THE ^CIDIAL STAGE OF RUSTS. 31 



Thomas A. Knight, president of the Royal Horticultural Society of 

 London, in 1813 recognized the importance both of the uredo stage 

 and of the barberry. He says (64, p. 85) : 



A single acre of mildewed wheat would probably afford seeds sufficient to com- 

 municate disease to every acre of wheat in the British Empire, under circumstances 

 favorable to the growth of the fungus. 



Knight adds: 



There is also reason to believe that the barberry tree communicates this disease to 

 wheat, and I have also often noticed a similar apparent parasitical fungus upon the 

 straws of the couch-grass in the hedges of cornfields. 



In 1818 a paper was published by the Royal Agricultural Society 

 of Denmark, which was the result of investigations by Schoeler (92, 

 p. 289) in Denmark from 1807 to 1816. He had planted grains 

 around barberries and found that rye and oats were liable to be 

 destroyed every year by rust; that when large and small barberry 

 bushes were planted in his field — 



The larger bushes did not give rise to rust when they lost their foliage in the process 

 of transplanting, but, on the contrary, the smaller bushes, which did not lose their 

 leaves so readily, did give rise to the rust in rye to a very marked degree. 



In 1816 Schoeler actually cut freshly rusted barberry leaves, 

 carried them in a box into a rye field, and nibbed them on rye plants 

 moist with dew. The plants were carefully marked and in five days 

 were found to be severely affected with rust, "while at the same 

 time not one rusty plant could be found anywhere else in the field." 



A German farmer performed similar experiments in 1818 (77, 

 p. 280; 52, p. 408). He gathered the dust (Staube) which fell from the 

 cup (Kapsel) on barberry leaves as he shook them and placed it on 

 rye plants far from the rye fields and where there were no barberries 

 in the neighborhood. After five or six days the plants thus treated 

 were attacked by rust, while there was nothing similar on any other 

 plants. He concluded that the dust from barberries blown by the 

 wind to grains causes the rust. 



While many botanists still believed that the rusts on barberry and 

 wheat belonged to different genera, some were sufficiently good 

 observers to believe that the Puccinia and Uredo were in some way 

 connected. In 1852 Tulasne (97, pp. 79-113) proved the genetic 

 relation between the summer rust (Uredo) and the autumn rust 

 (Puccinia) and also showed (97, p. 141) that the autumn spores of 

 many of the rust species, among which are Puccinia graminis and 

 P. coronata, go through a resting period from autumn until spring 

 before they will germinate. 



It remained for De Bary, in 1864-65, to publish the results of his 

 experiments, which actually proved heteroecism of Puccinia graminis 

 (12, pp. 15-49). He demonstrated in 1864 that the sporidia from 



216 



