R. N. Salaman 39 



1907. Very slight touch of disease on haulm, none in tuber. 



No seed. 



1908. Slight disease in haulm, none in tuber. Set seed freely. 



1909. No disease in haulm on September 3, but some later, 



considerable disease in tubers. No Seed. 



1910. Some disease in haulm in August. Selfed and crossed 



seed. 



The incidence of disease amongst the seedlings was remarkable, 

 those attacked by disease were in some cases consumed away and all of 

 them, excepting one which was but very slightly touched in the haulm 

 and quite free in the tuber, were most seriously damaged. Out of 40 

 seedlings 34 were diseased and six were untouched, to these might be 

 added the one only just touched by disease on a leaf or two, making 

 seven. The ratio of 33 : 7 is of course suggestive of a 3 : 1 ratio. 

 Resistance to disease being, as Biffen(2) found in the case of wheat, a 

 recessive. Further careful observation will be needed before anything 

 more definite can be asserted. It is a most striking fact that although 

 the parent etuberosuni plant was for 20 years and upwards noted for its 

 immunity to disease, yet directly its sexual life begins that immunity 

 goes. The chain of events, the fact that the F^ family contains a 

 number of immune plants, suggests that with the onset of sexual 

 activity some disturbance in the mechanism by which the plant had 

 hitherto security its immunity to Phytophthora had occurred — and that 

 the dominantly susceptible state of the plant apparently heterozygous 

 in this respect, has as it were been uncovered and its true nature laid 

 bare. 



The immune seedlings in 1910 demonstrated afresh their resistance 

 to Phytophthora. The etuberosuni seedlings were so planted that on 

 either side of an immune plant was a susceptible one, whilst immedi- 

 ately behind was a row of ordinary domestic potatoes. The susceptible 

 seedlings and the ordinary potatoes were devastated by disease. Before 

 the end of July the haulms of both these latter were destroyed. Up 

 till the beginning of September the immune plants were unscathed. 

 Signs were not wanting that the immune plants had been attacked but 

 had successfully withstood the enemj'. Pale spots were seen on some 

 of the green leaves during the height of the disease, whilst on these 

 spots on a few fading leaves colonies of Cladosporium epiphyllum were 

 found. The presence of the bright green healthy immune plants 



