PRESIDENTIAL ADDRESS. 45 



Some years later, Hiltner 1 in experimenting with peas r 

 found that repeated growth of the plants in a soil resulted 

 in the development of sickness. This however, seemed to 

 cure itself as it developed in the second and third crops, but 

 disappeared in the fourth, fifth and sixth. Treatment of 

 the soil with carbon disulphide caused the sickness to 

 reappear. This led him to think that the growth of the 

 plant causes an accumulation of toxic substances, but that 

 continued growth produces antitoxins which are destroyed 

 by carbon disulphide. Ponget and Ohouchak 2 confirmed 

 the observation of Koch, that extracts of lucerne-sick soils 

 were capable of injuriously affecting healthy plants, but 

 they concluded that the plants secreted substances toxic 

 to themselves. 



On the whole, the investigators who have worked with 

 soil-sickness, favour the idea of a plant secretion which is 

 toxic, and therefore discount the possibility of the cause 

 being due to an alteration of the mycological flora, with 

 the increased development of a toxin-producing micro- 

 organism. There appears to be more likelihood of an indirect 

 action being the true cause. Bolley 3 found a mouldy 

 Fusarium lini, infesting the roots of Linum in a flax-sick 

 soil, and considered that it produced the disease. It is 

 probable that many more similar examples could be found 

 of bacteria and moulds being directly responsible for the 

 various kinds of sickness by the secretion of toxins. The 

 conveyance of the disease by water, and the action of heat 

 and disinfectants, are against the probability of the sickness 

 being due to a toxin directly secreted by the plants. 

 Schreiner and Sullivan* isolated crystalline toxins from 

 wheat-sick and cowpea-sick soils which were toxic to these 

 plants, but this does not militate against the idea that the 



• Cent. Bakt. 2 te, 21, 536. a Comp. Rend. (1907) 145, 1200. 

 3 Bull. N. Dakota, Ag. Coll., No. 50. 4 Chem. Zeifc., 1908, 410. 



