54 



Parasitism and lmmnnit\ 



of his new crosses with bunt spores, and then select bunt-free plants from 

 the new generation, hoping thereby to secure an immune strain, or one at least 

 less susceptible than the parents. But in either case, it has yet to be proved 

 that immunity is absolute and complete, and that rich feeding or starvation, 

 for instance, or the severity of the infection may not break down the power 

 of resistance. Insufficient food is said to increase the susceptibility to infec- 

 tion in the human subject, but this does not seem to hold good for plants. 

 Brooks^ in his infection experiments with Botrytis cinerea used lettuce plants. 

 as they are extremely susceptible to attack. He found that leaves just be- 

 ginning to turn yellow were readily infected w^hile normal green leaves were 

 immune. When grown under different conditions of mineral starvation, 

 they behaved similarly as regards infection to those grown under normal con- 

 ditions. Just as Ward^ found that, wath Puccinia dispersa, the starvation of 

 the host had no appreciable effect upon its abiUty or inability to cause infec- 

 tion, so here, the work on Botrytis confirms Ward's view that " Whatever 

 may be the causes at work in the living cell which confer immunity or predis- 

 position on the species of host-plant, or which confer virulence or impotence 

 on the spore, they lie deeper than nutrition." 



Since Stinking Smut of wheat is so easily and certainly prevented by means 

 of pickling, there is not the same practical interest in producing a bunt- 

 resistant wheat, as in producing a rust-resisting wheat for instance. But it 

 must be remembered that there are other smuts, such as flag smut of wheat 

 and loose smut of wheat and barley, which are not so amenable to treatment, 

 and the methods by which a bunt-resisting wheat is obtained W'ill also be 

 applicable to the others. 



Relative Virulence of Infection by Spores of T. levis and T. tritici, 

 It is well known in connexion with cereal rusts that a wheat may be 

 immune or comparatively immune to one species of rust, and susceptible to 

 another, so that it became necessary to determine if wheats reputed to be 

 immune to bunt were equally so to both species. In the case of Florence 

 and Genoa they were liable to infection by both T. levis and T. tritici, but in 

 varying degrees. Experiments were carried out specially with two other 

 wheats, viz.. Dexter and Federation, to see how far the two species of bunt 

 differed in their capacity for infecting them. Sound seed was sown in each 

 case along with the infected, and the sound seed invariably yielded healthy 

 plants. The results are given in the following Table and compared with 

 those obtained from Florence and Genoa collectively : — 



Table III. 



-Relative Amount of Infection by T. levis and T. tritici 



RESPECTIVELY. 



