Oct., 1922] KLAPHAAK AND BARTLETT RESISTANCE TO MILDEW 
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The Fi plants of the reciprocal cross {Oe. pratincola hyb. immunis X " Oe. 
biennis Chicago'') were likewise of one type {Oe. pratincola) . All the plants 
obtained were susceptible to mildew, and gave an identical, susceptible F2. 
In these crosses the factors determining immunity are in the jS gametes, 
and immunity is therefore a patroclinic character. 
VII. Crosses between ''Oenothera biennis Chicago'' (susceptible) and 
Oenothera pratincola (''Lexington C") (susceptible). These crosses offer 
the same difficulty as the former in regard to the differentiation of types 
in the progenies, since the parents are themselves doubtfully distinguishable. 
In the cross ''Oe. biennis Chicago" X "Lexington C," all Fi plants with 
the exception of three were pratincola-Vike, the three exceptions being 
mutations. Two of these were similar (probably of one type) and showed 
a slight susceptibility towards mildew. The third was of a different type 
and was very susceptible towards mildew. All the typical plants were 
highly susceptible. The F2 was the same as the Fi. In the reciprocal 
cross (''Lexington C" X "Oe. biennis Chicago") all plants of the Fi and F2 
generations were of one type and very susceptible. 
The only statement that can be made in regard to inheritance of sus- 
ceptibility in these crosses in which both parents are highly susceptible is 
that the offspring are likewise highly susceptible. The two slightly sus- 
ceptible mutations show that a marked degree of resistance may be acquired 
as a result of mutational change. 
VIII. Crosses between Oenothera cinerescens (immune) and Oenothera 
pratincola hyb. immunis (immune). 
The Fi generation of the cross Oe. cinerescens X Oe. pratincola hyb. 
immunis consisted of two cinerescens types, one of them being similar to 
Oe. cinerescens in nearly all respects, the other a smaller or dwarf type. All 
plants were immune, and gave an immune F2 like the Fi, with a few muta- 
tions. 
In the reciprocal cross {Oe. pratincola hyb. immunis X Oe. cinerescens) 
the Fi generation consisted of only one type of plants (a small pratincola 
type), all of which were susceptible to mildew. The F2 showed a splitting 
into two types, one more dehcate than the other, but both pratincola-Yike 
and both heavily mildewed. 
These crosses show conclusively that the female a gametes of the hyb. 
immunis do not carry factors for immunity, but that the male /3 gametes do. 
Consequently, any combination to which hyb. immunis contributes the 
egg will be immune only providing the pollen parent produces male gametes 
with the immunity factor. 
IX. Crosses between Oenothera cinerescens (immune) and Oenothera 
pratincola ("Lexington C") (susceptible). 
All the Fi plants produced from the cross Oe. cinerescens X "Lexington 
C" were of the Oe. cinerescens type and similar to the larger one of the twin 
hybrids produced in the cross Oe. cinerescens X Oe. pratincola hyb. immunis. 
