Oct., 1922] KLAPHAAK AND BARTLETT RESISTANCE TO MILDEW 449 
B. In Oe. numismatica the a gamete carries the factor I (immunity to 
mildew), whereas the /3 gamete carries the factor i. 
C. The composition of Oenothera pratincola hyb. viscida is therefore 
aijSi, just as is the case in true Oe. pratincola. 
Now the interesting situation develops. Both true Oe. pratincola and 
hyb. viscida have thrown a mutation which closely simulates a certain wild 
elementary species, namely, Oe. Reynoldsii Bartlett (4). However, this 
mutation coming directly from Oe. pratincola is susceptible to mildew, 
whereas that from hyb. viscida is immune. Furthermore, the immunity 
of the mutation from hyb. viscida (called mut. simidans because it is in- 
distinguishable from Oe. Reynoldsii) is concerned with the jS gamete, since 
the cross Oe. pratincola hyb. viscida X mut. simulans yields an altogether 
immune hybrid closely resembling Oe. pratincola in morphology, but smaller 
in size, and, perhaps on account of its immunity, very different in coloration. 
The type comes true from seed, and has been so frequently used in crosses 
that it has been designated for convenience as Oe. pratincola hyb. immunis. 
This hybrid has the composition ai^l, and, as we interpret the situation, 
the immunity factor resides in the jS gamete by virtue of mutation of the 
iSi gamete originally entering into the composition of the line from Oe. 
numismatica to jSI, this mutation taking place at the time of origin of mut. 
simulans. That the jSi gamete of Oe. pratincola does not undergo such a 
mutation is shown by the fact that the mutation simulating Oe. Reynoldsii, 
which arises from pure Oe. pratincola, and is called mut. simulans rubricalyx, 
because it differs from the otherwise identical mutation from hyb. viscida 
in having red buds, is neither immune itself, nor can its (3 gamete impart 
immunity to crosses with the parent type. Thus, Oe. pratincola mut. 
simulans rubricalyx is not a type like hyb. immunis, but is merely a mildew- 
susceptible Oe. pratincola. It shows neither the immunity nor the small 
fetature of hyb. immunis, thus proving that the unique characters of hyb. 
immunis are due to the ^ gamete from Oe. mimismatica. 
These genetical facts are of no moment to the reader who is interested 
in the inheritance of the immunity after it has once arisen. They are a 
necessary part of the present record, however, since hyb. immunis has been 
extensively used in our crosses, being the one available form through which 
immunity could be transmitted to a cross through the pollen. It should be 
remarked that hyb. immunis breeds quite as true from seed as the other 
types used as parents. Constitution, ai^l. 
5. Oenothera cinerescens Bartlett (5). This species was collected at 
White Sulphur Springs, West Virginia, in 1912, and has been continuously 
in culture ever since. It is an outstandingly resistant type, and had been 
observed to be free from mildew for eight generations up to the time it 
was used for the crosses described below. Constitution, al^i. 
The Identity of the Fungus {Erysiphe Polygoni DC.) 
Salmon (7) has shown that Erysiphe Polygoni occurs on a great many 
