TRANSACTIONS OF SECTION K. 855 



cell8 contain to the exterior is facilitated by over-distension and super-saturation, 

 or by actual wounds, we cannot be surprised at the consequences when a fungus, 

 hitherto unable to enter the plant, suddenly does so. 



In spite of all the progress made towards an explanation of the origin and 

 course of an epidemic of rust, however, one serious inconsistency has always 

 puzzled men who have worked with it in the open and on a large scale. This 

 inconsistency concerns the outbreaks of epidemics over large areas, at periods, and 

 within intervals, which do not agree with the weather records and the described 

 biological facts. We know, speaking generally, the conditions of germination of 

 the spores, we know how long infection requires, and the latent period is known : 

 ■we know much as to the conditions which favour or disfavour the fungus 

 mycelium in the tissues, and, nevertheless, an outbreak of disease over large 

 areas sometimes occurs under conditions which appear quite inconsistent with 

 this knowledge. 



During his six years' study of the wheat rusts Eriksson was so impressed with 

 these difficulties that he has lately committed himself to an hypothesis which may 

 perhaps crystallise the ideas which have floated in the minds of several who have 

 been puzzled by these matters. 



The facts which seem to have finally impelled Eriksson to his hypothesis were 

 those of the distribution of the wild rusts and grasses. Having learnt which 

 grasses could infect the wheat, oat, barley, and rye i-espectively, he found cases of 

 epidemics occurring where it was impossible to fit in the facts with the view that 

 spores had been transferred from these grasses within the period required for 

 infection and development of the disease spots. Again, seasons occurred when all 

 the conditions pointed to the probability of a serious outbreak of rust, and no such 

 epidemic occurred. Further, experiments were made in which cereals of varieties 

 known to be susceptible to given rusts were planted in close vicinity to grasses 

 infected with such rusts, and, nevertheless, in seasons eminently suitable for the 

 outbreak of this particular rust on these particular cereals none appeared, or so 

 little that it was impossible to explain the outbreaks of this same rust on this 

 cereal elsewhere, during that season, as due to direct infection from the surrounding 

 grasses. 



More and more it became evident that the infective capacity of the rusted 

 grasses is small, and confined to restricted areas, and that the outbreaks in certain 

 seasons — rust-years — must be due to something other than wind-borne spores dis- 

 tributed by gales over the district. 



Three hypotheses can be suggested to account for the , non-spreading of the 

 disease on to susceptible cereals — (1) Indisposition to germinate on the part of the 

 spores ; (2) unfavourable weather for germination ; (3) some structural peculi- 

 arities of the leaves on which the spores fell, of such a nature that infection was 

 prevented. 



The results of many experiments showed that, as matter of fact, the spores 

 are often very obstinate, and refuse to germinate even when the weather is 

 apparently favourable, and Eriksson discovered during these experiments that 

 cooling the ripe spores on ice increased their germinating power. Neither of the 

 other two hypotheses mentioned could be brought into agreement with the results, 

 however. 



The conclusion was thus arrived at that an outbreak of rust cannot always be 

 referred directly to the normal germination and infection of wind-borne spores 

 from neighbouring centres of infection. 



In some patches of extremely susceptible cereals, the disease appeared simul- 

 taneously on plants isolated from all perceptible sources of infection, and on plants 

 not thus protected ; the date of outbreak in these cases — reckoned from the sowing of 

 the grain — was far too late to be explained by direct infection from spores on the 

 soil, or on the grain sown. Experiments demonstrated that if such spores had been 

 there, and germinal tubes formed as usual, the disease woiUd have shown itself 

 much earlier. 



These and numerous other inconsistencies drove Eriksson to look for an 

 * internal source of infection,' in spite of the improbability of any such existing, 

 and of its apparent incompatibility with scientific theory since De Bary's time. 



