308 Professor Marshall Ward, On the Question of 



The chief difficulty in connection with the whole matter turns 

 on the vagueness of the statements frequently made, and on the 

 lack of recognition even by plant-pathologists of the extremely 

 complex nature of the phenomena. On several occasions I have 

 attempted to narrow the issues involved, and to bring the subject 

 into the region of practical investigation 1 , but hitherto with but 

 small measure of success, for it is evident that two conflicting sets 

 of factors are concerned in every epidemic fungus disease — viz. 

 (1) the external conditions, which may favour the one organism 

 and place the other at a disadvantage in very many different 

 ways and degrees; and (2) the reactions of the two organisms 

 one to another, the host presenting obstacles to the entry of the 

 fungus or offering it attractions in all conceivable degrees, while 

 the fungus developes weapons of attack also in very various and 

 graduated forms 2 . 



Within the last few years much has been done, especially under 

 the stimulus of investigators into the problem of Wheat-Rust, 

 towards clearing the way for further progress. Dating from 

 De Bary's proof that the Rust of Wheat is a Uredine which 

 developes its uredospores and teleutospores on the cereals, and 

 is heteroecious — i.e. forms its secidium and aecidiospores on another 

 plant, viz. Berberis — much work has been accomplished, and many 

 questions raised of which some are still controversial. 



The results of this work have been to establish many other 

 cases of Heteroecism — e.g. between Gymnosporangium on Junipers 

 and Roestelia (the aecidium-form) on Pomaceae ; and to show that 

 the old Puccinia graminis, the Rust of Wheat, included a number 

 of forms or species of Puccinia which were not sufficiently dis- 

 tinguished by the earlier observers, but which have been shown to 

 be quite different when investigated by modern methods. 



For instance, it turned out that in addition to the true Puc- 

 cinia graminis (Pers.) which is heteroecious on Berberis, there 

 occur on our cereals at least two other species morphologically 

 distinct and now easily separated by those who examine their 

 microscopic characters. These species are P. coronata (Corda) 

 with apical processes on its teleutospores, and which is heter- 

 oecious not on the Barberry but on species of Rhamnus, and P. 

 rubigo-vera (D.C.) with brighter yellow aspect and heteroecious on 

 neither Berberis nor Rhamnus, but on plants of quite a different 

 Natural Order, viz. Boraginacese. 



Closer and more prolonged investigations, and especially the 

 industrious labours of Barclay in India, Eriksson in Sweden, Plow- 



1 See for instance " On the Structure and Life-history of Entylonia Eanunculi," 

 Phil. Trans. 1886; and " On some relations between Host and Parasite " (Croonian 

 Lecture), Proc. R. S., vol. xlvii. 1890, p. 393. 



3 See also Disease in Plants, London, Macmillan & Co., 1901, where a general 

 treatment of the whole subject is attempted. 



