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 do\vn 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 Forms 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 



