E. S. Salmon 151 



follows: H and G indicate respectively that the plant was growing either 

 in the hop-garden or, in a pot, in the greenhouse. signifies that the 

 plant remained absolutely free from mildew on any part of it throughout 

 the season. — indicates either that the plant was absent, or that no 

 record exists — sometimes because no observations were made or more 

 commonly because no infectible material (with c? seedlings in the hop- 

 garden) occurred. It was not found possible, as a rule, to estimate the 

 degree of susceptibihty shown in the greenhouse; the letter S stands for 

 "normal" or "full" susceptibility; in a few cases, marked S*, the plant 

 became so virulently infected, that extreme susceptibility appeared to 

 be indicated. In the hop-garden S^ indicates that only a minute trace of 

 mildew occurred; S^ that the infection was of medium intensity; S^ that 

 the infection was very severe, — those cases where the entire crop of hops 

 was destroyed by mildew being indicated by the added sign f . 



The 52 seedlings, whose annual records are given in Table IV, give 

 evidence as to certain facts which will now be discussed. 



(ff) Immunity under greenhouse conditions. 



Twenty-seven seedlings have showm persistent immunity to mildew 

 when grown in the greenhouse. The history of the behaviour of these 

 plants in the open, indicated in Table IV, will now be considered in 

 detail. The examination of the plants was made each year in October 

 (see above, p. 149). 



Ref. nos. V 91, V 92, V 93. These three seedlings, as soon as raised 

 from the seed, showed complete immunity in the greenhouse throughout 

 the seasons of 1916 and 1917. They were planted out in the hop-garden in 

 November 1917, and in October 1918 all three plants^ showed a minute 

 trace of mildew, in the case of V 93 consisting of a tiny patch of mildew 

 on the under-surface of one leaf only. In 1919 and 1920 V 91 and V 92 

 remained free from mildew; V 93 in 1919 had again a tiny "powdery" 

 patch of mildew, at the back of one leaf only, while in 1920 it was free. 



Ref. no. Z 2. This seedling was originally planted out in the hop- 

 garden in 1914. Cuttings taken in 1917-18, 1918-19 and 1919-20 proved 

 persistently immune in the greenhouse, and no mildew was observed on 

 the parent-plant in the hop-garden during the years 1917-20, when 

 infectible material was present each autumn, although its neighbour^ 



^ All three seedlings were then about 5 ft. high and non-flowering. 



- In every case where reference is made to a neighbouring plant (e.g. Z 16 next to Z 17, 

 Z 24 and Z 26 next to Z 25, and so forth) it is another seedling of the same origin. The seed- 

 lings were planted 3 ft. 6 in. apart in the row, and the lateral shoots of neighbouring 

 plants frequently became intertwined. 



Ann. Biol, vin 11 



