vi.] DISEASES DUE TO CERTAIN PARASITES. 173 



these diseases, and I must be content here with the 

 bare statement that these " cankers " are in the main 

 due to local injury or destruction of the cambium. If 

 the normal cylindrical sheet of cambium is locally 

 irritated or destroyed, no one can wonder that the 

 thickening layers of wood are not continued normally 

 at the locality in question : the uninjured cells are 

 also influenced, and abnormal cushions of tissue 

 formed which vary in different cases. Now, in 

 " cankers " this is put shortly what happens : it may 

 be, and often is, due to the local action of a parasitic 

 fungus ; or it may be and, again, often is owing to 

 injuries produced by the weather, in the broad sense, 

 and saprophytic organisms may subsequently invade 

 the wounds. 



The details as to how the injury thus set up is pro- 

 pagated to other parts how the " canker " spreads 

 into the bark and wood around are details, and would 

 require considerable space for their description : the 

 chief point here is again the destructive action of 

 mycelia of various fungi, which by means of their 

 powers of pervading the cells and vessels of the wood, 

 and of secreting soluble ferments which break down 

 the structure of the timber, render the latter diseased 

 and unfit for use. The only too well known larch- 

 disease is a case in point ; but, since this is a subject 



