ASPLENIUM PLENUM: A FERN WHICH SUGGESTS AN UNUSUAL METHOD OF SPECIES FORMATION

Morphological data, chromosome behavior, and occurrence strongly suggest that the Florida fern known as Asplenium plenum arose as a cross (AABjB2) between a sexual 2x species, A. abscissum (AA), and a "sterile" 3x hybrid, A. curtissii (ABlB2). A mechanism is described whereby such an origin is possible: A. curtissii, among other so-called "sterile" hybrids, has the ability to produce unreduced spore mother cells capable of becoming spores directly. Such spores are able to form gametophytes with viable sperms and have the potential of producing sporophytes apogamously. It is highly probable that this heretofore overlooked process has operated in the origin of the hybrid fern, A. plenum, and is the first to be reported in the literature. THE EXCEEDINGLY RARE Florida fern known as Asplenium plenum E. P. St. John was first discovered in 1936 in a limestone cavern near Lecanto, Fla. In 1938 St. John (Small, 1938) described this plant as a normal species, known then from that single locality. Small (1938) says that A. plenum "was at first considered a form of Asplenium curtissii, from which it differs in similar ways and to about the same extent as that fern differs from A. verecundum." In view of the studies to be described below the latter statement has proved to be significant. In the mid1950's specimens of A. plenum were discovered at a new locality (Indian Field Ledges in Sumter Co., Fla.) by E. S. Ford and Thomas Darling, Jr. In describing his observations at this locality, Darling (1961) wrote: "At the time of my 1957 visit I .. . supposed the entire lot to be A. curtissii. Not until I revisited Indian Field Ledges in 1958 and collected more specimens of these ferns did I suspect that two different species of Asplenium (A. curtissii and A. plenum) were represented." More recently Wagner (1963a, b) suggested that A. plenum is of hybrid origin and that its most probable parents are A. abscissum and A. cristatum. The purpose of the current paper is to present evidence in support of a new hypothesis for the origin of Asplenium plenum, which may involve an undescribed process in the formation of certain fern species. THE TAXA INVOLVED-The five taxa included in the complex under discussion are shown in 1 Received for publication 5 November 1966. This study was supported in part by National Science Foundation Grants G-10846 and GB-3366. The author thanks Professor Warren H. Wagner, Jr., for helpful suggestions; Dr. E. S. Ford, Mr. Thomas Darling, Jr., and Mr. C. E. Delchamps for sending living plants upon which this research is based; and the staff of the University of Michigan Botanical Gardens for cultivating the plants. Fig. 1: A. cr-istatum Lam., A. abscissum Willd., A. plenum E. P. St. John, A. curtissii Underw., and A. verecundum Chapm. The habitats of all are similar. As summarized by Wherry (1964), they grow on shady, wooded limestone ledges and walls of sinks. All five have been reported from at least one locality, Indian Field Ledges. Darling (1962) reported that "Asplenium abscissum and A. verecundum were common everywhere, and A. plenum was luxuriant on a long, low-lying boulder, with A. curtissii less plentiful on rocks to the southwest." Asplenium cristatum, however, was not recorded from this locality until Wagner (1963a) found a single plant. Recent studies on cultivated plants have resulted in the following data: Three are normal species, two being diploids and one a tetraploid; the other two are hybrids, a triploid and a tetraploid (Fig. 2-6). Both A. cristatum and A. abscissum are 2x with n = 36 and have 64 normal, monolete, meiotic spores per sporangium; A. verecundum is 4x with n = 72 and it -also has 64 meiospores per sporangium. Asplenium curtissji is a 3x hybrid with 108 chromosomes, forming mostly univalents at meiosis with an irregular number of bivalents, varying from 1 to 17 pairs in the observations made. In sporogenesis A. plenum also behaves as a hybrid but is a tetraploid, 2n = 144, and exhibits many more pairs, usually some 60-odd bivalents. Up to the present, where the spores produced by these two hybrids have been described, they have been called "abortive." Asplenium curtissii is obviously a simple hybrid between two normal species. It is analogous to dozens of such taxa now known in the literature of pteridology. This taxon was described by Underwood from the vicinity of Ocala, Fla., in 1906, although he did not regard it as a cross. As discussed by Wagner (1963a), the plant -is, [The JOURNAL for September (54:931-1059) was issued 17 October 19671 FAMERICAN JOURNAL OF BOTANY, Vol. 54: No. 9, 1967