Host and Parasite in certain Diseases of Plants. 



395 



the time, but some of which seem almost incomprehensible now, as 

 some of our present errors will appear in the future, it may be said 

 that no very exhaustive survey of these diseases as a whole was 

 possible until comparatively recent times. The successive attempts 

 of modern authors* have been almost entirely along one or other of 

 two lines : they have classified the diseases either (1) according to 

 the symptoms externally visible and the organs attacked, or 

 (2) according to the causes which seem most concerned in producing" 

 the disease. 



Whichever method is adopted, it is repeatedly found that large 

 assumptions have to be made and recognised in order to bring given 

 diseases into the sphere of treatment for the time being, and diffi- 

 culties of very peculiar nature continually make themselves felt. 



As an instance, we may take the well-known symptom of the 

 appearance of yellow leaves. ISTot only are yellow leaves charac- 

 teristic of many diseases due to fungi of the groups Uredineie 

 and Asconrvcetes (Peziza, Hysterium, Poly stigma,, 8fc), or to the 

 attacks of insects (e.g., Aphides), but they may indicate "something 

 wrong " at the roots — want of drainage, over-drainage, or lack of some 

 ingredient such as iron, or the presence of some noxious mineral, 

 to say nothing of parasites (Phylloxera, Melolontha, Agaricus 

 melleus, 



In cold weather in the spring yellow leaves may mean that the 

 temperature is too low for the production of the green chlorophyll ; 

 while frost is responsible for the yellowing of other leaves by a 

 totally different procedure — the acid substances in the cells are 

 enabled to diffuse through to the chlorophyll corpuscles and kill them. 



Yellow leaves often indicate the access of too little sunlight, but 

 they may be produced by too intense insolabion and consequent 

 destructive changes in the cells. 



Leaves injured by acid gases and poisonous substances in smoke 

 also turn yellow, and the yellow hue of autumnal leaves is well 

 known, while we have numerous yellow varieties of leaves among 

 cultivated plants, the causes for which are less clear. 



These are by no means all the cases observable, but they will suffice 

 to show how little can be inferred from a symptom which may be due 

 to so many causes, operating alone or in combination, be it said. In 

 fact, as a symptom, the yellowing of leaves is of scarcely any classi- 

 ncatory value, and we are driven to the conclusion that the leaves 

 of plants react to most injuries by turning yellow. 



It is much the same with other classes of disease named after the 

 prominent symptoms. What is usually termed " canker," for 



* The literature is for the most part in Hallier's ' Phytopathologie ' (1868), 

 Frank's ' Krankheiten der Pflanzen' (1880), and Sorauer's 1 Pflanzenkrankheiten * 

 <2nd ed., 1886). 



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