STRUCTURAL INVESTIGATIONS OF ASEXUAL REPRODUCTION IN NEPHROLEPIS EXALTATA AND PLATYCERIUM BIFURCATUM

Nephrolepis exaltata cv. Bostoniensis, the Boston fern, exhibits extreme stem dimorphism. The plant has orthotropic, dictyostelic shoots which bear pinnatifid leaves and plagiotropic, protostelic stolons which are aphyllous. Vegetative reproduction occurs by budding from primary and secondary stolons. Secondary stolons arise exogenously from derivatives of the apical cell of the primary stolon, whereas root primordia develop endogenously. Shoots develop in vivo when a creeping stolon makes contact with the substrate via extensive root proliferation. When stolon segments are excised and grown in vitro, secondary stolon primordia expand and initiate leaf primordia, forming new leafy shoots. In Platycerium bifurcatum, the staghorn fern, asexual propagation occurs on ageotropic roots ramifying among the basal nest fronds. Root bud initiation is marked by root tip hypertrophy following cortical parenchyma expansion. Root apical cell derivatives produce the bud apex; the root apical cell remains separate from the developing root bud. Superficially, vegetative reproduction in Nephrolepis and Platycerium appears to involve unusual organs. However, both ferns exhibit leafy bud development from distinct sites of origin, not from undetermined primordia or from direct transformation of root to shoot. Thus, distinctness of organ types is maintained in these two ferns and no evidence for interconvertibility of organ types has been found. BRANCH POSITION in the ferns is more diverse than in the seed plants. Bower (1923) described five major categories of branch organization among ferns: dichotomous, axillary, extraaxillary, adventitious on leaf laminae, and adventitious on roots. This contrasts with the typical axillary mode of branching in the seed plants, where a leaf subtends one or more lateral buds (Goebel, 1905). When branch positions other than axillary are found in the seed plants, these represent either 1) developmental modifications of axillary branching, e.g., tendrils in Parthenocissus inserta (Millington, 1966) or leaf-opposed buds in some rattan 1 Received for publication 26 July 1982; revision accepted 23 March 1983. This study was part of an undergraduate research project accomplished by JZB at the University of Minnesota and was funded in part by a University of Minnesota Graduate School Grant-in-Aid to AMH. Additional funding was provided by Wellesley College. We acknowledge the encouragement and help of Prof. R. Tryon and Dr. A. Tryon who also made constructive suggestions for the improvement of the manuscript. We also wish to thank E. Seling of the Harvard University Museum of Comparative Zoology for his help with the scanning electron micrographs. Finally, we wish to thank Prof. D. R. Kaplan for his interest in and encouragement of our work in fern morphology. 2 Present address: Department of Biological Sciences, Florida International University, Miami, FL 33199. 3Present address: 210 Bedford St., S.E., Minneapolis, MN 55414. palms (Fisher and Dransfield, 1979); or 2) a non-axillary type of branching which occurs in addition to axillary branching (Nolan, 1969; Tomlinson, 1973; Peterson, 1975; Fisher and Dransfield, 1979). Thus, although non-axillary branching is found in the seed plants, it does not represent the typical pattern of shoot organization. The diversity of branch organization in the ferns, along with such features as the relatively indeterminate nature of some fern leaves, has led to suggestions that a greater flexibility of organ determination exists in the ferns. Experimental manipulations of young leaf primordia at the shoot apex of Osmunda cinnamomea have shown that the youngest primordium (P 1) may develop as either leaf or bud (Haight and Kuehnert, 1971), although isolation of such leaf sites may be comparable to dividing the apex itself in parts. The rhizophore of Selaginella produces either a root or a leafy shoot, depending on external conditions (Webster and Steeves, 1967). Other experiments and observations used to document organ plasticity in the ferns are summarized in a recent review (White, 1979). Kaplan (1977), however, has argued effectively against organ flexibility in ferns. He points out that in all cases studied in sufficient detail, the ferns have been found to have as great a degree of organ differentiation as the seed plants.