46 Parasitism and Immiinitx. 



with the disease, afiordiug only temporary relief, and that the only permanent 

 and natural means for overcoming them lie in following Nature's methods for 

 preventing extinction of the species, by rendering the plants themselves im- 

 mune, or at least, partially immune to such attacks. Before this goal is 

 reached, numerous researches in the laboratory and experiments in the field 

 will require to be carried out, but already there are sufficient instances 

 known to justify our hope and that of Rust in Wheat may be taken as an 

 example. 



Rust Immunity. 



There are, at least, three different kinds of rust which attack wheat, the 

 Black Rust (Pwcci/im gramims,'PeTS.), the Brown Rust (P. triticina, Eriks.), 

 and the Yellow Rust {P. cjlumarum,Yiv\ks. and Henn.). Only the two former 

 occur in Australia, and it is the so-called Black Rust which is so injurious in 

 certain seasons, while the Brown Rust is comparatively harmless. It appears 

 that in Britain and America the Yellow Rust does considerable damage, and 

 it is this one which has recently been made the subject of exact experiment. 

 Mr. Biffen, of the Agricultural Department of Cambridge University, found 

 in his plots a strain of wheat which was immune to this rust, for although under 

 observation for four seasons, and surrounded by varieties susceptible to the 

 disease, it remained free and showed no trace of infection. This Avheat 

 belonged to the sub-species, Triticum cofupactum, or Dwarf Wheat. On the other 

 hand, there was a type known as Michigan Bronze which was so liable that 

 not a single individual escaped, and comparatively few ripe grains were ob- 

 tained. On crossing these two types, the first generation consisted of plants 

 badly rusted without exception, even the awns and grains being affected, but 

 fortunately, a number of ripe grains were obtained for subsequent sowing. 

 Every available grain was sown to produce the second generation, in plots 

 alongside the parent varieties. The result was that while every individual of 

 the susceptible variety was infected and the immune variety escaped entirely, 

 the crosses were badly rusted, but certain individuals stood out perfectly 

 clean, not showing signs of disease, even on the withering basal leaves. The 

 second generation of the cross was thus composed of plants either badly rusted 

 or immune, and the exact numbers were 1,609 diseased and 523 immune, or 

 a ratio of o.07 to 1. 



Biffen has found another wheat which possesses an even greater degree 

 of immunity to Yellow Rust than the American Club — the variety of 

 Triticum compactum already referred to. It is known as Einkorn, or 

 Triticum monococcmn, and is said to be exceptionally immune to the three 

 common rusts. 



There is a principle here which is found to hold for diseases other than 

 rust, and it opens up great possibilities in the way of breeding plants immune 

 to disease. There are certain varieties of wheat, for instance, which are more 

 or less immune to rust, but lacking in those useful qualities which the farmer 

 desires, such as strength of straw, holding of grain, prolific yield, &c. But 

 when it is discovered that these unit characters, as they are called, go in pairs, 

 such as susceptibility and immunity to rust, and that they can be combined 

 with other characters which obey the same law, by crossing, then it is seen 

 to be possible in three generations to obtain a pure strain by crossing, com- 

 bining these desirable qualities with immunity to rust. If two fixed strains 

 are crossed, all the possible forms obtainable appear in the second generation 

 if a sufficient number of plants are grown, and a certain proportion of each of 

 these forms is already fixed. The third generation will show those individuals 



