CRYPTOGAMS 



321 



364. Connection between barberry and wheat rust. - 



With the discharge of the secidium 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 secidium stage al- 



together. The para- 



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 



year after year in the 



Uredo Stage, 



spores surviving 

 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 



FlG - 45G - 7 A species f " <*?" f p P le " (Gym - 



nosporangum) , showing the uredo-teleuto stage of 

 the the apple rust fungus. (From a photograph by 



Prof ' ] 



