Oct., 1922] KLAPHAAK AND BARTLETT RESISTANCE TO MILDEW 453 
progenies a few plants are observed like the other reciprocal cross. These 
are metaclinic plants and are interpreted as combinations. In these 
cases the /3 gamete is the female (comes from the pistillate parent) and the a 
gamete is male (from the pollen parent). 
Groups of Crosses - 
I. Crosses between Oenothera mississippiensis (su-sceptible) and Oeno- 
thera cinerescens (immune). 
The Fi plants produced in the cross Oe. mississippiensis X Oe. cinerescens 
were, in each of two crosses made, all of the mississippiensis type and 
showed abundant infection with mildew, except that in one of the progenies 
there was one metaclinic plant of the type of Oe. cinerescens, which was 
immune. The F2 plants, obtained by self-pollination of typical Fi plants 
from both crosses, were entirely similar to the Fi plants, both in external 
morphological characteristics and in the degree of susceptibility. 
In the reciprocal cross {Oe. cinerescens X Oe. mississippiensis) twin 
hybrids of the Oe. cinerescens type were obtained, both types immune to 
mildew. There was one metaclinic plant, of the type of Oe. mississippiensis, 
which was susceptible. The self-pollinated matroclinic plants of the Fi 
gave no seeds, and the most essential data on the F2 are therefore lacking. 
The single metaclinic plant, however, gave seeds by self-pollination and 
produced, in the F2, susceptible plants similar to itself and to those of the 
cross Oe. mississippiensis X Oe. cinerescens. 
II. Crosses between Oenothera mississippiensis (susceptible) and Oeno- 
thera pratincola hyb. immunis (immune). 
In the cross Oe. mississippiensis X Oe. pratincola hyb. immunis, all 
the plants of the Fi produced were of the mississippiensis type, and immune 
towards powdery mildew. The F2 plants were again of the mississippiensis 
type, with some slight morphological differences between two cultures 
coming from two different individuals of the same Fi culture, which, how- 
ever, were not detected as different when self-pollinated. All the plants 
obtained in both cultures were immune. 
In the Fi of the reciprocal cross {Oe. pratincola hyb. immunis X Oe. 
mississippiensis), all the plants with one exception were matroclinic, 
except for lack of mildew resistance. (The one exceptional plant was a 
mutation.) All the plants were susceptible and of the type of Oe. pratincola. 
The F2 plants from the reciprocal cross, Oe. pratincola hyb. immunis 
X Oe. mississippiensis , were of three types, all closely resembling Oe. 
pratincola. All the plants were susceptible, as in the Fi. 
III. Crosses between Oenothera mississippiensis (susceptible) and 
''Oenothera biennis Chicago'' (susceptible). 
In the cross Oe. mississippiensis X ''Oe. biennis Chicago,'' all Fi hybrids, 
with the exception of one, proved to be matroclinic, while the exceptional 
