1916 J Babcock: Studies in Juglans, III 



73 



more, the demonstration of such a genetic relation between 

 quercina and californica must be accepted as ample proof that 

 the first quercina individual, at least, originated by mutation. 

 Since the mutant factor is recessive to its normal allelomorph, 

 it is highly probable that calif ornica trees which are known to 

 produce quercina seedlings are not to be considered as mutating 

 individuals. Such a tree is /. calif ornica, "Garden Grove No. 

 16", which was referred to in the preceding number of this 

 series. This tree may be either the result of a mutation in one 

 of the gametes that produced it, in which case it would, of 

 course, be heterozygous, or it may be a heterozygote produced 

 by hybridization between quercina and calif ornica. The same 

 is true of the other two calif ornica trees known to have produced 

 quercina. But the genetic relation between the variety and the 

 species shows that the first production of quercina at least must 

 have been caused by a mutation in one genetic factor and that 

 this change occurred in all probability in a germ cell of the 

 grandparent of the first quercina tree. 



A question arises as to the interpretation of the test of 

 the particular californica tree, Garden Grove, No. 16, reported 

 in the preceding paper. In 1913 the unguarded seed from this 

 tree produced about 5 per cent of quercina seedlings. The fact 

 that the seed was not self-pollinated under control and so may 

 have been crossed with nearby trees may serve as an explanation 

 of this result. However, another explanation is indicated. This 

 tree is very late in developing its pistillate flowers, the stigmas 

 becoming conspicuous after most other trees have shed their 

 pollen and after this tree has shed much of its own pollen. 

 Now if only a portion of its pistillate flowers are self-pollinated 

 the remainder probably develop apogamously. That would 

 account for fewer quercina seedlings than would be expected 

 from self -pollinated seeds in case this tree is heterozygous for 

 the quercina factor. For only one-fourth of the self -pollinated 

 nuts would produce quercina seedlings, whereas all apogamous 

 seeds would produce californica individuals. Presumably all such 

 apogamic plants would contain the diploid number of chromo- 

 somes. Hence they would be heterozygous like their parent and 

 for this reason they would be californica in type. 



