1908.] 



Rust in Wheat. 



245 



in the numerous airspaces in the foliage leaves. By pushing 

 out small suckers or haustoria into the actual cells of which the 

 leaf is built up it drains the tissues of foodstuffs, but it does 

 not kill them immediately, as, for instance, the fungus causing 

 potato disease kills the leaves of the potato. The normal 

 functions of the leaf system are thus interfered with con- 

 siderably, and infected plants cannot elaborate so great a 

 supply of carbohydrates, proteids, &c, as healthy ones. When 

 the time comes for the developing grain to draw upon the plant's, 

 reserve of foodstuffs it can only obtain a small supply. One 

 of the results of this is seen in the shrivelled grain produced 

 by a diseased crop, which is often so light that a large percentage 

 is blown out whilst the grain is being cleaned. At the same 

 time the straw is discoloured by the dark masses of teleuto- 

 spores and it suffers in weight and texture. 



All attempts to cope with these rust epidemics have proved 

 fruitless, and at present there seems little likelihood of satis- 

 factory remedial measures being discovered. Under these 

 circumstances the method suggested a century ago by Knight 

 seems the only practicable one, namely, to breed varieties 

 which shall be resistant to the attacks of this rust. The 

 experiments described below were planned to test the possibility 

 of this. 



The fact is well known amongst those who grow a number 

 of kinds of one and the same crop that w r hen disease puts in 

 an appearance some suffer far more than others and some may 

 prove almost completely disease-free. This suggests that 

 disease -resistant varieties should be searched for and pro- 

 pagated in preference to susceptible varieties. Efforts have 

 been made to do this with certain crops, such as the potato, 

 and the sustained attempts to find a disease-resisting variety 

 have met with a certain amount of success which augurs well 

 for the future. Where such experiments are carried out a 

 difficulty generally arises just when success seems within 

 the experimenter's reach. The variety when found, 

 though perhaps excellent as a disease resister, may be deficient 

 in other essential respects. It may not, for instance, give 

 sufficiently large crops or the quality of the crop may be 

 unsatisfactory, a failure in either of these respects being 

 sufficient to condemn the variety from the point of view of 



