may sometimes be found passing 
through the dead tissue is usually similarly affected. In the © 
phloem, between the concentric browned zones, large numbers 
of crystals occur which physical and chemical tests show to be 
calcium oxalate (Pl. IX, figs. 8 a, b and c). 
The occurrence of these zones is not invariably associated 
with bark canker, but they have been found in a number of 
cases so that they appear to be caused by Myxosporium corti- 
colum. 
