LARCH AND SPRUCE FIR CANKER. 



27 



rapid drying. The first indication of the success of an inocula- 

 tion was usually manifested during the sixth to eighth week 

 after the experiment, when the outer dead bark became cracked 

 and raised, due to accelerated growth of the living cortex under- 

 neath ; and at the expiration of about ten weeks a few pustules 

 of conidia, accompanied by a small number of usually imperfectly 

 developed ascopbores, appeared if the weather continued moist ; 

 but, as a rule, it was not until the following year, in April or 

 May, that well-formed cups were produced. 



In addition to the kind of wound described above, I have 

 proved by repeated experiments that a pin-prick makes a wound 

 sufficiently large for the purpose of a successful inoculation, if 

 spoi'es are placed in the drop of liquid oozing out of the wound. 



May is the month during which artificial infection takes most 

 readily, and I imagine that the same holds good in a state of 

 nature, for the following reasons. Well -developed ascophores 

 are most abundant during this month, and there is a super- 

 abundance of sap which readily oozes to the surface through the 

 smallest puncture it is possible to make with a tine needle. In 

 this extruded sap the ascospores readily germinate and enter the 

 living tissues of the tree. 



The large percentage of instances where the canker appeared 

 only at the point of artificial inoculation proved that the disease 

 was in reality the outcome of such inoculation ; nevertheless, in 

 two instances where four-year-old larches, gi'owing in pots, and 

 obtained from a locality where the disease was supposed to be 

 entirely absent, were inoculated, I was surprised to find that in 

 one tree the canker appeared simultaneously at three different 

 points, two of which had not been artificially inoculated ; and 

 that in the second tree, artificial inoculation produced no result, 

 but the canker appeared on another part. 



The only explanation that can be suggested is that, in this 

 instance, the disease must have existed in the locality where the 

 trees were raised, and that infection had taken place from floating 

 spores before the trees came into my possession. 



That the ascospores when expelled from the asci in the cup do 

 actually float in the atmosphere, was definitely proved by the 

 following experiment, which was repeated on several occasions. 

 Ordinary glass slips used for microscopic purposes, having about 

 a square inch of their surface on one side smeared with dilute 

 glycerine, which does not dry up when exposed to the air, were 



