Studies on the fern genera bolbitis and egenolfia. II. The gametophytes and the juvenile sporophytes

Abstract

An account of the spore-morphology, development and structure of the prothalli and morphology of the juvenile sporophytes of 19 spp. (12 of Bolhitis and 7 of Egenolfia) is given. The spores are monolete, with a smooth exine and enveloped by a granulose (rarely smooth) or reticulate perine which is characteristically folded. The average spore-size is 22-34 32-40 22-34 μ. in Bolbitis, and 28-36 36-49 26-37μ in Egenolfia. On spore germination, a 3-6-cells long filament is formed and develops into a prothallial plate by longitudinal divisions in the anterior cells. Generally an obconical apical meristematic cell is developed from one of the anterior marginal cells, either soon after plate formation or a little later. The anterior region of the thallus expands, later becoming cordate, and a multicellular meristem replaces the apical cell in the usual way. An apical cell stage is sometimes omitted, more frequently in Egenolfia than in Bolbitis. A midrib is developed when the thalli are about 2 months old. Prothalli reach maturity in 6-8 months in most species (3-4 months in B. presliana). The mature prothallus is of the cordate type, with a tendency, in most species, to elongate slightly with age: the midrib is 6-8 cells thick and the wings are spread out. Collenchyma-like thickenings of wing cells are found in some species of Bolbitis. The prothalli, in most species, are naked: towards maturity, sparse, marginal hairs are developed in B. heteroclita and B. presliana, and club-shaped, marginal, multicellular hairs in B. semicordata, B. subcrenata and E. sinensis. The early juvenile leaves are simple with a lobed lamina and supplied by a solitary, vascular strand forked 2 or 3 times. A midrib is found from the 5th-7th leaf onwards: the lamina is pinnately dissected and the venation is free and pinnate. As the size of the successive juvenile leaves increases, the lamina becomes bipinnate with small ultimate lobes. The protostelic vascular cylinder of the juvenile rhizome becomes dorsally grooved and later gutter-like, with the leaves supplied by paired leaf traces originating from either margin of the gutter. Soon the vascular cylinder becomes siphonostelic by the margins of the gutter coming together, and later, the leaves are borne in 2 alternating rows as in the adult plants. The leaves on the siphonostelic rhizome are markedly larger and simply pinnate as in the adult plants, the transition being rather sudden. Branching associated with leaf insertions occurs only after the simply pinnate condition is attained. Unicellular papillate hairs are found on the lamina of all the juvenile leaves. Uniseriate club-like hairs are found towards the bases of the earlier leaves, and later, spread over the main veins. Large acicular hairs are found on the lamina from the 3rd or 4th juvenile leaf onwards, and persist till the adult simply pinnate form is attained. Bolbitis and Egenolfia are closely related genera, allied to Elaphoglossum on the one hand and Lomariopsis and Teratophyllum on the other: the group appears to be more or less an isolated one, but may be remotely related to the Rumohra-Polystichum line of evolution of the 'Aspidiaceae'.