36 Chapter II 



actually succeeded iii communicating an artificial immunity to varieties 

 of the potato which were most susceptible to decay by immersing 

 them for several hours in solutions of certain organic acids. On the 

 other hand, when he treated varieties endowed with a well-marked 

 natural immunity with alkaline solutions, the tubers became very 

 susceptible to the decay set up by the bacillus. 



The struggle between the potato and the Bacillus coli reduces 

 itself, then, to the chemical reaction between the alkaline cell-secre- 

 tions of the micro-organism and the acid secretions of the potato. 

 This general fact, according, to Laurent, explains the part played by 

 certain manures in determining the susceptibility or the resistance 

 manifested by the potato arid many other plants against infective 

 diseases. 



We know that the addition of phosphates to the soil increases the 

 immunity of certain cultivated plants. These substances are greedily 

 absorbed by the roots and produce acid salts which are dissolved in 

 the cell-juice. The nitrogenous manures, on the other hand, both 

 potassic and lime, diminish the resistance of the same plants, 

 probably from the fact that they bring about a diminution of the 

 acidity of the cell-juice. 



But these manures can act difierently on difierent plants. Thus 

 the same phosphates which confer immunity on the potato against 

 bacterial decay render the Jerusalem artichoke more susceptible to 

 the attacks of the Sclerotinia, 



Laurent explains this fact as due to the difierence in the reaction 

 of the medium, which favours the action of one or the other of the 

 [39] soluble ferments of the two parasites. The ferment of the bacillus 

 digests the cell-membrane in an alkaline or feebly acid medium, 

 whereas the hyperacidity which results from the absorption of the 

 phosphates prevents this digestion and consequently aids the plant in 

 its struggle. On the other hand, the ferment of Sclerotmia, as is seen 

 from the researches of de Bary, will digest cellulose even in a dis- 

 tinctly acid medium. The hyperacidity, induced by the phosphated 

 manure, in this case favours the parasite and enables it to gain the 

 upper hand in the struggle with the tissues of the artichoke. 



In addition to neutralising the microbial products the acids of 

 the cell-juice also act injuriously on most bacteria, which will only 

 develop in neutral or alkaline media ; it is for this reason that 

 bacterial diseases are so much rarer in plants than in animals. 



The secretion of cell-juices is consequently a very important 



