Host and Parasite in certain Diseases of Plants. 437 



not only on the cases of parasitism referred to, but also on the 

 behaviour of the host in its struggle for existence with the factors of 

 the inorganic environment, generally. 



The question as to how far this view of the matter may be extended 

 to other parasitic diseases of plants cannot be answered at present. 

 Obviously the reflections excited will suggest lines of enquiry, and I 

 may appropriately bring these remarks to a conclusion by a few brief 

 comments on what is known as to the behaviour of other classes of 

 parasitic fungi in this connexion. 



Omitting the Schizomycetes, partly because they have a literature 

 to themselves, and partly because they rarely* occur as parasites in 

 the cells of plants, possibly owing to the acidity of the sap ; and the 

 Myxomycetes, of which Woronin's Plasmodiophoraf is the best and 

 most curious case ; we have pronounced parasites (capable of produc- 

 ing epidemics) among the Peronosporece, one of which (Phytophthora 

 infestans) has been more studied, probably, than any other true 

 fungus parasite, at any rate so far as its life-history is concerned.]; 



Much that has been stated in this lecture would, apparently, apply 

 to the potato disease, and in view of the extreme interest that neces- 

 sarily attaches to that malady, I draw attention to the following points 

 of interest. 



Suppose we take a potato plant the leaves of which are very 

 slightly marked with minute disease spots, and divide it into two 

 halves as exactly alike as possible, and place each half in a tumbler of 

 water; the two tumblers, with their half-plants, are then placed in an 

 ordinary room, side by side, at a temperature of about 20° 0., and one 

 is covered close with a bell-jar and the other left uncovered. In a 

 short time — often a few hours — the covered leaves become black and 

 rotten with the disease, whereas the uncovered one will go on looking* 

 fresh for several days, though it also succumbs at once if covered. § 



The question arises whether the rapid spread of the fungus and the 

 rot it causes here are simply owing to the increased supply of water, as 

 the tissues become turgid in the saturated atmosphere under the 

 bell-jar ; or whether we have not here again, in addition, a case where 

 the diminished access of oxygen to the interior of the tissues of the 

 host results in an accumulation of organic acids and other substances 

 which make the' excessively turgid cells and thin, watery cell-wails 

 more than usually easy prey to the parasite. 



* Exceptions probably occur in the case of Wakker's hyacinth rot, the American 

 " pear-blight," the " peach yellows," and a few others. See de Bary's ' Lectures on 

 Bacteria,' 1887, p. 177, and the • Reports of the U.S. Department of Agricul- 

 ture,' especially No. 9, 1888. 



f Pringsheim's 4 Jahrb. f. wiss. Bot.' (1878, vol. 11, p. 548). 



X See de Bary ' Morph. and Biology of the Fungi ' for the chief literature. 



§ See de Bary ' Die gegenwartig herrschende Kartoffelkrankheit,' 1861, p. 55, 



