E. S. Salmon 1«3 



SUMMARY. 



1. The "wild hop" {Humulus Lupulus L.) is composed of a number 

 of forms^ which show distinctive physiological, or "constitutional," 

 characters, as measured by the grade of susceptibility to the attack of 

 the mildew Sphaerotheca Himiuli. These characters vary from extreme 

 susceptibihty, shown both in the open and under greenhouse conditions, 

 to a high degree of resistance in the open and complete immunity in the 

 greenhouse. Of 291 ? seedUngs, 165 seedlings, or 56-70 per cent., show 

 extreme susceptibihty, while 18 seedhngs, or 6-19 per cent., are "com- 

 mercially resistant" in the open. The remaining seedlings fall into 

 groups representing intermediate grades of susceptibihty. 



2. Of 480 seedhngs, ^ and ?, 27 seedhngs, or 5-63 per cent., are com- 

 pletely immune — and 7 seedhngs, or 1-46 per cent., semi-immune — under 

 greenhouse conditions. 



3. The majority of the seedUngs which show complete immunity 

 under greenhouse conditions show a high degree of resistance in the 

 open. 



4. The distinctive degree of susceptibihty possessed by a seedhng, 

 both as shown in the open and under greenhouse conditions, has shown 

 no change after the plant has been cultivated for five years in a manured 

 hop-garden. 



^ The question as to whether these forms possess distinctive morphological characters 

 is reserved for another occasion^ 



{Received May 27, 1921.) 



