14 



periments in the laboratory led gradually to the knowledge that in many 

 cases of disease no specific parasites were present but universally distributed 

 fungous and bacterial forms. The further the studies advanced, the more 

 cases were listed in which inoculation with spores of the most common 

 molds, as Botrytis, Penicillium, Cladosporium etc., also the most widely dis- 

 tributed soil bacteria. Bacillus subtilis and B. vulgatiis, develop disease in 

 healthy tissue. And finally was recognized the importance of the question 

 how organisms universally present could at times be parasitic in their mode 

 of life and, at other times, saprophytic. Corollary to this question is one 

 which was deduced from rapidly increasing discoveries in many experiments 

 with the same methods of infection ; certain varieties or even individuals 

 were resistant while others succuml)cd easily to the parasitic attack. \Miat 

 is the cause of such differences? 



Some of the investigators brought forward the theory of virulence as 

 an explanation of such cases. It was emphasized that in each separate case 

 parasitism as a struggle between tw^o organisms had depended necessarily 

 upon which was the stronger. If the weapon of attack of the parasite, for 

 instance, be an enzyme, able to dissolve the cell walls of the host, then it 

 would be explicable that this process would take place more quickly in pro- 

 portion to the increase of solvent ferment formed in any given unit of time. 

 Since it was now possible to prove experimentally that the strength of the 

 attack varied in cultures of different nutritive substances, it could be said 

 that, w^here it became the active agent of disease and its production of enzy- 

 mes especially abundant, it must have been especially virulent. Bacterial 

 cultures furnished the greatest number of examples of change in virulence. 

 Yet such cases w^ere also determined with fungi. De Bary's statement con- 

 cerning the frequently encountered mold, Balryiis cinerca, is well-known. 

 He states that the mycelium must develop by the customary saprophytic 

 form of nutrition up to a certain strength before it becomes parasitic and 

 successfully attacks the living parts of the plants. I succeeded in getting 

 like results with the conidia of this fungus. Masses of spores were strewn 

 on delicate Begonia leaves and kept very damp. After several days it was 

 possible to observe that, where these spores had lain in thick masses, the leaf 

 had become diseased, showing a browning of the tissue. Where the spores 

 had lain isolated, however, no attack could be discerned. The action of the 

 ({uantity of ferment excreted by the individual spores therefore proved in- 

 sufficient, while the excretion from a mass of spores brought about infection. 

 It can thus easily be understood that parasites, like every other organism, de- 

 velop most strongly when the nutritive conditions are most favorable and 

 that the stronger and the more abundant the formation of their vegetative 

 organs, the greater the excretion of the enzyme and accordingly the increase 

 in strength of their attack. Therefore their virulence is raised. 



But these processes are not sufficient to explain the fact that in one field 

 when a number of varieties are grow^n in a single plantation, certa-n ones 

 may be completely destroyed while others standing next are but little injured. 



