180 
structure of the leaves of the host plant, or on any recognizable 
excretion from them; at any rate the microscope shows none 
such, and experiments with B. mollis and B. sterilis de- 
monstrated that the germ tubes grow readily in filtered extracts, 
boiled or unboiled, of the leaves. [he matter evidently depends 
on the influence of the previous nutrition of the Fungus, as well 
as on the reactions of the species attacked, and presents problems 
of great complexity. 
It is possible to grow pure cultures of the grass as well as of 
the Fungus for weeks and even months in closed and in erated 
tubes. =“ Seeds,” sterilised by various reagents, have been 
germinated in large tubes, on cotton-wool supplied with pure 
nutritive materials, and excellent plants thus raised out of contact 
with any but filtered air. Such pure cultures have been infected 
with uredo-spores, and the progeny utilized for similar re-infec- 
tions, thus ensuring pure cultures of both host and parasite. 
“ Seeds,” from infected plants, thus treated do not give rise to 
infected plants ; and unless spores are sown on the leaves of the 
latter no pustules are developed. Moreover, pustules only arise 
in these cases on the spots where the spores were sown, and 
within the usual incubation period. These facts seem to militate 
against any theory of internal or intra-seminal sources of 
disease. It is hoped that by longer series of such pure cultures 
more definite information on several obscure points will be 
obtained. 
At present the evidence points to the following conclusions. 
The acts of infection and incubation occupy about ten days, and 
many exigencies may prevent the germination of the spores, the 
entry of the germ-tubes into the stomata or the successful growth 
of the mycelium in the tissues. 
Experiments go to show that the lack of certain minerals— 
é.g., potassium or phosphates—cause a starvation of the Fungus; 
partial etiolation of the host, or any other hindrance to free 
nutrition, assimilation, transpiration, &c., also act detrimentally to 
the well-being of the mycelium. 
Every degree of virulence of attack seems to be shown by the 
spores in the behaviour of their germ-tubes, either on different 
varieties or species of host, or on the same host in different 
conditions. 
It appears that in certain cases the germ-tubes may kill the 
tissues, and cause them to turn black and shrivel up as if 
corroded. In such cases the parasite can make no further 
progress, and infection fails: In other cases a mycelium 1s 
developed, but lies dormant and produces no spores; we may 
compare the Fungus in this case to a plant in poor soil, or 
suffering from drought, &c. 
