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. Thes? 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 easily 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 lulled 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 



