HYBRIDIZATION,POLYPLOIDY AND ADVENTITIOUS GROWTH IN THE GENUS ASPLENIUM

Gametophytes of Asplenium platyneuron, A. rhizophyllum and A. monlanum were grown from spores under controlled conditions. Factors inhibiting and promoting germination and growth were determined. Leaves cut from sexually produced sporophytes of A. platyneuron, A. rhizophyllum and A. montanum were cultured under controlled conditions on agar slants. The leaves produced neoplastic growths of two kinds: (1) two-dimensional growths, (2) three-dimensional growths. The former developed into normal-appearing, rhizoid- and gametangia-producing, heart-shaped gametophytes having a diploid chromosome complement. The formation of two-dimensional growths was favored by conditions drastically reducing the energy supply of the initiating sporophytic cells and by destroying the correlation of the leaf. Three-dimensional growths most often developed into sporophytes unless conditions were limiting; in the latter case they transformed into two-dimensional growths. Both two- and three-dimensional growths developed from fragmented leaves excised from buds growing near the tips of acuminate fronds of A. ebenoides. The high frequency of two-dimensional adventitious growth leads to the conclusion that apospory may provide a mechanism for the production of fertile allotetraploids, by the fusion of diploid gametes of adventitiously produced diploid gametophytes, in the genus Asplenium and perhaps other fern genera, in contrast to that which has been previously suggested in the literature.