INVESTIGATIONS ON THE CONTROL OF DISEASE IN PLANTS. 323 



from their place of origin, lose their immune properties, and in many 

 instances are exceptionally susceptible to disease under such conditions. 

 Marked instances of this condition of things are found in the varieties 

 of wheat immune, as grown in Australia, but, when imported into the 

 United States, found highly susceptible to the disease to which it was 

 immune at home. The same experience followed the introduction 

 of immune Indian cowpeas to the United States. Such instances 

 could be multiplied almost indefinitely, and appear to suggest that 

 immunity appears in some way to be influenced by the conditions 

 under which it was produced, food, climatic conditions, &c. Again, 

 in attempting to produce immune forms, the idea appears to be that 

 of modifying the plant in such manner that its fungus parasite is 

 unable to attack it, forgetting or ignoring the fact that a fungus is 

 as capable of modifying its mode of attack as its host-plant is 

 of repelling it. I have demonstrated that fungi known only as 

 saprophytes can be educated to become rampant parasites, so much 

 so that they can no longer live as saprophytes, but, by reversing the 

 process, can be brought back again to a saprophytic condition. Others 

 have done the same, and, knowing the adaptability of fungi generally, 

 I would back fungus to win against any process of immunity that 

 can be acquired by higher plants, granted time and opportunity. 



In replying, the lecturer pointed out the difficulty of obtaining suffi- 

 ciently critical evidence to deal with these views. That a rapid change 

 on the part of the fungus was likely seemed to be negatived by historical 

 evidence, for the oldest known wheat in cultivation, ' Einkorn,' was 

 characterized by an extraordinary degree of immunity, not merely to 

 our common rust, but to two others as well. Yet these parasites had 

 presumably had the opportunity of changing their habits from the 

 time the second town of Troy was built up to the present. The 

 question of the failure of a variety known to be disease-resistant in 

 one locality when transferred to another required careful considera- 

 tion before it was assumed that the failure was due to any change in 

 its power of resistance to disease. The question of failure was largely 

 an economic one, and if the variety in question did not succeed as 

 well as those already established in the new locality it naturally went 

 out of cultivation. Such was the case with Farrer's Wheat 1 Bobs * 

 in South Africa : it simply proved unsuitable for the new climatic 

 conditions. That the failure was not due to any changed disease- 

 resistance was shown by the fact that Pole Evans used this variety 

 as a parent plant in his cross-breeding experiments, owing to its im- 

 munity to the very rust for which Farrer claimed it possessed a 

 great power of resistance in Australia. 



The lecturer did not think inferior strains were more disease- 

 resistant than those of good quality. That it was a rare character 

 combined with the many that go to make superiority was readily 

 intelligible. A more definite answer could be given only after 

 attempting to combine disease-resistance with " quality." 



