1899] THE CEREAL RUST PROBLEM 339 



That Eriksson himself can only support his theory on negative 

 evidence is shown by his own candid remark as follows (5) : — " These 

 results [those given above] prove beyond doubt that the disease must 

 come from internal germs inherited from the parent plant. But in 

 what form are these internal germs of disease living ? Is it easy to 

 follow and identify them with the microscope ? Not at all. They can 

 only be detected just before the breaking out of the young pustules." 



A theory propounded by one who has for many years made a special 

 study of the " rust " problem has naturally attracted much attention, 

 and Eriksson's experiments have been repeated by observers in different 

 countries, the result being in every instance opposed to the theory. 



Bolley (7) holds that there is no ground for the mycoplasmic 

 theory, his reasons for so doing being founded on similar experiments 

 to those on which Eriksson founded his theory. Cereals were grown 

 until quite mature, in structures specially arranged to prevent inocula- 

 tion from rust spores, with the result that the plants remained perfectly 

 free from disease, whereas every specimen of unprotected plants of the 

 same kind growing close to the protected plants were badly rusted. 



Mr. G. Nicholson, F.L.S., curator, Royal Gardens, Kew, kindly 

 procured for me one pound of " Horsford Pearl " winter wheat. This 

 variety was selected, because Eriksson says (5) : — " We are warranted 

 in suggesting that the predisposition of the Horsford wheat to yellow 

 rust may be explained by assuming that between this variety of wheat 

 and the yellow rust an extremely vital mycoplasma-symbiosis is to be 

 found." 



This was experimented with as follows : — 



Half the quantity was used the first season. Two flower-pots, one 

 containing ordinary Kew soil, the other old stable manure mixed with 

 a very small quantity of soil similar to that used in the first pot, were 

 prepared ; no sterilisation was attempted in either case. An equal 

 weight of wheat was soon in each pot. Each pot was placed on a large 

 plate, a thick layer of cotton-wool was placed round the edge of each 

 plate, and on this layer of cotton-wool a tall glass globe rested, each 

 globe having an opening at the top plugged with cotton-wool. The 

 glass globes were not removed for a period of twelve weeks, the neces- 

 sary water being supplied by wetting the cotton-wool outside the bottom 

 of the glass globe. At the expiration of twelve weeks the wheat in 

 both pots had grown to the top of the glass globes, and in both pots 

 was found to be perfectly free from " rust." At this stage these experi- 

 ments terminated. 



The remainder of the half-pound of wheat not sown in the plant 

 pots was sown in a mixture of rotten manure and soil placed in a shady 

 corner out of doors. At the expiration of nine weeks pustules of 

 " rust " appeared on some of the leaves, and when the plants were 

 about two feet in length nineteen per cent of the plants bore rust 

 pustules on the leaves ; but this " rust " on examination proved to be 



