28 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



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 glycerine, which, on being placed in suit- 

 able 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 sdow resting on the branches ; (h) 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 laricis, 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. 



I have constantly observed that in the month of May — when 

 practically all infection occurs — the proportion of sap is much 

 greater in seedlings and quite young trees than in old ones. 

 What I mean exactly is this, that if branches of equal age of a 

 young and an old tree are wounded by inserting a needle or a 

 lancet, sap oozes to the surface in the case of the young tree, but 

 not in the old one. 



It is this sap that retains floating spores, and also serves as a 

 nutrient solution in which they readily germinate. 



Commencing with the causes which lead lo the development of 

 canker in seedlings and young trees, the larch aphis (Chermes 

 laricis, Hartig) is most responsible ; in fact, I consider it to be 

 no exaggeration to state that if the aphis could be obliterated, 

 larch canker, as a destructive epidemic, would cease to exist in. 

 this country. As to whether the interdependence between aphis 



