BREEDING OF WHITE PINE AT THE INTERSPECIES LEVEL 547 



vegetative growth on young grafts in early spring. The resulting new 

 growth from axillary buds then produced male flowers. We have had a 

 similar experience recently with young grafts that grew too large for 

 proper inoculation with blister rust. Perhaps this method can be 

 applied to other white pine species, at least to those of the group with 

 which we are dealing. This could be of help in more rapid breeding work, 

 and also with the precocious receptor strains envisaged above. It should 

 be possible gradually to develop such precocious receptor strains of all 

 the economically important white pine species and thus to accelerate 

 resistance breeding on an international scale. 



LITERATURE CITED 



Chiba, S. 1965. Experiments on the flower induction on the grafts of 



Pinus stvobus. Oi i Inst, for Forest Tree Improv. Tech. Note 59. 3 p. 

 Critchfield, W. B., and E. L. Little, Jr. 1966. Geographic distribu- 

 tion of the pines of the world. U.S. Dep . Agr. Forest Serv. Misc. 



Publ. 991. 9^ p. 

 Dormling, I., A. Gustafsson, and D. von Wettstein. 1968. The experimental 



control of the life cycle in Picea abies (L.) Karst . I. Some basic 



experiments on the vegetative cycle. Silvae Genet. 1": 44-64. 

 Heimburger, C. 1961. Comments. Recent advances in botany 2: Sec. 14: 



1699-1703. 

 Little, E. L., Jr., and F. I. Righter. 1965. Botanical descriptions of 



forty artificial pine hybrids. U.S. Dep. Agr. Forest Serv. Tech. 



Bull'. 1345. 47 p. 

 Spaulding, P. 1925. A partial explanation of the relative susceptibility 



of the more important American white pines to white pine blister rust. 



Phytopathology 15: 591-597. 

 Stone, E. C, and J. W. Duf field. 1950. Hybrids of sugar pine by embryo 



culture. J. Forest. 48: 200-201. 

 Tubeuf, C. v. 1926. Blasenrost der Weymouthskiefer . Zeitschirft fUr 



Pf lanzenkrankh. 56: 143-146. 

 Tubeuf, C. v. 1933. Studien Uber Symbioise und Disposition sowie tlber 



Vererbung pathologischer Eigenschaften unserer Holzpf lanzen . IV. 



Disposition der fUnfnadeligen Pinus-Arten einerseits und der verschie - 



denen Ribe s -Gat tun gen , Arten, Bastarde und Gartenformen andererseits 



fUr den Befall von Cronartium ribicola. Zeitschrift ftlr Pflanzen- 



krankh. und Pf lanzenschutz 43: 433-471. 

 Wright, J. W. 1959. Species hybridization in the white pines. Forest 



Sci. 5: 210-222. 

 Wright, J. W. 1964. Flowering age of clonal and seedling trees as a 



factor in choice of breeding system. Silvae Genet. 13: 21-27. 



FLOOR DISCUSSION 



KEDHARNATH: Dr. Heimburger, as one who has had really wide experience 

 in species hybridization of pines, do you visualize the time when you can 

 use hybrid seeds between two species when you have identified two very 

 good clones, and if so, how do you propose to go about it? 



HEIMBURGER: This of course has been proposed by John Wright. When he 

 first published his results on white pine hybridization in Philadelphia, 

 he thought he could mass produce hybrid seed for direct use in forestry. 

 I don't believe very much in that, because there may be some hidden 

 pests. There may be a fungus or insect that doesn't attack the native 



