Bacteria in their Relation to Vegetable Tissue. 31 



Besides the possible sources of immunity and resistance which we 

 have already considered, we have the action of the living plasma of 

 the plant. Concerning the exact nature of this force we have but 

 little positive knowledge, although it is probably chemical in its 

 action. We know that the plant endowed with vital activity is more 

 resistant toward outside influences than the same dead structure; also, 

 that protoplasm which is in an active state is much less subject to 

 the attacks of disease than quiescent or inactive protoplasm. This 

 has been demonstrated in the case of a number of tree-destroying 

 fungi that are only able to overcome the tissue cells during the winter 

 season of rest. 1 Anything that tends to impair the normal exercise 

 of the vital functions of the protoplasm predisposes the plant to the 

 attack of outside organisms. It is quite unlikely that this so-called 

 lowering of the general vitality affects to any considerable extent the 

 physical means of resistance. It is much more probable that it is 

 the repelling ability of the living protoplasm that is weakened, and 

 thus less resistance is offered to disease. To this action Marshall 

 Ward attributes a large share of the resistance of the plant against its 

 parasitic foes. 



Speaking of fungi, he says, so long as the protoplasm can over- 

 come, by respiratory oxidation or otherwise, the small amounts of 

 poison generated by the parasite, the hypha does not pass, but when 

 the poison exceeds this power of repulsion, then it effects an entrance 

 into the cell. 2 



In here presenting the sources which seem to be operative in the 

 production of the resistance and immunity of plant-tissues, examples 

 have also been given illustrative of this condition with fungi as well 

 as bacteria. While the conditions necessary for the best develop- 

 ment of these two classes of vegetable life are often different, there 

 can be but little question that the same means of protection which 

 the plant possesses, operates in many cases against the one quite as 

 effectually as against the other. The refractoriness of higher plants 

 against bacteria has many more points in common with the same 

 phenomena against fungi, than it has with the action of the animal 

 body against bacterial life. 



1 Hartig: Die Baumkrankheiten, S. 87, 112, 33 u. A. 



2 H. Marshall Ward : Journ. Roy. Soc., 1890, 213. 



