28 



wound by means of a deposition of cork. Further protective means are oils, 

 resin, balsams, milky juices and gums exuding from injuries. 



Metschnikoff thoroughly treats of Laurent's^ studies which are 

 mentioned in connection with other bacteria in the second volume of this, 

 work. At this point, however, we will emphasize especially the immunity 

 I)recautions against bacterial attacks. The species of the Colibacillus, with 

 which Laurent worked, secretes a ferment dissolving the cellulose of the 

 j.otato tuber and produces also sap with alkaline reaction, the presence of 

 which is necessary for the process of assimilation on the part of the bacteria. 

 Xow, to be sure, Bacillus Coli communis is naturally not a plant parasite but 

 it can be changed into one. This happens when it is first cultivated on po- 

 tatoes whose resistance has been weakened by having been dipped into alka- 

 hne solutions As a result of such cultivation the bacillus can act as a plant 

 parasite when carried over to the same species of potato. The struggle be- 

 tween the Colibacillus and the potato depends therefore really on the chemi- 

 cal action of the alkaline secretion of the bacillus on the acid cell sap of the 

 potato. After fertilization with potassium salts and phosphates, carrots and 

 potatoes resist the bacillus. On the other hand, a phosphate fertilization 

 showed in (Topinambur) that this plant then became more susceptible to the 

 Jjotrytis form of Sclerotinia Lihertinia. 



Just as clearly by strong nitrogen fertilization potatoes are made less re- 

 sistant to wet rot. According to our observations abundant fertilizing with 

 nitrates, ammonia salts or stable manure, causes even the most resistant 

 species to succumb to the potato rot. Laurent explains the difference in the 

 action of parasites under the same method of fertilizing by the fact that with 

 bacteria the secreted ferment can attack the cell membrane only in alkaline 

 juices or weakly acid ones. An increased acidity of the cell sap, incited by 

 the formation of acid salts resulting from phosphate fertilization, renders 

 the plants immune to this fission fungus. I obtained the same results for 

 phosphoric acid by fertilization experiments on sugar beets, in which the 

 Bacillus betae was widely disseminated and had produced the bacterial for- 

 mation of gum or tail rot. The rapid increase of bacteriosis with the abun- 

 dant use of fertilizers which contain nitrogen might be explained in this 

 way : — that the acid of the cell sap is thereby decreased. According to de 

 lUiry, the conditions for Sclerotinia are exactly reversed. Their ferment 

 dissolves the cell wall only in an acid fluid. Most mycelial fungi act 

 similarly. 



If, by a change of constitution of the cell sap, sometimes a factor of im- 

 munity presents itself and, at other times, a condition predisposing to para- 

 sitic disease, we are referred by Metschnikoff (1. c. p. 39) to a further pro- 

 cess. He cites the investigations of van Rysselberghe- who found, 

 especially in the epidermal cells of Tradescantia that if these cells were 



1 I^aurent, Recherohe.s exptTimentalo.s sur les maladies des plantes. Anna!, 

 de rinst. Pasteur. Cit. Zeitscher. f. J'Hanzenkr. 1900, p. 29. 



- Osmotische Reaktion der I'fianzenzellen. Memoires couronnes de I'Academie 

 r. d. Belgique. Briissel 1899. 



