THE LARCH CANKER 33 



efficient channels for the conduction of organized food 

 material sent down from the leaves. 



The canker as a pathological structure. The type of 

 structure known as a canker is the outcome of the action 

 of a fungus on a stem. The same general result may be 

 produced in different trees by different fungi (cf. canker 

 of apple and ash caused by Nectria ditissima) . But fungi 

 which are capable of producing cankers must have four 

 properties which, together, distinguish them from most other 

 parasites. These features may be summed up as follows : 



(i) They must be capable of killing living tissues, i. e. either 

 they must be parasites in the ordinary significance of the 

 word, or they must be capable of secreting some substance 

 which kills the cells in their vicinity so that the fungus can 

 penetrate them. 



(ii) They must be essentially ' rind fungi ', i. e. fungi 

 whose mycelia flourish in the cortex and phloem of the stem 

 and kill the cambium ; and if the hyphae enter the wood 

 at all, they must be incapable of spreading extensively in 

 this part of the stem, or reinfecting the phloem ; otherwise 

 the hyphae would soon spread right through the tree and 

 kill an entire section of the stem. 



(iii) The mycelium must be perennial in the tissues of the 

 cortex and phloem. 



(iv) The mycelium must spread from cell to cell extremely 

 slowly, so that its rate of tangential extension is approxi- 

 mately equal to the rate of increase in girth of the stem. 

 If the mycelium spread too fast, it would soon extend the 

 whole way round the stem, whereas a canker can only be 

 produced as the result of continued growth of the stem 

 whilst being attacked. And if the mycelium spread too 

 slowly it would never get through the phloem and reach 

 the cambium. When a fungus with these four characteris- 

 tics attacks a host, a canker is the necessary result. 



The canker of the larch usually obtains its hold on the 

 stem within the first six years of its growth. The fungus 

 gains admission in what way will be shown later and 

 flourishes in the cortex and outer phloem, where there are 



1888 



