276 Ward . — On Relations between Host and Parasite 
perhaps the most important — it being always understood that 
sufficient moisture is present. 
The effects to be looked for, about the tenth day, on those 
parts of the leaves on which Uredospores have been sown, 
are outbreaks of the well-known sienna-brown or orange- 
brown tufts of Uredospores similar to those sown. These 
appear first as minute yellow pimples or pustules, pushing up 
the epidermis and then bursting through as they rupture it at 
the stomata. The outbreak of such pustules is the proof of 
successful infection. 
Preceding the actual outbreak of pustules, often by several 
days, we have a pale yellowish discoloration of the leaf, at the 
spot infected, and it not unfrequently happens that such pale 
flecks are alone formed, and never advance so far as to develop 
spores* Sections of these infection-flecks show the presence 
of the mycelium in the tissues, and the loss of colour is due to 
two causes — the destruction of chlorophyll in the attacked 
cells, and the filling up of the intercellular spaces by the 
hyphae, thus rendering the mass more translucent than when 
filled with air. 
In yet other cases I find an active discoloration of another 
kind produced at the infected spots. The tissues turn yellow 
and then brown or black, rapidly shrivelling as if corroded. 
This is due to the actual death of the cells and withering of 
the tissues at the infected spots, and at first I thought it must 
be owing to some other fungus having got in. It is so in 
some cases, but in the majority of those to be considered it 
appears to be due, rather, to the infecting tubes and hyphae 
being too destructive to adapt themselves to the host-tissues, 
and must be regarded as a sign of failure of infection, because 
the Uredo-mycelium is unable to advance in the dead area, 
and of course no pustules are developed. We might compare 
the unsuccessful mycelium here to a pot-plant placed in 
unsuitable soil, and therefore incapable of flowering. 
It will thus be intelligible that four cases may occur as the 
incubation period runs its course. At the one extreme we 
may find no visible effect at all : the spores have either not 
