Host and Parasite in certain Diseases of Plants. 399 



in the case of the comparatively simple diseases referred to above ; but 

 when the problem consists in disclosing the life-history of a micro- 

 scopic fungus on the one hand, and then in discovering its relations 

 to the plant (the biology of which is always assumed to be known) on 

 or in which it passes the whole or part of this life-history, on the 

 other, the matter rapidly attains unexpected proportions. Yet this is 

 never the whole, or necessarily the major part, of the real problem — 

 the nature of the disease — and before that can be even approximately 

 solved we have to obtain an insight into the influence of the non- 

 living environment on both the host-plant and the parasitic fungus, 

 an inquiry which may assume appalling proportions before it is far 

 advanced. 



Nor is this the end, though it is quite sufficient to account for the 

 fact that we never know all about any of these diseases. 



There is a factor — or set of factors — which always tends to baffle 

 the inquirer into these matters, and that is the internal disposition* of 

 the parts of the organisms concerned ; call it what we will — constitu- 

 tion, inherited disposition, &c. — the fact remains that the host-plant 

 and the parasite alike exhibit peculiarities of behaviour that cannot 

 be explained in the present condition of science as directly due to 

 the action of any external agency of the environment, although we 

 are no doubt right in concluding that it is the outcome of the cumu- 

 lative results of the vicissitudes of the species and its ancestors in the 

 long past. 



But it is just the reactions of this constitution, and its variations 

 induced by changes in the physical environment, which are so often 

 and so persistently overlooked, although the attempt to understand 

 any disease is hopeless, unless we take them into consideration. I 

 hope to show, in the course of this lecture, how the modern study 

 of the pathology of plants differs in methods from that of our prede- 

 cessors, especially in this very particular — the recognition of the 

 reactions of the host to its living and non-living environment, as 

 apposed to the reactions of the parasite to its living and non-living 

 environment, and, further, of the truth that disease is the outcome of 

 a want of balance in the struggle for existence just as truly as normal 

 life is the result of a different poising of the factors of existence. 



Of course, inasmuch as the abnormal state of affairs, while detri- 

 mental to the host, is the best possible for the parasite, we have here 

 the elements of a paradox ; but there is no real confusion of ideas 

 here; we are concerned with a particular case, illustrative of the 

 struggle for existence, in which a given set of variable factors of the 

 environment favour one organism at a time when they disfavour 

 another. 



* Sachs, ' Lectures on the Physiology of Plants,' pp. 189—204. 



