60 Mendelian Inheritance and YeUoio Rnst in Wheat 



the imfortunce of distinguishuig between mere inoculation and infection 

 proper, and showed that it is the power of the hyphae to form haustoria 

 which must be taken as the real index of the infective capacity of the 

 rusts. Later, Miss Marryat(iO) carried out a very useful comparative 

 histological examination of the foliage of certain immune and susceptible 

 wheats which had been inoculated with Yellow E,ust. Her results fully 

 confirmed Ward's previous work, and also clearly proved that different 

 degrees of imnniiiity exist, as is indicated by the relative length of the 

 struggle which goes on between the fungus and the host cells before 

 the former is defeated. 



Extensive breeding experiments in recent years have taught us that 

 characters like rust resistance, which are the outcome of complex physio- 

 logical processes, are nevertheless subject to the laws of Mendehan 

 inheritance. Colour in plants and baking "strengtli" in wheat are 

 examples of this. With reference to colour, Miss Wheldale(2i) says: 

 "There is little doubt that the formation of anthocyanin does involve 

 a series of progressive reactions each of which is controlled by a certain 

 enzyme." Also Keeble and Armstrong(9) present what they beheve to 

 be convincing evidence in favour of the hypothesis "that pigmentation 

 is the outcome of the action of oxydase on chromogen." Yet, in spite 

 of the complexity of the reactions involved, tlie appearance of definite 

 colours has been shown by numerous experiments to obey the Jlendelian 

 laws. 



The foundations of our knowledge concerning the inheritance of rust 

 resistance were laid some years ago by Biffen(i, 2 and 3) in England, and 

 Nilsson-Ehle{ii) in Sweden. Biffen's experiments showed that resistance 

 to Yellow Rust was inherited as a sim])lo Mendelian recessive character. 

 Thus, in a cross between a susceptible and an immune variety, the 

 hybrids were all as badly rusted as the susceptible parent. In the F^ 

 generation one-quarter of the plants were highly resistant or immune, 

 while the remainder were rusted to various extents. Unfortunately, in 

 the F^ generation many of BifTen's cultures suffered from various causes, 

 and the mortahty was so high as to render the results inconclusive in 

 certain respects. It was clear that rust-free F, plants produced only 

 rust-free individuals in the next generation, and also that some of the 

 susceptible F2S gave rise to susceptible plants only. But there still 

 remained considerable doubt as to the behaviour of the progeny of the 

 su.sceptible F2 plants taken as a whole. The primary object of the work 

 here described was to clear up the points of uncertainty just referred to. 

 It was decided to go over the whole ground from the beginning, and, 



