50 T O R R E Y A 



races of Petunia have complex polygenic and heterogenic heredity of the intra- 

 race self- and cross-incompatibilities. Also in the constitution of both the 2n 

 and the 4n varieties of hybrid origin in Petunias there are both qualitative and 

 quantitative differences in the recombinations of genes, chromosomes, and 

 genomes which determine the specificity in the diverse parental species. No 

 doubt these may determine further inter-relations on other than a purely quan- 

 titative 2n and 4n relation. Tjebbes (1) reports he obtained few seed and only 

 weak seedlings in reciprocal inter-crosses between 2n "P. nyctaginiflora" (P. 

 axillaris) and two 4n cultivated races. Steere (2) made inter-pollinations be- 

 tween a pure race of P. axillaris and 4n cultivated races. To some he obtained 

 little seed of which none germinated. But one race having strongly colored 

 flowers and blue pollen (characters of the P. violacea) was highly cross-fertile 

 especially when it was the pollen parent. 



Summation for Petunia axillaris 



For the cultures of Petunia axillaris here reported the quantitative doubling 

 of each of the genomes in diploid plants results in two profound, different, and 

 discontinuous changes in the reactions of reproduction. 



1. The two closely related populations, one diploid and the other derived 

 tetraploid. exhibit an inter-group sterility barrier. The 4n X 2n gives no cap- 

 sules ; the 2n X 4n gives capsules with seeds that are empty and not viable. 

 The 4n gains a specificity status similar to that of the 4n and the 2n when 

 there are no incompatibilities and similar to that of certain well-established 

 natural species. 



2. In the 4n intra-relations there is inactivation of the secondary 5 factors 

 which operate effectively in the 2n. Thus the 4n population gains fen intra- 

 group reproduction that is complete for the self- and intra-cross relations be- 

 tween members. 



New York Botanical Garden 



Literature Cited 



1. 1931. Tjebbes, Klas. Orientierende Untersuchungen iiber Fertilitat bei Petunia. 

 Botaniska Xotiser 1931:174-184. 



2. 1932. Steere, W. C. Chromosome behavior in triploid Petunia hybrids. Am. Jour. 

 Bot. 19:340-356. 



3. 1936. Muntzing, Arne. The evolutionary significance of autopolyploidy. Here- 

 ditas 21 :263-378. 



4. 1938. Stout. A. B. The genetics of incompatibilities in homomorphic flowering 

 plants. The Botanical Review 4:275-369. 



5. 1939. Crane, M. B. and Thomas, P. T. Genetical studies in pears I. Journal of 



Genetics 37:287-299. 



6. 1941. Stout, A. B., and Chandler, Clyde. Change from self-incompatibility to 



self-compatibility accompanying change from diploidy to tetraploidy. Science 

 94:118. 



