Host and Parasite in certain Diseases of Plants. 425 



with references to any special forms, excepting in so far as the cita- 

 tion of authorities necessitates this. 



In the first place, the mycelium and conidia are not only capable of 

 growing and flourishing in artificial nutritive media, but they often 

 refuse to do otherwise — at least while young. If the conidia are 

 sown in such media as the juice of grapes or other fruits, or in solu- 

 tions containing an organic acid, sugar, asparagin, and traces of 

 mineral salts, enormous cultures may be made for weeks, and millions 

 of new conidia, sclerotia, &c, obtained, provided certain conditions 

 are fulfilled. Among these conditions are the following : — The 

 temperature must not be high, and ma t oe relatively low (best about 

 15° C.) ; the solution must not be alkaline or neutral, but should be 

 somewhat acid ;* sugar of some kind — and preferably a glucose — 

 must be present ; and the nitrogenous materials may be offered as 

 asparagin or peptone with advantage. 



It will be noted that just those external climatic conditions which 

 we have seen to be disturbing to the well-being of the green host- 

 plant are either favourable to the fungi we are concerned with, or are 

 at any rate not in the least inimical to their development. 



Thus the oxygen respiration of the fungus goes on at all tempera- 

 tures from 0° C. to 30° C. and higher, and, although we still want 

 information as to details, experiments have shown that the mycelia 

 flourish at temperatures considerably below the optimum for higher 

 plants. f 



Moreover light, so indispensable for the carbon assimilation o f . the 

 green host, is absolutely unnecessary for the development of the 

 fungus. X 



Then, again, the dull, damp weather and saturated atmosphere, so 

 injurious to higher vegetation if prolonged, because they email inter- 

 ference with the normal performance of various correlated functions, 

 as we have seen, and render the plant tender in all respects, are dis- 

 tinctly favourable to the development of these fungi. 



Consequently the very set of external circumstances which make 

 the host-plant least able to withstand the entry and devastation of a 

 parasitic fungus like Botrytis, at the same time favour the develop- 

 ment of the fungus itself. 



As already said, it had long been assumed that these forms of 

 Botrytis are saprophytes, and the ease with which they may be culti- 



* See Marshall Ward, "A Lily Disease" ('Annals of Botany,' vol. 2, 1888, p. 

 334) ; also cf. de Bary (' Bot. Zeitg.,' 1886, col. 400). 



f See also Hoffmann (' Jahrb. f. wiss. Bot.,' vol. 2,1860, p. 267) and Zopf. 

 " Encyklopaedie der Naturwiss." (Schenk's ' Handbuch,' vol. 4, 1889, pp. 471 — 472). 



X According to Klein ('Bot. Zeitung,' 1885, col. 6), the conidia of Botrytis cinerea 

 are only developed in the darkness of night, but this is certainly not the case with 

 other species. 



VOL. XLVII. 2 I 



