Roo.(Hotcr. J»2 



Plants may be placed in three arbitrary categories, namely: 



(1) Absolutely resistant. Not invaded. 



(2) Partially resistant. Invaded. 



(a) larvae fail to develop, 



(b) larvae develop but fail to mature, and 



(c) larvae mature, but the females are reduced in numbers 

 and are not very prolific. 



(3) Susceptible. Large numbers of highly prolific females are 

 produced. 



Other considerations entering into the problem are whether or not the 

 roots produce the hatching factor, whether or not the roots produce an 

 additional and quite separate substance responsible for attracting 

 larvae to them, and whether or not the roots respond to invasion by 

 the production of giant cells. 



Screening tests are based on pot work which assesses best efficiency, 

 not tolerance of invasion. Table h shows the results of investigations 

 by VJilliams into the larval invasion of resistant plants. All the 

 plants examined were invaded, formed giant cells, and produced subse- 

 quent stages in the proportions shown in columns (3) to (7). Generally, 

 the unmodified larva, as found in the soil (arbitrary stage one), made 

 up the greatest percentage of forms observed. A small percentage of 

 males which appeared normal were also noted, together iirith the virtual 

 absence of forms distinguishable as females; although in the root 

 samples under test, one small immature female was observed on the roots 

 of each of two plants. The occasional appearance of a few cysts on 

 resistant plants is sometimes a source of embarrassment in the deter- 

 mination of the ratios of resistant to susceptible plants. More must 

 be said of this later. Evidently, resistance is only partial (category 

 (2) c), and this holds equally for the foior lines of S. andigena (lines 

 HI - HU), for the crosses of S. vernei with S. stenotomum (the culti- 

 vated diploid potato), and for S. vernei itself. 



The final column in Table k gives the rate of invasion/gm. of root. 

 Fibers and stolons are less resistant than roots. The falling off in 

 the rate of invasion from the susceptible tj^Des at the head of the 

 column to the S. stenotomum x S. vernei crosses at the foot is apparent, 

 rather than real, and may be connected with the month in which the 

 observations were made. The very low figures for S. vernei , obtained 

 in mid-Auguat, may possibly indicate somewhat greater resistance to 

 invasion, but requires confirmation. 



In experiments conducted upon the production of the hatching factor, 2? 

 plants of S. andigena, drawn from the four lines (HI, H2, H3, and HU), 

 all produced a root diffusate active to a greater or lesser degree, 

 although the average was a little below the activity of S. tuberosum 

 varieties, Langvxorthy, Red Gladstone, and Golden Wonder. S. stenotomimi 

 X S. vernei crosses and S. vernei itself produced root diffusates of 

 low activity, but there was some evidence for segregation of this 

 factor in the former, one plant in twelve producing diffusate fully as 



