V. 



Plant Diseases and Bacteria. 



BOTANISTS have always been justly proud of the fact that they 

 had a fully-established germ-theory of plant diseases at a time 

 when this explanation of the diseases of man and animals was only 

 beginning to dawn on pathology; that they showed the way, and in 

 many cases (not the least important) invented the methods which the 

 modern bacteriologist has re-discovered and certainly perfected. This 

 botanical germ-theory was, in one respect, more obvious and easy of 

 discovery than the animal one. It dealt with fungi, which though 

 minute and elusive enough to defy all but the most patient and cunning 

 investigation, are yet far more tangible organisms than bacteria. 

 The brilliant work of de Bary and others in tracing the life-histories 

 of parasitic fungi through generations inhabiting successive host- 

 plants of diverse kinds has no parallel among the investigations of 

 the bacterial diseases of man and animals. On the other hand, 

 botanists must admit the reproach that, while they have concentrated 

 their energies on the micro-organisms that produce the disease — 

 naturally enough, since these are plants as well as the hosts — they 

 have neglected the study of the diseased host, and that consequently 

 plant pathology in the strict sense is in a comparatively backward state. 

 This, fortunately, has shown signs in recent years of being remedied 

 by increased attention ; and one of the fruits of such attention most 

 to be desired, viz., a better understanding of what is meant by 

 immunity and predisposition, can be reached only by this path. There 

 is hardly any subject in the whole of scientific literature and 

 conversation about which there is more vague writing and talk than 

 predisposition. There is a very small measure of bread and "an 

 intolerable deal of sack." How it may be exactly with human beings 

 and animals, I am not qualified to say, and I expressly exclude them 

 from these remarks ; but no burning and shining light on the subject 

 has reached botanists from this source, and I strongly suspect, from 

 my insufficient knowledge of the matter, that here too there is little 

 to be said but much to be doubted. 



Some years ago, when there was more public, and perhaps less 

 private, attention paid to such subjects as the diseases of the potato, 

 cereals, and other crops, it was a favourite view of farmers, gardeners, 

 and even of more exalted personages, that these cultivated crops had 



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