284 DISEASE IN I'LANTS. 



poisoned and destroyed, as in the killing off of 

 seedlings in " Damping Off" : near the other ex- 

 treme we have cases where the foreign protoplasm 

 of the parasite, although it gains complete access 

 to that of the host, merely stimulates the latter to 

 greater activity and itself works for its own ends 

 in conjunction with it e.g. Plasmodiophora. In 

 such instances we must figure to ourselves the cells 

 of the root of the Crucifer handing on food-materials 

 to both masses of protoplasm that of the Plas- 

 viodiophom and that of the cell into which it 

 penetrates ; and it is immaterial whether both ob- 

 tain the food-materials directly, or, what seems 

 more likely, the fungus only at second hand and 

 by the medium of the host's protoplasm. In any 

 case, the latter is for a long time at least not 

 poisoned or maimed, or in any perceptible way 

 injured by excreta from the fungus-protoplasm, 

 although it is evident that each must excrete 

 various metabolites which may soak into and 

 be taken up by the other : on the contrary the 

 host-protoplasm grows larger, attracts more food 

 supplies, rhakes larger cells, and is evidently 

 stimulated to greater activity for the time being, 

 its behaviour reminding us of the stimulation of 

 cells by means of slight doses of poison referred 

 to previously. We must therefore assume that 

 the general course of building up and breaking 

 down of its protoplasm-molecules go on as usual 

 or nearly so in both the host cell and the 

 invader ; and that the assimilatory, respiratory, 

 excretory and other functions are carried on in 



