26 



duced. In the previous divisions we have considered investigations showing 

 that different groups of substances produced in the plant cells, as, for in- 

 stance, sugar, act attractively for certain fungi in definite concentrations and 

 repellantly in others. The number of these groups of substances is deter- 

 mined by very different factors, as will be shown more thoroughly in the next 

 chapter. This metabolism will be found favorable for the nutrition of the 

 parasite or unsuitable for it, according to the quantity produced. 



In order to cite at least one example in this connection, we will refer to 

 the investigation of Viala and Pacott^ on the black rot of the grape. 

 The cultures, undertaken with the fungus Gu'ignardia Bidzvcllii which pro- 

 duces the disease, determined that the development of the fungus is depen- 

 dent primarily on the sugar content of the nutrient substratum and its organic 

 salts. Only young leaves were affected. They contained 1.75 per cent, tar- 

 taric acid and 4.3 per cent, glucose, while the old leaves showed only traces 

 of these substances. The berries were susceptible from the time they began 

 to swell and this susceptibility continued up to the beginning of the ripening 

 stage. During this time they contained 32 to 24 per cent, of acid and 11 to 

 56 per cent, of sugar. During ripening the acid content falls from 9 to 2 per 

 cent., but the sugar content increases so greatly that the fungus can no longer 

 attack the berries. The conditions for the white rot fungus, however, are 

 exactly reversed. By this relation is explained the strikingly different resis- 

 tant capacity of different kinds of grapes. In the same way is explained the 

 circumstance that black rot epidemics generally occur in summer after 

 periods of cold weather with subsequent light rainfall. At this time the acid 

 content is especially large and the formation of sugar scanty. Similar fluct- 

 uations in the concentration of the cell sap combined with the phenomena of 

 perforation of the membrane, the varying processes of tension in rhe tissues 

 and other mechanical changes also in the plants cause a state of greater sus- 

 ceptibility to weather extremes. The more recent investigation is endeavor- 

 ing to find more macroscopic and microscopic characteristics also demarking 

 the stages of susceptibility to injurious parasitic attacks. 



The conditions pictured in the preceding example of ihc increased tend- 

 ency of the grape to become susceptible to the black rot fungi are entirely 

 normal developmental phases which are influenced by the weather. On this 

 account we may speak of such states as normal predisposition. In contrast 

 with these we should distinguish as abnormal predisposition the case in which 

 the plant or one of its organs has fallen into a condition of weakness or of 

 disease from other influences and in this conception of one cause of disease 

 is first given the desired point of attack. As an example, w^e will call attention 

 to the infection of leaves affected with honey dew by the black fungi, to the 

 attacks of the so-called parasites of weakness and the migration of wood- 

 destroying fungi from wounded surfaces. 



1 Viala, P., et Paeottet, Sur la culture du black rot. Compt. rend. Paris 1904. 

 Vol. CXXXVIII, p. 306. 



