Host and Parasite in certain Diseases of Plants. 429 



walls ; but as soon as the protoplasm of a cell succumbs, it signifies 

 its defeat by collapsing, and then its own more or less acid cell sap 

 filters through to the walls and hyphse. If this view is correct, and 

 the evidence supports it entirely, it is clear that any variations in the 

 host-plant which lead to weakening the outposts — cuticle and cell- wall 

 — or diminishing the fighting power of the protoplast, increase the 

 advantages of the hypha to a corresponding extent; and we have 

 seen that such variations exist when circumstances cause the cells to 

 become more watery and turgid, the cell-walls thinner and softer, and 

 so forth. But, no doubt, the most important event is the lowering of 

 the resisting power of the protoplasm, as must happen whenever 

 external changes — such as low temperature, murky weather and a 

 saturated atmosphere, &c. — combine in lessening the activity of 

 respiration and assimilation, and consequently bring about the accu- 

 mulation and possible filtration of organic acids, glucose, aspa- 

 ragin, &c. ; for, in the first place, the lowered metabolism means less 

 resisting power, no matter what hypothesis we adopt ; and, secondly, 

 the organic acids themselves prove an internal source of weakness if 

 they become too abundant, and filter through the partially dis- 

 organised protoplasm. The protoplasm, then, has its powers di- 

 minished or destroyed by the accumulation and inhibitory action of 

 products of its own activity. 



Fortunately, however, we have something more than the above 

 evidence, strong as it is, to support this view. Muller-Thurgau 

 found, in the case of grapes devastated by Botrytis cinerea* that the 

 Botrytis mycelium lives on the sugar, acids, and soluble nitrogenous 

 substances of the living cells ; but he also discovered, by means of 

 numerous comparative analyses, that the mycelium consumes espe- 

 cially the organic acids, the sugars to a less extent, and the soluble 

 nitrogenous matters were all converted into insoluble nitrogenous 

 substances. f No doubt some of the destruction of the acids is to be 

 put down to direct oxidation, but much is due to assimilation by the 

 fungus. Similar events were found to occur when the Botrytis was 

 cultivated as a saprophyte on the juice of grapes. 



It is, however, not difficult to give experimental proof of the accu- 

 racy of the statements that the entrance of the hypha into the cell is 

 dependent on the condition of the protoplasm. 



If the mycelium of one of these fungi is placed on an uninjured 



* Thiel's < Landwirthsch. Jahrb.,' 1888, pp. 83—160. 



f Penicillium behaved very differently : it took the sugars in greater proportion 

 than the acids, and left the juices more acid than before. Botrytis, on the other 

 hand, left the juices less acid than before, and more concentrated owing to the 

 evaporation of water from the injured grapes, and it is interesting to note that 

 these diseased (so-called edelfaule) grapes yield the best and sweetest wines of the 

 Rhine district. 



