THE LARCH CANKER 53 



during their first growing season that justifies the conchision 

 that shoots do not become infected until the first year's 

 growth is complete. This is in keeping with the theory 

 already enunciated that DasyscypJm requires dead tissue 

 to grow on while it is secreting those substances by which 

 it kills the hving 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 Idnds occur on young 

 branches, which may all help in furthering infection. These 

 wounds may be caused by : 



1. Frost. The late frosts of ten 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 wdll 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, 



