Host and Parasite in certain Diseases of Plants. 



431 



the weather improved ; and I have noticed the same fact in other 

 oases as well. 



Invig oration of Mycelium and Conidia by Saprophytic Mode of Life, 

 and Differences in Behaviour of Successive Generations. 



We have seen from the foregoing that the relations of the host to 

 the parasite may depend very much on the condition of the former, as 

 induced by the complex action of the environment. I have now to 

 show you that the variations from a normal which culminate in an 

 epidemic are not confined to the host; but that the parasite also 

 exhibits phenomena leading to the same result. 



That the mode of nutrition influences the vigour and size, &c, of a 

 fungus is a fact well known ; but it is a comparatively recent dis- 

 covery that certain fungi, usually saprophytic in their habits, may be 

 educated as it were to parasitism.* Thus, Penicillium glaucum, usually 

 regarded as the type of a saprophyte, causes rotting in fruits, bulbs, 

 &c, when its spores penetrate into a wound in the living organ ;f 

 and many other fungi usually met with as saprophytes are capable of 

 assuming a parasitic mode of life if opportunity arises,J e.g., species 

 of Mucor, Pythium, Nectria, Agaricus, &c. 



The most instructive of all is the genus Botrytis, however, for it is 

 apparently possible to carry the process of " educating " this sapro- 

 phyte to habits of parasitism much further than in any other cases 

 known. 



It was pointed out as early as 1874, by Zimmermann,§ that Botry- 

 tis cinerea, long known as a common saprophyte on fallen dead 

 vine leaves, passes from the rotting debris on the ground to the 

 healthy living lea7es of several plants and develops spots on them ; 

 and the same fungus has been found as a parasite on the male in- 

 florescences of junipers, thujas, and other Conifers, || as well as else- 

 where. But a still better case than any of these is the occurrence of 

 a severe epidemic on the gentians in the Jura during the wet summer 

 of 1888.^" The infection of the plants occurred in the young parts 



* The converse is also true to a certain extent. See B. Meyer " Ueber die 

 Entwicklung einiger parasitischen Pilze bei saprophyfcischer Ernahrung" (' Landw. 

 Jahrb.,' 1888, vol. 17) ; and Brefeld, ' Botan. Unters. ix. Hefenpilze,' Part V, 1883. 



t See Sorauer, ' Pflanzenkrankheiten,' vol. 2, p. 92 ; Muller-Thurgau (op. cit.) 

 says Penicillium causes a speckling of living grapes. 



% See de Bary, ' Comp. Morph. and Biol, of the Fungi,' pp. 379—380. 



§ " Ueber verschiedene Pflanzenkrankheiten " ('Hamburger Grarten-und Blumen- 

 zeitung,' 1874). 



|| Klein, 'Verhandl. d. Zool.-Bot. G-esellsch. zu Wien' (vol. 20, p. 547), and 

 Sorokin, 1 Mykologische Skizzen' (Charkow, 1871). 



% Kissling, ' Zur Biologie der Botrytis cinerea ' (Dresden, 1889, p. 6). It is worth 

 notice that this epidemic occurred in the same dull, cold, damp summer (1888) as 

 the one on lilies in this country. 



