1890.] Host and Parasite in certain Diseases of Plants. 213 



February 27, 1890. 



Sir G. GABRIEL STOKES, Bart., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The Croonian Lectnre was delivered as follows : — 



Crooxiax Lecture. — " The .Relations between Host and 

 Parasite in certain Epidemic Diseases of Plants." By 

 H. Marshall Ward, M.A., F.R.S., late Fellow of Christ's 

 College, Cambridge, Professor of Botany in the Forestry 

 School, Royal Indian Engineering College, Cooper's Hill. 

 Received February 27, 1890. 



(Abstract.) 



Pointing out the intimate relations between the study of plant 

 physiology and pathology, the lecturer briefly referred to the existing 

 modes of classifying the diseases of plants, and the difficulties they 

 involve. Broadly speaking, there are diseases due to soil, climate, and 

 the influence of the non-living environment on the one hand ; and 

 those due to the attacks of living organisms (parasitic fungi, insects, 

 &c.) on the other. Some interesting cases were briefly discussed, and 

 the fact brought out that several causal factors co-operate in pro- 

 ducing any disease. 



With regard to fungus diseases, there are especial complexities, 

 because we have to learn (1) the life history of the fungus, and 



(2) understand the biology of the host-plant, and this means we must 



(3) also discover what influences in each case are exerted by the 

 variations of the environment (heat, light, moisture, &c.) in each 

 case. Even then there is an unknown variable (4) in the internal 

 changes going on in both the host and the parasite. 



After reviewing, with the aid of illustrations 'and experiments, 

 some of the principal functions of the normal tissues of a green plant, 

 the effects of variations in temperature, intensity of light, amount of 

 water in the air, &c, were discussed. The chief points are that under 

 certain conditions, e.g., a low temperature, feeble light, and when 

 the atmosphere is saturated with moisture, the plant may be less 

 able to withstand the inroads of a parasite, because its protective 

 cell-walls are thinner and more watery, its cell sap abounds in sub- 



