. together. The para- | 
CRYPTOGAMS 391 
364. Connection between barberry and wheat rust. — 
With the discharge of the excidium spores, the part of the 
life cycle of the fungus spent on the barberry comes to an 
end, and it is ready to begin the uredo-teleuto stage over 
again as soon as it finds a suitable host. Where there are no 
barberries, it is capable of propagating without them, either 
by adapting itself to some other host plant, or by omitting 
the ecidium stage al- 
sitic habit being an 
acquired one, the 
fungus, like some ani- 
mal organisms that 
we know of, can often 
be “educated” by 
force of circum- 
stances into tolerat- 
ing, and even thriv- 
ing upon, foods which 
under other circum- 
stances it would re- 
ject. The wheat rust 
is known to be ca- 
pable of propagating bi ae 
yearafler year in tho se, Ame o oeet eee er 
uredo stage, the the apple rust fungus. (From a photograph by 
spores surviving Prof. F. E. Lloyd.) 
through the winter on volunteer grains and grasses; and in 
no other country in the world does rust do greater damage 
to the wheat crop than in Australia, where the barberry 
is practically unknown. This power of accommodation 
possessed by many parasites is one of the difficulties the 
agriculturist has to contend with in the development of rust- 
proof varieties. 
365. Polymorphism. — Plants that pass through different 
stages in their life history are said to be polymorphic, that 
