30 THE LARCH CANKER 
by de Bary (1886), and later by Kissling (1889), Smith 
(1902), and Brooks (1908), in the case of Botrytis cinerea. 
Various views have been entertained as to the substance 
which Botrytis secretes, but recent work by Blackman and 
Welsford (1916) and W. Brown (1915, 1917) has shown 
that oxalic acid plays no significant part in the parasitism 
of this fungus. These authors have further demonstrated 
that the only active substance secreted by the germ tubes 
of the fungus is an enzyme which destroys the cellulose 
walls of the host. Whether or no oxalic acid plays a part 
in the parasitism of Dasyscypha, there seems to be little 
doubt that the fungus should be regarded rather as a plant 
poisoner than a true parasite. It first kills the cells in its 
neighbourhood and then grows into them. 
(ii) The tissues in the proximity of the fungus grow 
faster than usual, causing a swelling which is noticeable in 
young cankers. A specialized form of this activity is the 
formation of new cork layers. These may surround an 
infected area and inhibit further advance of the fungus. 
Cases have been observed where the mycelium has been 
successfully prevented from reaching the phloem by such 
a cork layer, and probably this frequently happens, though 
it casily escapes observation, as the general healthy appear- 
ance of the stem is so little disturbed. The tree always tries 
to isolate the fungus, and when the mycelium has penetrated 
the phloem and killed the cambium at any point, phellogen 
layers are instituted, which cross the cortex radially and 
then traverse the phloem in an oblique direction, and this 
-phellogen forms a ring of cork which tends to prevent the 
canker spreading in a lateral direction. The cork layer 
formed by such a phellogen for a time completely prevents 
the further growth of the mycelium, and in some sections 
I have seen the tissues on one side of a cork layer healthy 
and apparently normal, whereas on the other side the cells 
were brown and had contracted, leaving large intercellular 
spaces filled with hyphae. Often, however, such cork 
layers may be found embedded in the brown attacked 
tissues, showing that the fungus has succeeded in passing 
