NATURE OF DISKASK. 137 



cell-walls, but that of the living protoplasm also : 

 if it cannot do the latter it must remain outside, 

 as a mere epiphyte, or at most an intercellular 

 endophyte. If it can do neither it must either 

 content itself with a saprophytic existence or fail, 

 so far as that particular host-plant is concerned. 

 Its inability to enter may be due to there being 

 no chemotropic attraction, or to its incapacity to 

 dissolve the cell-walls, or to the existence in the 

 cell of some antagonistic substance which neutral- 

 ises its acid secretions, destroys its enzymes or 

 poisons, or is even directly poisonous to it. 



Moreover when once inside it does not follow 

 that it can kill the cell. The protoplasm of the 

 latter may have been unable to prevent the fungus 

 enemy from breaking through its first line of 

 defence the cell-wall, but it may be quite capable 

 of maintaining the fight at close quarters, and 

 we see signs of the progress of the struggle in 

 hypertrophy, accumulation of stores, and other 

 changes in the invaded cells and their contents. 



Finally, the invested or invaded cell may so 

 adapt itself to the demands of the invader that a 

 sort of arrangement is arrived at by which life in 

 common Symbiosis is established, each organism 

 doing something for the other and each taking 

 something from the other. In this latter case, 

 which is often realised e.g. lichens, leguminous 

 plants and the organisms in their root-nodules, 

 mycorrhiza, etc. we leave the domain of disease, 

 which supervenes indeed if the other symbiont is 

 lacking. 



