SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 805 



presence of a number of other plants, which may not be attacked at all. 

 This observation was first made by Liebscher. 10 Nemas are capable of 

 locating their host plants at considerable distances, moving even against 

 the water-flow. 11 The nemas are even capable of distinguishing closely 

 related plant species. This was explained by the fact that although 

 different species of plant-parasitic nemas feed on a wide range of host 

 plants, a given population of one species will, if possible, always attack 

 first the kind of host plants that its ancestors lived on. If this host 

 plant is not available, host plants of near relationship (taxonomical, 

 chemical) are sought and attacked. If the ancestors of a given 

 population lived on a number of host plants for many generations, the 

 population is polyphagous. If the ancestors of a population lived 

 for many generations on a single species or variety of plant, their de- 

 scendants will always attack that host plant and, only in exceptional 

 cases, other plants, unless the host is absent; such a population is 

 monophagous. 



The growing plants seem to produce some root secretions which are 

 carried by the soil water and act as stimuli upon the nemas. The latter 

 perceive the stimuli by means of a special sense organ (amphid). The 

 nema then moves towards the points of higher concentration of the 

 stimulating fluid until the host plant is reached. 



A detailed review of the relation between parasite and host plant, 

 especially in respect to rusts, is given elsewhere. 12 



Animal and plant diseases caused by bacteria that may be found in 

 the soil. A number of bacteria capable of causing various animal dis- 

 eases have been isolated from the soil, where they find a natural habitat 

 or persist only for longer or shorter periods of time. It is sufficient to 

 mention that Bac. anthracis (Pasteur, Koch), Bac. tetani (Nicolaier, 

 Sanfelice), Bac. chauvoei (Arloing, Pellegrino), Bad. pestis (Jersin), 

 Bad. typhi (Mace) will persist in the soil for many months or even 

 years. Pathogenic bacteria were often found to be actually capable of 

 developing in the soil. Pasteur showed that earthworms can spread 



10 Liebscher, G. Beobachtungen iiber das Auftreten eines Nematoden an 

 Erbsen. Jour. Landw. 40: 357-36S. 1892. 

 « Baunacke, 1922 (p. 344). 



12 Zimmermann. Sammelreferate iiber die Beziehungen zwischen Parasit 

 und Wirtspflanze. Centrbl. Bakt., II, 65: 311—418. 1925. 



13 Prausnik, \V. Die Hygiene des Bodens. Handb. der Hygiene (Rubner 

 usw.), 1: 520-562. 1911 (Int. Mitt. Bodenk., 4: 239); Bail, O., and Breinl, F. 

 Versuche iiber das seitliche Verdringen von Verunreinigungen im Boden. Arch. 

 Hyg., 82: H. I. 1914. 



