Babcock: A New Walnut 



45 



interest to l3iolo.c;ists. The three qiter- 

 t"ma-producin[r trees above mentioned 

 are all southern California Black Wal- 

 nuts and would be classified botanically 

 as Jtiglans californica. The newly dis- 

 covered source of qiiercina is the nor- 

 thern California Black Walnut, which 

 has been named Julians californica var. 

 hinds a Jepson.^ It is a foiTn so 

 different from the southern type of tree 

 that it might easily be mistaken for a 

 distinct species. The circumstances in- 

 volved in this new appearance of 

 qnercina are too complicated to be 

 related in full here. Let it suffice to say 

 that without doubt quercina has sprung 

 from hindsii as well as from californica. 

 Since quercina comes from californica by 

 mutation it is practically certain that 

 hindsii produces the new variety by the 

 same process. Although it does not 

 follow of necessity, yet these facts would 

 seem to indicate that hindsii also origi- 

 nated from californica by mutation. A 

 full statement with illustrations con- 

 cerning this latest development in the 

 history of the oak-like walnut will be 

 made as soon as it is feasible to do so. 

 It is worthv of note that this is a 



mutation in which there has been a 

 change in all gross characters — size, 

 shape, color and texture of leaves, size, 

 form and number of parts of flowers, 

 color of bark, habit of growth, etc. 

 Such a transformation has been called 

 aggregate mutation and cases have been 

 reported in cotton, tomato, tobacco and 

 evening primrose. The walnuts and 

 their relatives are recognized by botan- 

 ists and paleontologists as among the 

 oldest of the angiosperms. Many spe- 

 cies of walnuts existed in the various 

 periods of geological history back to the 

 Cretaceous. The known occurrence of 

 one such aggregate mutation as pro- 

 duces quercina suggests that some if not 

 all the species of walnuts existing or 

 extinct may have originated in a similar 

 manner. 



That this new walnut is not a natural 

 hybrid between oak and walnut, is indi- 

 cated by the negative results of my 

 experiments in artificial hybridization 

 and proved by the demonstration of its 

 origin through mutation, which has oc- 

 curred not once but many times in at 

 least four different trees of the Cali- 

 fornia Black Walnut. 



' Bui. S. Cal. Acad. Sci. vol. 7, p. 23, 1908. 



Prepotence in Plant Breeding 



The work of breeding new flowers, fruits and vegetables from natural or artificial crosses, has 

 been, and I believe can still be, carried out excellently with a minimum of Mendelian theory. 

 The most important idea, I think, is that of prepotence, or transmitting power, or strength of 

 heredity, as it is sometimes called. This rather loose term, so far as I have seen, has been used 

 to embrace at least the following different Mendelian cases: 



(1) The dominance of a character in a first-generation hybrid. (The dominant parent is 

 prepotent for the character.) 



(2) The presence in the prepotent parent of a number of separate dominant characters, or of a 

 number of characters inherited as a dominant unit. (An important case of prepotence.) 



(3) The excess of dominants from the cross of a positive homozygote with a recessive, over 

 those from the cross of a heterozygote with the recessive. (The pure-bred is more prepotent 

 than the mongrel.) 



(4) The presence of the dominant character in all the progeny of the back cross of a hybrid with 

 its dominant (prepotent) parent, whereas only a fraction show the recessive character when the 

 hybrid is crossed with the recessive parent. 



(.S) In cases of imperfect dominance, the difference in appearance, as well as in transmitting 

 power, between the homozygous dominant and the heterozygote. (Recognition of prepotence 

 by external characters.) 



(6) The large excess of dominants in the progeny of selfed hybrid plants. (Influence of the 

 prepotent grandparent.) 



(7) The constancy of a selfed homozygous dominant, compared with a hybrid. (The former is 

 prepotent.) 



(8) The constancy of a selfed recessive compared with a hybrid. (The pure-bred has greater 

 transmitting power.) 



If, then, the breeder of improved plants uses pedigreed lines (centgener plots, or ear-row tests) 

 instead of mixed cultures, and selects in each generation the plants which are most prepotent 

 for the partictdar characters he needs, he can, I think, usually leave Mendelian formulae to those 

 who are working to discover new facts. 



John Belling, 

 Florida Agricultural Experiment Station. 



