804 



PRINCIPLES OF SOIL MICROBIOLOGY 



by fungi fills in the gap between parasitism and symbiosis. Some of the 

 mycorrhiza are probably more symbiotic; others are often looked upon 

 as parasitic in nature; still others first attack the plant and then are di- 

 gested by the plant juices. In the last instance we have a case of bal- 

 anced parasitism; the root-tubercles of Arbutus unedo are caused by a 

 fungus first ectotrophic, then endotrophic; the digestion of the fungus 

 by the root tubercles confers immunity upon the root system as a whole. 7 

 On the other hand we find cases, like the infection of the thallus of the 

 liverwort Pellia epiphylla with a species of Phoma, 8 where the fungus 

 kills the protoplasmic contents of the infected cells. Since the plant can 

 be grown without the fungus, the relationship is largely parasitic. The 

 mycorrhiza of forest trees may be classed between these extremes. 



The relation of the soil organisms to the plant can thus be arranged 

 schematically as follows: 



Obligate parasitism - 

 (certain bacteria, 

 smuts, etc.) 



■ Balanced parasitism - 

 (certain mycorrhiza) 



Largely parasitic, — > 

 although plants may 

 derive some benefit 

 (certain mycorrhiza) 



Saprophytic microorgan- 

 isms (organisms de- 

 composing soil organic 

 matter, etc.) 



Parasitism combined - 

 with soil saprophy- 

 tism (Fusaria, Rhiz- 

 octonia) 



Symbiosis (legume - 

 bacteria and legumi- 

 nous plants, Pavetta 

 indica and other 

 plants with gall- 

 forming bacteria, 

 lichen-formations, 

 etc.) 



The problem of host selection and host specialization has been studied 

 carefully in the case of plant infesting nematodes. 9 The chief species of 

 these organisms attack a large number of host plants. However, 

 different populations of a nema species may prefer different host plants; 

 when different host plants are growing in a given soil area the soil nemas 

 will attack the one preferred, leaving the others unattacked even though 

 they are favored host plants of the particular species. If peas or oats 

 are grown on a soil for a number of successive years, Heterodera schachtii 

 becomes so adapted to this host that it will attack this plant in the 



7 Rivett, M. F. The root tubercles in Arbutus Unedo. Ann. Bot., 38: 661- 

 677. 1924. 



8 Ridler, 1922-1923 (p. 277). 



9 Steiner, G. The problem of host selection and host specialization of certain 

 plant infesting nemas and its application in the study of nemic pests. Phyto- 

 pathol., 15: 499-534. 1925. 



