426 Prof. H. Marsliall Ward. The Relations between 



vated in artificial solutions, as above, tended to support that view : 

 moreover, many attempts to directly infect living plants with the 

 conidia failed — the conidia, if merely placed in a drop of water on a 

 healthy leaf, simply germinated and died, and very often nothing 

 more came of it. Nevertheless, odd instances of infection were 

 recorded here and there, and the whole matter became a great 

 puzzle,* until several points of startling importance came to light. 



In the first place it turned out that, although the germinal tubes of 

 certain of the Pezfzet-forms could not penetrate into the living leaf 

 of the host directly — whereas they plunged forthwith into the tissues 

 of a dead organf — nevertheless the mycelium developed from such 

 spores, provided it was vigorous and well nourished by previous 

 culture as a saprophyte, could do so, but in many cases only provided 

 the tissues of the host were in a favourable condition. This last 

 proviso was found to be necessary, because in some cases the my- 

 celium easily infected young growing internodes, &c, but could not 

 penetrate into the more fully developed older parts of the same plant. J 

 This threw some light on the curiously capricious behaviour of the 

 fungus in green-houses, where seedlings, cuttings, young internodes, 

 &c, were often attacked and destroyed, while older parts escaped, 

 though without any regularity of behaviour. 



The key to the mystery appeared to be offered when it was dis- 

 covered that the invigorated mycelium, well nourished by cultivation 

 in a solution such as that mentioned above, excretes a ferment which 

 possesses the power of swelling and dissolving cellulose, and that this 

 ferment is formed at the tips of the hyphse,§ and thus enables them to 

 enter the cell- walls, as they were actually seen tojdo. It becomes 

 intelligible now why these hyphse sometimes can and sometimes 

 cannot quickly enter the cell-walls of a plant : when the cell- walls 

 are thin and watery, and especially if small quantities of organic 

 acids are present, the fungus hyphas can easily attack and dissolve 

 them, but in cases where they are thick and tough, owing to paucity 

 in water and no traces of acids, the hypha has no chance. Just such 

 differences as these would occur in the case of young and old organs 

 respectively, or of partially etiolated or thoroughly matured tissues 

 respectively. 



But in addition to piercing the walls, and "at first living in the 



* We shall see that the occasional infection depends on (1) condition of host, (2) 

 "whether any soluble food-materials pass from the leaf into the drops of water, and 

 (3) the slate of the conidia. 



f See de Bary, "Ueber einige Sclerotinien und Sclerotinien-Krankheiten " ( c Bot. 

 Zeitung,' 1888, col. 410). 



X See de Bary, ' Bot. Zeitg.,' 1886, col. 440—441. 



§ See Marshall Wa^d, "A Lily Disease" ('Annals of Botany,' vol. 2, pp. 339 — 

 343). 



