48 University of California Publications in Agricultural Sciences [Vol. 2 



J. calif ornica growino: in the wild. However, since in certain 

 known interspecific hybrids the F^ plants all resemble the female 

 parent, it was deemed advisable to grow at least the second 

 generation from all these trees. In 1913 thirty of them bore 

 nuts, which were collected and stratified. Also in the spring of 

 1913 large paper bags were placed on most of these trees enclos- 

 ing flowers of both sexes in order to insure self-pollination of a 

 few flowers on each tree. These bags were shaken vigorously 

 several times during anthesis and upon examination of several 

 flower clusters the stigmas were found to be well covered with 

 pollen. Very few nuts developed under bag, but this may not 

 have been due to self-sterility on the part of the individual trees. 

 A period of very warm and humid weather caused most of our 

 walnut flowers that were developing under bag, both in Southern 

 California and at Berkeley, to drop. From the self-pollinated 

 nuts secured on these supposed hybrids 75 seedlings from 24 

 different trees are growing. These, together with 2001 seedlings 

 from naturally pollinated nuts representing 30 different trees, 

 give a total of 2076 second generation seedlings that have been 

 examined. Among them all not a single individual has been 

 found that resembles the new variety. Neither is there any indi- 

 cation of oak characters in these Fo seedlings. Therefore as yet 

 we have no evidence that the F^ trees are true hybrids. A suffi- 

 cient number of Fo plants will be retained to grow the F3 gener- 

 ation from each F^ tree. 



Now the question arises as to the nature of the supposed Fj 

 hybrids. I consider the conditions under which the work of 

 pollination was performed in 1908 to have been practically ideal. 

 The pistillate flowers were bagged long before any walnut trees 

 were shedding pollen. On one of the two trees I am certain 

 that no pollen was being shed even at the time I pollinated. 

 This tree was fairly well isolated from other walnuts, some of 

 which were shedding pollen at that time. Moreover, no nuts 

 developed in the check bags on this tree. Now if natural polli- 

 nation was prevented both before and after the application of 

 oak pollen, why are not these trees hybrids ? Are they the result 

 of abnormal embryogeny due perhaps to stimulation by the oak 

 pollen but without the occurrence of fertilization ? It should 



