452 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
and that such immunity acts as a dominant unit factor in heredity. Our 
data in regard to the inheritance of immunity or susceptibility concern a 
certain biologic strain of Erysiphe Polygoni, abundant upon Oenothera at 
Ann Arbor, Michigan. Other strains of Erysiphe might conceivably infect 
our "immune " types. In general, however, the types immune at Washing- 
ton, D. C, also proved to be immune in Michigan. 
General Statements in Regard to the Crosses Made 
Of the five strains of Oenothera selected, each was crossed with the four 
remaining ones, and each was self-pollinated ; together there were five 
self-pollinations, and 20 cross-pollinations in the first season. 
It might be asked if crosses between two immune or two susceptible 
strains would not be superfluous. The genetical relations of the Oenothera 
have been proved to be different in so many instances from those of other 
plants that all the possible crosses were made. The results obtained showed 
that in one case susceptible plants were obtained by crossing immune 
strains (in Oe. cinerescens X Oe. pratincola hyb. immunis). In no case 
were immune plants obtained in a cross between two susceptible species. 
Before going into detail regarding the crosses made and the Fi genera- 
tions produced, it will be well to state that the system used of designating 
the crosses is the conventional one. The pistillate parent is always named 
first, followed by the name of the pollen parent. 
Since the prevalence of zygotic sterility is surely significant in connection 
with the explanation of genetical phenomena in Oenothera, it is perhaps of 
interest to state the germination data for the seeds of the five strains. It 
should be strong y emphasized, however, that seeds of very low viability 
are usually those produced too late in the season to ripen normally. In 
other words, the high proportion of bad seeds is partly due to environmental 
factors. Abundant seeds were obtained in every case. The highest ger- 
mination obtained was in Oe. mississippiensis, with 43 percent germination; 
the lowest was in one culture of Oe. cinerescens, with no germination, 
probably an example of the effect of immaturity; another culture of the 
same species, but from a different individual plant, showed about 10 percent 
germination. The other three strains germinated as follows: "Oenothera 
biennis Chicago,'' 23.6 percent; Oe. pratincola hyb. immunis, 29 percent; 
Oe. pratincola ("Lexington C"), 29.4 percent. 
From each of the crosses, whenever possible, about 500 seeds were 
sown, and of the plants obtained 100 were potted off and later planted in 
the field. The data included in this paper extend to the F2 generations 
obtained by the self-pollination of typical Fi plants. 
A few words in regard to metacliny will not be out of place at this time. 
As has been said, an Oenothera hybrid is an combination and usually 
similar in most of its characters to one of its parents. Sometimes in hybrid 
