64 
MILDEWS, RUSTS AND SMUTS 
iiredo- or summer-spore form, enables the fungus to extend 
the area of its distribution. Towards the end of the season 
the production of uredospores gradually ceases, and teleu- 
lospores are produced from the same mycelium. It is not 
unusual to hnd a mixture of uredospores and teleutospores 
in the same sorus. In the spring the teleutospores ger¬ 
minate and give origin to secondary or promycelium spores, 
which infect young barberry leaves and give origin to the 
aecidium stage again. 
When heteroecism was discovered by De Bary, it was 
hoped that a means of preventing the serious ravages 
caused by wheat rust had been discovered, by removing 
one of the two host-plants on which the fungus lived, hence 
a raid was made on barberry bushes. This idea was founded 
on the assumption that both host-plants were absolutely 
necessary for the continuance of the fungus, an assumption 
which did not prove to be correct, and it is now known that 
the aecidium stage can be left out without interfering with 
the continuance of the species. This is also true of other 
species. In Piiccinia graminis, and in other species also, 
the uredo stage is capable of perpetuating itself, independent 
of any other form of spore, so long as its host-plant is vigor¬ 
ous, hence in warm countries where the host-plant can 
grow all the year round, the uredospore condition is always 
present, and even in this country uredospores are met with 
on wild grasses during the winter season. The succession 
of uredospores and teleutospores of Puccinia graminis 
from the same mycelium in the wheat plant, is in accordance 
with the view entertained by Klebs, who has demonstaated 
that the particular phase of development is determined by 
the chemical condition of the food, the amount of moisture 
present and the relative density of the medium in which 
the plant is growing. So long as the wheat plant is in 
active growth, its chemical and physical condition will 
remain practically unchanged, and thus the uredospore 
condition will continue, but when the period of ripening 
approaches, the chemical conditions will change, as will 
also the general texture of the plant, conditions which do 
not' favour the further development of uredospores, but 
which, on the other hand, favour the formation of teleuto¬ 
spores. 
Parasitism 
Depending on the nature of the food, fungi are, broadly 
peaking, grouped under two headings : those that obtain 
heir food from dead organic matter, as dead wood, heaps 
