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. 
(iti) 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 D 
