in the Brontes and their Brown Rust. 299 
thesis. The dormant mycelium is dormant not because it 
has established a symbiosis with the host, but because it has 
failed to establish any such relations. It is like a plant in poor 
soil, or in a medium too dry or too cold, &q,, for its welfare. 
It drags on a starved, stunted existence which will end in 
death unless the environment improves, and nurses it again 
into vigorous growth. We must rather consider the flourishing 
mycelium, fully adapted to its host-plant, as having established 
successful symbiotic relations, for, as I pointed out long ago, 
a Uredine when flourishing in a leaf does not act as a devas- 
tating parasite, but as one which slowly taxes its host, and 
even stimulates the cells for some time to greater activity. 
This is often shown by the persistent green colour and activity 
of the cells immediately round a pustule at a time when the 
rest of the leaf is yellow and faded. The same conclusion is 
borne out by the well-established fact that the best developed 
pustules and Uredospores are derived from the most flourish- 
ing green leaves. In other words, we get a better nourished 
and better developed mycelium in a leaf which can supply it 
with plenty of food ; just as we get a finer plant by cultivating 
it highly in good soil and surroundings. 
It will be observed that I lay some stress on the origin of 
the spores, a point far too much overlooked in all discussions 
regarding these parasites. Most observers have seemed to 
regard adapted parasitism as if it concerned chiefly the 
Fungus. I look upon the influence of its host as an equally 
important factor: though here and there the Fungus also 
varies from the normal. 
The Uredospores derived from Bromus mollis attack the 
leaves of another plant of B. mollis so successfully because 
their food-supplies and previous environment have affected 
their protoplasm in some way which makes it easier for 
their germ-tubes and mycelium to grow in tissues which afford 
them the same nutriment and present the same obstacles, &c., 
as they have hitherto enjoyed, or been confronted with. 
They can also flourish in B. secalinus , B. racemosus , B. com - 
mutatus ) because here also the food-supplies, &c. offered are 
x 2 
