i894. PLANT DISEASES AND BACTERIA. 131 



or of predisposition, on the one hand; or, to put the matter otherwise, 

 as to the gradations in the aggressive behaviour of a parasite to the 

 different varieties or even individuals of a host. It is just here that 

 we leave sure ground in most cases, but there is one remarkable 

 instance, at all events, of which our knowledge is sufficient — that of 

 Pythium, which, as a facultative parasite, attacks flowering plants. 

 The host here displays degrees of predisposition or power of resistance 

 in proportion to the amount of water it contains — a condition not in 

 itself sickly. We know further — it is abundantly evident that sick- 

 liness is not always necessary to predisposition in plants, but, on the 

 other hand, it may frequently be so. At one extreme we have the 

 parasite which is the complete master of its host under favourable 

 external conditions — as much its master as the animal that devours 

 it in more summary fashion ; and, at the other extreme, we have 

 parasites which need such adventitious aids of access to their hosts 

 as wounds and abraded surfaces, etc. The subject is a very wide 

 one, and merely the salient points are being touched on here, but I 

 must cite the facultative parasitism of moulds that grow on fruits, to 

 be referred to again later. 



One of the most interesting general truths in all this web of fact 

 is the exemption of living plants from the assaults of bacteria. 

 I reserve certain possible exceptions, such as Wakker's hyacinth 

 disease. This exemption has been generally accounted for by the 

 acid reaction of plants, and I might venture to suggest that, since 

 more causes than one commonly co-operate in such matters, the com- 

 paratively strong illumination of the vitally active above-ground 

 parts of plants may partially aid in this direction. Recent researches 

 have demonstrated the exceptional powers of sunlight as a retarder 

 and destroyer of Bacteria,' and Professor Ward, who has re-examined 

 the whole question fruitfully, has pointed to the protective powers of 

 the pigments of such parasitic fungi as Uredineae, etc., against this 

 influence — a protection not shared by bacteria. 



The question of the relations of bacteria to plant tissues has 

 recently been discussed by Dr. H. L. Russell^ in a thesis for his degree 

 at the Johns Hopkins University. He complains at the outset of the 

 neglect or the sceptical reception by European writers of the case of 

 the pear-blight caused by Bacillus amylovorns which, he contends, has 

 been established by Professor Burrill,and he cites in an appendix tables 

 ot bacterial plant diseases, many of them, it is to be feared, in like 

 case with the pear-blight. In some of these diseases the bacteria 



^See Raum. " Der gegenwartige Stand unserer Kenntnisse iiberden Einfluss des 

 Lichtes auf Bacterien, etc." Zcitschr. /. Hygiene, Bd. vi., i88g. He cites some 150 

 papers dealing with the influence of light on bacteria, etc., and more or less agreeing 

 in this — that light exerts an adverse influence on pathogenic bacteria, and a stimu- 

 lating one on healthy processes in the animal body. 



2 " Bacteria in their Relation 10 Vegetable Tissues" (The Friedenwald Co., 

 Baltimore). 



K 2 



