20 Transaclions British Mycological Society. 



each type of fungus : in many cases it is intercellular, with 

 occasional haustoria piercing \he cells: thus Phytophthora, 

 the potato disease, is confined mainly to the intercellular spaces 

 and the cell walls, while Pythium hyph^e enter the cells, feed 

 on the contents and pass to others. 



In many or most tree and root parasites the fungus tirst 

 gains entrance by wounds. Fomcs annosus, a root rot, 

 enters between the bark scales and reaches the living cortex 

 killing the parenchyma by means of ferments; it penetrates 

 the wood and travels up to the stem through the woody 

 elements, lateral hyphal branches entering the medullary 

 rays and the adjoining tracheids : the cells are delignified by 

 the fungus from within outwards, the delicate lamellae remain- 

 ing for a while as a skeleton and then also disappearing. The 

 same action on the cells is effected by Polyporus squaniosus. 

 Different fungi have, however, different methods of attack. 

 Marshall Ward* found that Stcreum hirsutum causes deligni- 

 fication of the wood first, while CoUybia vchitipcs consumes 

 the cellulose and leaves the lignin behind. 



The injury done to the host consists in the seizing of its 

 oxygen, water and food supply, and by means of ferments 

 converting them into substances required by the fungus. 

 That the ferments may be also most virulently toxic was 

 proved bv Brownt who injected certain host plants with 

 an extract from the Botrytis fungus and found that it 

 produced exactly the same effect as the living fungus : in 

 half an hour after treatment the cells of a potato had become 

 disorganized and the brown or black colouration shewed 

 that the contents had been killed. The cells were macerated 

 by the ferment which dissolves the middle lamella ; but the 

 death of the cells only takes place at a late phase of the 

 process of disorganization of the cell walls. The extract 

 cannot, however, dissolve the cuticle : entrance must first 

 be gained by mechanical means. It was found that the 

 ferment could be rendered innocuous by heat, by violent 

 agitation or bv alkalis. In a study of yeast enzymes 

 Bokorny:): found that these were aft'ected by poison in the 

 same way as protoplasm, but they are more resistant so that 

 it was possible to kill the protoplasm and }'et leave the 

 enzyme uninjured. 



Reaction of the ho.<t cell. In many cases of disease, as in 

 Botrytis, the fungus is directly toxic in action and the cells 

 are Cjuicklv destroved. This is not however ahvavs the case, 



* Marshall Ward. 

 t Brown, '15 & '16. X Bokorny, '06. 



