GENERAL CONSIDERATIONS 293 



plants appear to be less complex than interactions between micro- 

 organisms in the soil, primarily because a smaller number of spe- 

 cies is involved. Undoubtedly the spores of many species germi- 

 nate at the surface of the plant, but only those of the pathogenic 

 species succeed in producing infections. Once the lesions are 

 formed, however, saprophytes may enter. In some instances 

 tissue plantings from young lesions are found to yield pure cul- 

 tures of the pathogen, but at a later date the tissues always yield 

 a mixture of the pathogen and one or more secondary invaders. 

 Still later it may be impossible to isolate the primary fungus, the 

 secondary ones may also have been eliminated, and the tissues may 

 be completely occupied by tertiary species. It is highly probable 

 that successions of this sort do not result simply from exhaustion 

 of specific food materials by the several organisms concerned. A 

 solution of the problem of these interactions must be based upon 

 an understanding of the physiology of each organism concerned, 

 especially of their enzyme-producing abilities and the metabolic 

 products they form. Only a beginning has as yet been made in 

 this field of research. 



The existence of several species of fungi in the same lesion may 

 also be interpreted to indicate that the conception of mono- 

 etiology of disease in plants, as in animals, is altogether too narrow 

 and may actually lead to misinterpretations. 



LITERATURE CITED 



Abraham, E. P., E. Chain, C. M. Fletcher, A. D. Gardner, N. G. Heatley, 



and M. A. Jennings, "Further observations on penicillin," Lancet, 2: 7, 



177-188, 1941. 

 Allen, M. C, and C. M. Haenseler, "Antagonistic action of Trichoderma 



on Rhizoctonia and other soil fungi," Phytopathology, 25: 244-252, 



1935. 

 Arrillaga, J. G., "The nature of inhibition between certain fungi parasitic 



on citrus," Phytopathology, 25:161-115, 1935. 

 Asthana, R. P., and L. E. Hawker, "Influence of certain fungi on the 



sporulation of Melanospora destruens Shear and of some other Ascomy- 



cetes," Ann. Botany, 50: 325-343, 1936. 

 Bamberg, R. H., "Bacteria antibiotic to Ustilago zeae" Phytopathology, 



27:881-890, 1931. 

 Bary, A. de, Beitrage zur Morphologie und Physiologie der Pilze, 4: 85, 1881. 

 Boyle, C, "Studies on the physiology of parasitism. X. The growth re- 

 actions of certain fungi to their staling products," Ann. Botany, 38: 113- 



135, 1924. 



