Host and Parasite in certain Diseases of Plants. 



393 



Croonian Lecture. — " On some Relations between Host and 

 Parasite in certainEpidemic Diseases of Plants." By H. 

 Marshall Ward, F.R.S., Professor of Botany, Royal 

 Indian Engineering* College, Cooper's Hill. Received 

 and read February 27, 1890. 



Introduction — Relations hehueen Physiology and Pathology. 



I thought I could not better respond to the honour of the invita- 

 tion to give the Croonian Lecture this year than by choosing a 

 subject from the domain of plant pathology, which should, at least, 

 have the merit of being of general interest and importance, and 

 it seemed probable that an account of some of the more conspicuous 

 features and recent results of the study of certain fungoid dis- 

 eases might be so placed before you that it should illustrate not 

 only the kind of progress which plant pathology is making, but 

 also show how dependent that progress is, and must be, on the 

 advances of physiology. Moreover, I hope to be able to demonstrate 

 '"hat the connexion between these two modern branches of science 

 is (in botany, at any rate, and I have no reason to doubt that the 

 truth applies to the animal kingdom as well) so close and so mutual 

 that the problems which arise daily appeal to students of both depart- 

 ments, and necessitate that each shall know what the other is about. 

 This, of course, is not the same as saying that either branch of study 

 is deficient in its special questions ; but it cannot be too much 

 insisted upon that, while the facts and generalisations of pathology 

 often throw light on physiological questions, the enquirer into the 

 pathology of plants has to pause at almost every step and ask some 

 question in physiology, and his progress may be slower or more rapid 

 in proportion as the answer is obscure or the reverse. 



For, after all, the pathology of plants embraces those phenomena 

 of abnormal life-processes which can go qn in the long series of 

 changes between normal, healthy, vigorous life, and the cessation of 

 that life as such, i.e., death ; and it is obviously impossible to study 

 these abnormal life-processes (pathological) without reference to the 

 normal ones (physiological). In other words, then, pathology is the 

 study of disturbed or abnormal physiological processes, and I thought 

 that it would be possible to interest you in some of the phenomena of 

 abnormal plant life, and especially in the working of some of these 

 factors which result in producing certain diseases, in which fungi 

 play the prominent part, and which occasionally assume the nature 

 of epidemics so suddenly that the phenomena continually prove too 

 much for the inherent credulity of those who are not in the habit of 



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