THE HONEY FUNGUS 159 
The method of infection. Parasite and host are con- 
stantly at war. At one stage in the contest the parasite 
has the advantage, at another the host. And it is the 
business of the pathologist to find out the line of defence 
where the host has the best chance of success against its 
aggressor, as assistance may there be of greatest advantage. 
Now, it is generally in the opening stages of the conflict 
that the plant is at its strongest. It is the obstacles it 
presents to the entrance of fungi, its cuticle and bark, 
which save it from what would otherwise be universal defeat 
and destruction. At the same time, parasites have become 
adapted so as to overcome these obstacles in a variety of 
ways. 
The spores of a few fungi can, on germinating, actually 
pierce the cuticle of leaves and stems; many germ tubes 
have acquired the power of finding stomata and entering 
the plant through them, and spores of other fungi germinate 
on the stigmas of flowers and grow down into the ovaries 
by the same route as the pollen tubes. But those fungi 
which attack the trunks of trees have a much more formidable 
barrier to pass, since in the older parts of stems the epidermis 
is replaced by the bark, which is composed of many layers 
of cork cells, which are almost impermeable to fungi. 
Though the earlier pathologists seem commonly to have 
accepted the theory that fungi pierce the sound bark of 
trees, no authenticated instance of this has ever been 
recorded, and the trend of recent opinion has been more 
and more in the direction of admitting the possibility of 
infection only by wounds or by outflanking the bark pro- 
tection. Willkomm, for instance, thought that the larch 
canker fungus gained admission to the cortex through 
undamaged bark, a view that was disproved by Hartig. 
But Hartig, in his turn, thought that Fomes annosus could 
attack healthy unwounded roots, which now appears ex- 
tremely improbable. 
The mode of infection employed by Armillaria mellea 
has never been very critically examined. The question is 
a difficult one, and exceptional care and subtlety will be 
