EXPERIMENTAL PRODUCTION OF PARASITES. 43 



From other experiments the following general results were obtained : 



The variety Blanehard is very sensitive. Pousse Debout is very resistant. Tubers of Chave 

 from I and IV were most attacked, those from V least. Tubers of Chardon from IV were seriously 

 injured, those from I and II only a little, and those from III not at all. In tubers from plots I and 

 IV a black zone was observed between the attacked and the healthy tissues. As this stain was not 

 noticed elswhere, Laurent attributed it to the nitrogenous product formed by the bacteria at the 

 expense of the tissues. 



Corresponding experiments made at 20 to 22 C. on Simson and Chardon potatoes and chicory 

 and carrot, gave very similar results. The inoculating material came from a third passage through 

 Early Rose, and was, therefore, quite virulent. All tubers and roots from plot IV were rapidly 

 attacked. The variety Simson was much less resistant to this bacillus than to B. fluoresce ns putidus. 

 Carrots from plots V and III and chicory from plots I, V, and III, resisted most strongly, while 

 those from IV were always completely attacked. 



Comparative experiments were also made on tubers coming from ( 1) a field dosed with 800 kg. 

 sulphate of ammonia, 800 kg., superphosphate, and 400 kg. each of kainite and sulphate of lime 

 and (2) a field which had received 80,000 kg. of barnyard manure. Tubers from (1) rotted completely 

 at 35 within 5 days after inoculation with virulent bacilli. Those from (2) rotted, but less rapidly. 

 Tubers from unfertilized land resisted the rot better than either of the other lots. Hence, Laurent 

 concludes that the use of fertilizers, by allowing an exaggerated absorption of nitrogenous com- 

 pounds, favored bacterial invasion. He states that lime diminishes the resistance of the potato, 

 carrot, and chicory. Nitrogenous fertilizers and potash salts had analogous but less striking 

 effects. On the other hand, phosphates, and to a lesser degree sodium chloride, increased the 

 resistance. 



The results with lime led Laurent to believe that differences in resistance were due to a modi- 

 fication in the acidity of the cell-sap. Experiments were undertaken to test this as follows: 



Tubers of Chave and Chardon, from plot III, known to be resistant, were cut in two and plunged 

 for 5 hours in the following solutions made up with distilled water: 



Potassium sulphate 2 per cent. 



Calcium sulphate 1 per cent. 



Ammonium sulphate 2 per cent. 



Asparagin 2 per cent. 



At the same time tubers of Early Rose and Marjolin from plot IV, known to be sensitive to 

 rot, were similarly plunged in the following solutions: 



Neutral sodium phosphate 2 per cent. 



Neutral potassium phosphate 2 per cent. 



Neutral ammonium phosphate 2 per cent. 



All the tubers were inoculated with the bacillus from a fourth passage through Early Rose, 

 and were kept in the thermostat at 35 . The first lot (resistant varieties) was uninjured, the second 

 lot (sensitive variety) was attacked. Exposure to these solutions, therefore, did not alter the resistance 

 of these varieties. Even exposure of the cut flesh of Marjolin and Blanehard for 40 hours to 1 per 

 cent acid potassium phosphate did not protect them when inoculated; after 15 hours at 30 C. they 

 were badly rotted. 



Resistant tubers of Chave and Chardon from plot III were immersed 3 hours in lime water, 

 1 per cent potash or 1 per cent soda solutions. At the same time sensitive tubers of Early Rose 

 and Marjolin from plot I were plunged in 1 per cent solutions of tartaric, citric, and lactic acid. 

 All were then inoculated with the bacillus from a fifth passage through Early Rose. After 12 hours 

 at 35 all were infected; the alkalis rendered the resistant ones susceptible and the acids did not 

 protect the sensitive ones. When the dose of organic acids was increased, however, the organism 

 did not succeed in penetrating the tissues, the acidity of whose cell sap was thus artificially increased 

 This is supposed to be due to the fact that the enzyme which dissolves the middle lamella?, acts 

 on the potato only in a slightly acid or else in an alkaline medium. 



As shown above, the cut surfaces of resistant tubers were rendered sensitive by immersion in 

 1 per cent alkaline solutions. The total acidity of the cell-sap does not, however, furnish an indica- 

 tion regarding the mechanism of immunity, for tubers of Preciosa and Zeland, two refractory 

 varieties, have an acidity (tested by phenolphthalein and estimated in milligrams of sulohuric acid 

 per 100 cc.j expressed by 231.1 and 289.0, while Blanehard and Early Rose, two little resistant 

 varieties, have an acidity represented by 317.8 and 387.1. 



Experiments were then made by immersing slices of little resistant varieties of potato for 12 

 hours in the juice of two resistant ones obtained by great pressure (300 atmospheres) to test the 

 protective effect of this juice. The results obtained were somewhat contradictory. The juices 



