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 wa}^ 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 secidiospores 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 secidium-form) on Pomacese ; 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. Boraginacere. 
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 Entyloma Ranunculi,” 
Phil. Tram. 1886; and “ On some relations between Host and Parasite” (Croonian 
Lecture), Proc. R. S., vol. xlvii. 1890, p. 393. 
2 See also Disease in Plants, London, Macmillan & Co., 1901, where a general 
treatment of the whole subject is attempted. 
