200 Potato Diseases 



conclusively to the non-parasitic nature of the disease. Hence the 

 question of infection does not occur. 



A similar conclusion has been reached by W. A. Orton from a study 

 of leaf-roll in seedling varieties in the United States. 



Professor Lefroy and the writer found that the seedling plants used 

 for infestations exhibited considerable variability not only in the habit, 

 shape of leaf, and other external characters but in physiological charac- 

 ters as evidenced by selection by the White Fly (Aleurodes) and by the 

 uneven response exhibited by the plants when under similar experimental 

 conditions. These physiological characters are not in any way associated 

 with particular external plant features. The tubers themselves, as the 

 writer^ has pointed out, also exhibit considerable variability in shape, 

 kind of eye, and other characters, whilst Doby^ has recently shown that 

 tubers from plants affected with leaf-roll give a higher reaction with 

 respect to oxidase, peroxidase, and tyrosinase ; also they had a slightly 

 higher ash content and less starch and protein. 



Taking everything into consideration it is very evident, firstly, that 

 physiological variability plays an important role in the curl problem, 

 and secondly, conditions of culture. There appear to be certain optima 

 that favour the production of good plants, as at Dunbar in 1910, below 

 these optima the percentage of bad plants increases. Pathological 

 symptoms are the outward expression of the response made by particular 

 plants to conditions which do not suit them. 



The value of selection under the circumstances must now be con- 

 sidered. In the course of recent experiments, at Wisley, small tubers, 

 whether from good or bad plants, produced bad plants as a rule — tubers 

 from bad plants being usually small, produced bad plants. Of greater 

 importance, however, was the result obtained from tubers that had been 

 specially selected as desirable for seed purposes, obtained from the Dunbar 

 ground which produced a heavy crop in 1910. These tubers produced 

 both good and bad plants and a high percentage of the latter. The 

 occurrence of bad plants, however, did not bear any relation to any 

 particular characters of the tubers which produced them. Selection 

 becomes, therefore, an exceedingly difficult matter, since there appear 

 to be no external tuber-characters that can be used as a guide in selecting 

 favourable physiological plant-characters. Selection on the basis of 

 favourable external characters (shallow eyes, etc.) leaves entirely to 

 chance the selection of favourable internal properties. 



' A. S. Home in Jour. Roy. Ilorl. Soc, xxxix (1914), p. 50G. 



- G. Doby in Zeit. fur Pflanzenkrankheiten, Bd. xxi, xxii (liUl, 1912). 



