A Cytological and a Genetical Study of Petunia IV. Pollen Grains and the Method of Studying Them

Abstract

There is a growing tendency among taxonomists to consider pollen grains of diagnostic value. The significance of the morphology of pollen grains in reference to hybridity and to polyploidy has also been emphasized by students of plants especially during the past ten or more years. As recorded by the senior author (1924) the nand zn. number of chromosomes for the ordinary bedding petunias is 7 and 14 and to this was added, in 1928, the statement that Petunia axillaris (Lam.) BSP. (P. nyctaginiflora juss.) is characterized by 7(n) and 14(2n) chromosomes. We would now add to this list the true-breeding species Petunia violacea Lind!., and P. alba Hort. Petunia parviflora juss. is the smallest flowered petunia yet described but has, contrary to our expectations, 9 and 18 chromosomes. These counts are based on division stages in microsporogenesis and in root tips. Skalinska and Cuchtman (1927), Matsuda (1927), and Malinowski (1928) have recorded 7(n) and 14(2n) chromosomes for P. violacea, but as shown in a recent paper (Ferguson and Ottley, 1932) their studies were unquestionably based on unstable garden forms rather than on the true species.Pollen grains of three natural species, of one horticultural species, and of many cultivated strains, including polyploids and species hybrids, have been studied in large numbers and under varying conditions; but only those of P. axillaris (PI4), P. violacea (P9), P. parviflora (P7), and two tetraploids of distinctly different origin will be discussed in this paper. Pollen grains in relation to chromosome number and to heterozygosity will be considered in a later paper. The large-flowered tetraploid was selected from a family of 4n plants grown at Wellesley and known in our cultures as population one (PI) ; while the small-flowered tetraploid was derived from seed collected in California. This plant is a representative of our population four (P4). MOUNTING MEDIAThe typical pollen grains of Petunia are bilateral in origin. If observed in dry air, they are elliptical or lens-shaped in outline when the longer axis is perpendicular to the line of vision, that is, when the grains lie horizontally on the slide. Normally there are three "germ pores," each located on a furrow which extends the entire length of the grain. These furrows are equally distant one from the other and give the grains a somewhat triangular 1 Published at the expense of the author, out of the order determined by the date of receipt of the manuscript.