456 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
All plants produced were immune to mildew. The F2 plants were similar 
to the Fi and immune. 
In the reciprocal cross, "Lexington C" X Oe. cinerescens, all Fi plants 
were of the Oe. pratincola type and susceptible. The F2 split into two types, 
both in general like Oe. pratincola. All the plants of both types were 
susceptible. 
X. Crosses between Oenothera pratincola hyb. immunis (immune) and 
Oenothera pratincola ("Lexington C") (susceptible). 
All Fi and F2 plants of the cross Oe. pratincola hyb. immunis X "Lex- 
ington C" were of the type of Oe. pratincola and susceptible, while all 
plants in the Fi of the reciprocal cross, Oe. pratincola X Oe. pratincola hyb. 
immunis, were of the hyb. immunis type and immune. Among the F2 
plants of the reciprocal were several mutations and a few metaclinic plants, 
the latter being highly resistant. All other plants, including the mutations, 
were immune. 
Conclusions 
In several of the foregoing cases in which metaclinic plants were pro- 
duced in crosses between immune and susceptible parents, the immunity- 
factor combination which would insure susceptibility or immunity in one 
particular type seems not to insure the same effect in another type. In- 
vestigations are now started to prove, in these cases, whether or not the 
expression of the immunity factors is influenced by morphological characters. 
In other words, is it possible that types may exist in which susceptibility 
is so great that one I factor will not confer immunity, whereas in other types 
the factor complex, without I, is so highly resistant that the addition of I 
confers complete immunity? The explanation of the phenomena presented 
by metaclinic plants must be deferred. As far as the normal hybrids are 
concerned, the results are all consistent and lead to definite conclusions. 
The results accord perfectly with the following hypotheses: 
1. The factor for immunity (I) is dominant. If it enters the zygote 
from either side, the plant produced is immune. 
2. In all the five strains involved in the experiments, the eggs are 
different from the sperms. The former are a gametes, the latter j8 gametes. 
A few exceptions to this general rule are indicated by the rare appearance 
of metaclinic plants in the progenies. Whereas a normal hybrid is an 
combination, the metaclinic hybrid is ^a. 
3. In both the immune strains, the immunity is due to an unbalanced 
factor for immunity in the zygote. In Oe. cinerescens this factor is strictly 
associated with the a gamete, and in Oe. pratincola hyb. immunis with the 
jS gamete. 
4. Representing immunity and susceptibility by capital I and small i 
respectively, the zygotic composition and reaction to mildew of the five 
strains are as follows: 
