396 Prof. H. Marshall Ward. r Ihe Relations between 



PlG-. 2. Oaks in the neighbourhood of a manufacturing town, the leaves of which 

 were damaged by acid gases. The injury results in the production of yellow 

 spots on the leaves, and the latter eventually turn wholly yellow and brown, 

 and die. From a photograph taken August 8th, 1882. 



instance, is in different cases referred by different authorities to the 

 agency of excessively low temperature (frost*), or of insects, "j" or of 

 fungi, J or of two of these combined, to say nothing of other causes. If 

 we now ask how the matter stands with regard to the method of clas- 

 sifying diseases of plants according to their chief causes, the answers 

 are no clearer. It is the custom to proceed somewhat as follows : — 



There are, first, diseases due to the action of the non-living en- 

 «nvironment (soil, climate, mechanical injuries, &c.) ; and, secondly,. 

 diseases due to the attacks of living beings (parasitic insects, 

 fungi, &c). 



Now, leaving out of account altogether certain totally unexplained 

 diseases, such as some forms of "gumming," &c.,it becomes apparent 

 that we are liable to all kinds of errors unless we recognise that no 

 one factor ever accounts for a disease ; it is not so obvious, however, 

 that the changes which result in disease are usually due to several 

 factors acting in concert or successively, and I shall try to show that, 

 even in marked cases, it is by no means always easy to decide which 



* Sorauer ' Pflanzenkrankheiten ' vol. 1, 1886, pp. 305 — 448. 



f Frank, « Krankheiten d. Pflanzen,' 1880, p. 719. 



X Hartig, ' Lehrb. d. Baumkrankheiten,' pp. 89 and 109. 



