THE LARCH CANKER 53 
during their first growing season that justifies the conclusion 
that shoots do not become infected until the first year’s 
growth is complete. This is in keeping with the theory 
already enunciated that Dasyscypha requires dead tissue 
to grow on while it is secreting those substances by which 
it kills the living cells in its vicinity. 
By the end of the first year of growth a larch shoot has 
made a continuous cork layer, and as long as this remains 
unbroken it is quite impervious to the hyphae outside it. 
The only weak points in this armour are at the leaf bases 
(see p. 10), and though no artificial infections have proved 
successful at such points without wounding, yet it is just 
possible that in an extremely small percentage of cases 
infection does actually occur here. 
In addition, wounds of various kinds occur on young 
branches, which may all help in furthering infection. These 
wounds may be caused by : 
1. Frost. The late frosts often cause ruptures in the cork 
protection of young shoots, presumably through the swelling 
of the saturated cortical cells inside. Such wounds are 
usually seen as whitish specks, the white colour being due 
to resin which is secreted through the wound. The mycelium 
will tolerate a considerable percentage of resin in its sub- 
stratum, but an almost pure resin layer, such as is here 
formed, is impermeable to it. It is thus unlikely that 
infection will take place through these wounds, except 
immediately after their formation. 
2. Hail. Hail-stones often make wounds on young 
stems and may even break off young shoots. These wounds 
also are quickly covered by resin, and after a very short 
interval they are protected against the canker fungus. 
3. Chermes. Massee’s infection experiments, which showed 
the possible connexion between canker and attacks of 
Chermes abietis, have been cited above, and the punctures 
made by the Chermes should be included among the list of 
wounds. Further evidence on this point may be looked for 
as a result of the experiments initiated by Burdon (1908), 
in which larches infected with Chermes were to be compared, 
