Johnson — Willow Canker. 159 



One peculiar feature which appeared in several different cankers 



is worthy of note. After a pycnidium has emptied its contents, its 



floor rises up into its cavity, -and forms a second pycnidium with 



contents, replacing the one the cavity of which it has obliterated 



(Plate XV., fig. 1). Invagination is common in marine Algse, 



and in some fungi, where a sporangium, emptied of its contents, 



becomes invaded by the turgid cell below, which becomes, in its 



turn, a sporangium. I have not before seen invagination of a 



pycnidium. 



Infection Experiments. 



I have seen the canker-spots on so many different rods, all show- 

 ing the same kind of bodies as those described, that I am fully satis- 

 fied as to the cause of the local rottenness of the rod. On July 6th, 

 1903, I infected, under sterilization conditions, several perfectly 

 healthy young plants of several species of Salix, supplied by 

 Mr. F. W. Moore, m.r.i.a., the Keeper of the Eoyal Botanic 

 Gardens, Glasnevin. On the 29th of September, while the control 

 plants were quite healthy and normal, the infected ones at the 

 points of infection showed signs of canker-formation, with myce- 

 lial hyphse and pycnidia, which were too young for complete 

 identification. As far as the experiments went, they tended to 

 show the reappearance of the disease. 



Prevention of the Disease. 



To carry out experiments in the prevention of the disease 

 by fungicides requires ground such as is provided in the Govern- 

 ment Pathological Stations I have seen in France and Germany. 

 In the absence, as yet, of such facilities here, I have been confined 

 to laboratory experiments. Mr. J. Adams, b.a., the demonstrator 

 of Botany in the Eoyal College of Science, has made a number 

 of experiments on the ascospores and conidia, treated with Bor- 

 deaux mixture, 2 per cent. ; sulphide of potash, 0'25 per cent. ; 

 or with formalin, 0*5 per cent, solution. In each case the 

 spores were soaked for an hour in the solution, and then cultivated 

 in a boiled extract of willows. The formalin proved most fungi- 

 cidal. Neither ascospores, conidia, nor the mycelium in the willow 

 stem, germinated after treatment with it. 



Field experiments are needed before one can make a definite 

 recommendation for practical purposes. 



