WIIKAT CULTURE. 345 



It should be specially noted that wheats that are Kust-resistant or Rust- 

 escaping under the conditions obtaining in Australia are not so when grown 

 in Europe, and, conversely, varieties that, are Kust-resistant in Europe arc 

 not Rust-resistant in Australia. We must concentrate all attention on 

 breeding plants suitable for local conditions. We have the wheat, the Rust, 

 and the environment maintaining, as it were, an equilibrium; alter the 

 environment and the equilibrium of the other two is at once upset. 



Susceptibility to Rust disease, together with resistance to the same, form 

 perhaps one of the most important instances to which Mendelian method 

 has yet been applied. Using a variety susceptible to the Rust (Puccinia 

 glutnarum) and another practically immune to its attacks, Professor Biffen. 

 who worked in England, found that the first cross (Fl Generation) between 

 the susceptible and the rust-resistant variety was not perceptibly less 

 attacked than the rusty type. When, however, a further series of plants 

 (the F2 Generation) was raised from the Fl Generation, ordinary segrega- 

 tion (that is, the sorting out of characters in the F2 Generation) was found 



Fig. 13. — A small part of a section through a Wheat Leaf. 



Showing a uvedospore (6) of Puccinia graminis resting- on the surface, germinating, and sending its 

 branching filament (d) into the leaf through the breathing poie or stomata (a). {After Bolley 

 and Pritchard.) 



to take place, and the green resistant plants, standing among the yellow ones, 

 formed a very striking spectacle. It has not yet been shown to what the 

 resistance is due, but it has been suggested — and some experiments that 

 have been made strengthen the hypothesis — that wheat plants containing a 

 larger amount of acid in their sap are the more resistant. 



While temperature and water content in many cases play an important 

 part in the incidence of plant disease, the supply of food and other materials 

 in the soil is often of the greatest importance. In experiments conducted 

 at Rothamsted and at Woburn it has been shown that the susceptibility of 

 wheat to yellow rust has been increased by growing the plants with large 

 amounts of available nitrogen. Conversely, by starving highly susceptible 

 varieties, plants practically immune can be grown; but the yield is not 

 great enough to be profitable. From experiments carried out by Spinks 

 we now have to recognise that merest traces of different salts in the soil 

 may profoundly modify the plant's susceptibility to disease. The question 

 has arisen as to whether immunity to disease and susceptibility to disease 

 are characters inherited by plants. Tf so, it would seem possible by cross- 

 breeding to associate the valuable feature of immunity with the othor 



