Wormald. — Further Studies of the ‘ Brown Rot ’ Fungi. 1 . 307 
cankers examined were on twigs 3 to 7 mm. in diameter, and in most cases 
had extended laterally about half-way round the twigs ; one of the largest 
of these was 4*4 cm. in length, extending from the node i-8 cm. upwards 
and 2-6 cm. downwards. 
The infected tissues undergo a disintegration resulting in gummosis, 
the gum frequently being so copious as to exude in drops (Plate XIII, Fig. 2). 
A characteristic feature of these lesions is a necrosis of the young xylem 
elements far in advance of the fungal hyphae. Mycelium is to be found 
only in the tissues of infected shoots and in the cankers, while disintegration 
of the xylem elements can be traced for several centimetres (as far as 10 cm. 
in one case) from the cankers. On cutting a twig transversely at a short 
distance above or below a canker the disintegrated xylem is seen as a row 
of dark dots, just visible to the naked eye and easily seen with a pocket lens 
(see Plate XIV, Fig. 10), forming an arc parallel with, and about 0-5 mm. from, 
the cambium layer, and situated on the same side of the twig as the canker. 
Microscopic examination resolves the dots as gum ‘pockets’ in the xylem. 
In longitudinal radial section through a cankered node the necrosis is seen 
as a dark line extending upwards and downwards from the canker and 
again parallel with the line that indicates the position of the cambium 
layer (Figs. 8 and 9). It was present even when there was no visible 
canker at the node apart from the base of the withered shoot itself. As 
with the cankers themselves, this necrosis of the xylem extends, as a rule, 
farther below the node than above it. 
Observations on the extension of the gum 4 pockets ’ from the original 
lesion were recorded in four cases, as follows : 
Description of the original lesion. 
Extension of gummosis 
( xylem necrosis ) from the 
original lesion. 
1. No canker visible on outside. Shoot dead and tissues brown as 
far as and including xylem formed in the current year. Hyphae 
found in the shoot only. 
Upwards. 
2*i cm. 
Downwards. 
1-5 cm. 
2. No canker visible on outside. The inner basal tissues of the 
shoot not brown ; the younger tissues of the shoot brown and 
containing hyphae. 
0-3 cm. 
1*5 cm. 
3. Canker present, extending two-thirds round the twig and upwards 
from the node for i-6 cm., downwards for 2-4 cm. 
5.5 cm. 
8-5 cm. 
4. Canker present, two-thirds round the twig, extending upwards 
5.0 cm. 
io*o cm. 
from the node for 1.3 cm., downwards for i-8 cm. 
No organism has been found in these disintegrated tissues beyond the 
cankers, and the necrosis appears to be brought about by an enzyme 
(secreted by the fungus) which diffuses along the young vessels and causes 
their hydrolysis. Whatever may be the cause, the action ceases early in the 
growing season, for if, in the winter following infection, a twig is cut across 
at 2 or 3 cm. above or below a canker, the disintegrated vessels are found to 
be embedded in the wood and confined to the inner edge of the ring of 
