CHAPTER V. 



DISPOSITION OF PLANTS TO DISEASE. 



§ 11. We must here distinguish between an internal or 

 inherent disposition dependent on the constitution of the living- 

 protoplasm of the host-cells, and an external or accidental dis- 

 position arising from anatomical peculiarities or from the con- 

 ditions of environment. 



The condition of inherent disposition has as yet been little 

 investigated. In many cases it must be allowed that resting 

 cells are more disposed to disease than those in full activity 

 of life. Thus De Bary,-^ basing his conclusions on the observations 

 of Davaine and J5refeld, points out that various species of Mucor, 

 Pcnicillium, and allied forms penetrate into ripe juicy fruits, 

 and remarks : " Observation of the fruits shows that the fungi 

 develop more easily, the nearer the vital powers of the plants 

 attacked are to their lower limit, and at this point the conditions 

 of saprophytic vegetation make their appearance." ^ Davaine 

 also found that the vegetative organs of several succulent plants 

 show the same phenomena as the fruits. As further examples 

 may be mentioned that fungi can frequently penetrate withering 

 plant-organs while they could not infect the fresh living tissue. 

 Hartig o])served on Feziza Willkommii that the mycelium of this 

 bark-parasite advanced and killed the tissues only while the host- 

 cells were in a condition of vegetative rest, not during their 

 active period. 



' Morpholoriy and Biology of the Fiiiuii, English Edition, p. 380. 

 ^Welimer (BeitriUjt z. Kennf. einheimi.irher Pike, Jena, 1895), has contributed 

 new facts to this subject, which are referred to later. 



