Oct., 1922] KLAPHAAK AND BARTLETT RESISTANCE TO MILDEW 457 
Oe. pratincola hyb. immunis, ai8l, immune, 
Oe. cinerescens, al/3i, immune. 
Oe. mississippiensis, ai^i, susceptible. 
''Oe. biennis Chicago'' (a strain of Oe. pratincola), ai^i, susceptible. 
Oe. pratincola ("Lexington C"), ai/Si, susceptible. 
5. The composition and reaction to mildew of the several Fi hybrids 
must therefore be as formulated below: 
Oe. mississippiensis X cinerescens, ai^i, susceptible. 
Oe. cinerescens X mississippiensis, al^i, immune. 
Oe. mississippiensis X Oe. pratincola hyb. immunis, aifil, immune. 
Oe. pratincola hyb. immunis X Oe. mississippiensis , aijSi, susceptible. 
Oe. mississippiensis X "Oe. biennis Chicago,'' oi/Si, susceptible. 
"Oe. biennis Chicago" X Oe. mississippiensis, ai/3i, susceptible. 
Oe. mississippiensis X pratincola, aifii, susceptible. 
Oe. pratincola X Oe. mississippiensis, ai/Si, susceptible. 
"Oe. biennis Chicago" X cinerescens, ori/Si, susceptible. 
Oe. cinerescens X "Oe. biennis Chicago," al/3i, immune. 
"Oe. biennis Chicago" X hyb. immunis, ai/Sl, immune. 
Oe. pratincola hyb. immunis X "Oe. biennis Chicago," ai^i, susceptible. 
"Oe. biennis Chicago" X pratincola, ax^i, susceptible. 
Oe. pratincola X "Oe. biennis Chicago," aifSi, susceptible. 
Oe. cinerescens X hyb. immunis, aI/31, immune. 
Oe. pratincola hyb. immunis X Oe. cinerescens, aiSi, susceptible. 
Oe. cinerescens X Oe. pratincola, nliSi, immune. 
Oe. pratincola X Oe. cinerescens, ai^i, susceptible. 
Oe. pratincola hyb. immunis X pratincola, ai^i, susceptible. 
Oe. pratincola X hyb. immunis, ai/3I, immune. 
ez;gf3; ca56 //^^ reaction of the hybrid to mildew conformed exactly to 
expectations, according to the formulation above. 
6. On account of their peculiar type of heterogametism, immunity due 
to a single factor must breed as true as that due to a factor pair. Of the 
total number of 20 hybrids, 13 were susceptible, 3 had a single factor for 
immunity, derived from the maternal parent, 3 had a single factor for 
immunity, derived from the paternal parent, and one only had double 
immunity, derived from both parents, 
7. In accord with the hypothesis of immunity advanced above, com- 
bined with the hypothesis of heterogametism, the F2 generation by self- 
pollination of Fi plants should be the same, in regard to immunity or 
susceptibility, as the Fi. This conclusion has been amply proved, 
LITERATURE CITED 
I . Atkinson, G. F. Sorting and blending of unit characters in the zygote of Oenothera 
with twin and triplet hybrids in the first generation. Zeitschr. Indukt. Abstamm.- u. 
Vererbungsl. 16: 193-238. 1916. 
Quadruple hybrids in the Fi generation irom^/^'^^^^^^nula,n^^^(^^ 
lothera 
pycnocarpa with the F2 generations and back and^ inler-crosses. Geneticsv^ >J3- 
260. 1917- KV nnj r.rx 
Bartlett, H. H. The status of the mutation theory, \^!bth espfeml rgJetfen(}Qg9enot^r 
Amer. Nat. 50: 5i3-=;29. 1916. \ 
