Larch and Spruc e Canker. 



179 



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 

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

 suspended by means of thread, at distances varying from 6 to 

 18 inches from cankered spots bearing well-developed ascophores. 

 Examined after twelve hours' exposure numerous spores were 

 found entrapped in the glycerin, which on being placed in 

 suitable medium germinated readily. Similarly prepared slips 

 of glass suspended haphazard in a plantation where canker 

 abounded also caught floating spores. No conidia were observed 

 on any of the slides. 



The wounds occurring in nature through which inoculations 

 take place may be grouped under four headings : (A) wounds 

 caused by wind, or by snow resting on the branches ; (B) extru- 

 sion of sap caused by late frosts ; (C) nibbling of the cortex by 

 insects, and more especially the punctures made by the larch 

 aphis (Chermes /aria's, Hartig) ; (D) wounds made near the base 

 of the stem when planting young trees. 



As a broad rule, it may be stated, that when trees under ten 

 years of age are attacked by canker, they are either killed out- 

 right, or are so deformed that if they continue to grow, they are 

 of very inferior value from an economic standpoint. The reason 

 for this is that in the case of seedlings or very young trees, the 

 main stem is the part usually attacked, whereas in older trees 

 the bark of the trunk becomes so rigid that it is impervious to 

 the punctures of aphides, and it is not so readily influenced by 

 frost ; and the only chance of admission to the cambium of the 

 trunk is when branches are either broken off or more or less 

 cracked at the point where they leave the trunk. 



M 2 



