A Synthetic "Trigeneric" Hybrid, X Asplenosorus pinnatifidus X Phyllitis scolopendrium var. americana

Abstract

Hybrids in artificial cultures have contributed to the understanding of the systematic morphology of ferns. Since the classic work of Margaret Slosson (1902), which first proved the origin of the natural hybrid x Asplenosorus ebenoides (R. R. Scott) Wherry (=Asplenium platyneuron (L.) B.S.P. x Camptosorus rhizophyllus (L.) Link), the techniques of growing fern gametophytes and producing crosses have improved, and many new developments have ensued. In 1957, Wagner and Whitmire provided the first demonstration of the conversion of a sterile allodiploid fern to a fertile allotetraploid. In 1968, Lovis reconstructed a fertile hybrid species of fern (Asplenium ( x )adulterinum Milde). In this article, all taxa, fertile and sterile, of interspecific origin will be referred to as nothospecies, and indicated by the use of the times sign if sterile and with parentheses around the sign if fertile-i.e., (x), a convention proposed by C. Werth (pers. comm.); divergent species will be referred to as orthospecies and will lack the multiplication sign. Many important experiments on Aspleniaceae were accomplished in European laboratories especially, as discussed and summarized by Reichstein (1981). In almost all cases, such experimental hybridizations were carried out to test some hypothesis of the origin of a given nothospecies. By comparison, many fewer hybridizations have been undertaken simply to find out what a cross between taxon A and taxon B might look like, and what combining, for example, a creeping rhizome with an upright caudex, or hairs with scales, or discrete sori with acrostichoid sori, might yield morphologically. Yet such questions may bear upon our understanding of the determinants of structure and form; we may be able to gain insights that would otherwise be unavailable. The plants involved in this report are all members of the spleenwort family, Aspleniaceae, always popular objects for culture work and hybridization experiments because of their conveniently small size, ease of culture, rapid growth, and often very distinct forms. As a matter of fact, the bizarre hybrid that we briefly describe below was formed by accident. A terrarium containing numerous gametophytes of the lobed spleenwort, (x )Asplenosorus pinnatifidus (Muhl.) Mickel (= mountain spleenwort, Asplenium montanum Willd. x walking fern, Camptosorus rhizophyllus (L.) Link) from south of Shoals, Martin Co., Indiana (kindly provided by Warren P. Stoutamire) was opened at the same time spores of the