8 - Reproductive biology and evolution

Summary

Reproductive processes and propagules

Bryophytes

In this final chapter we shall consider first the reproductive biology of polar cryptogams, as an introduction to a discussion of life strategies and their significance in the origin and evolution of the polar floras. The emphasis is of necessity on bryophytes, as there have been few relevant studies on lichens (Smith, 1984a).

The bryophytes life history involves heteromorphic alternation of a haploid gametophyte and a diploid sporophyte, the latter permanently attached to and, nutritionally, partially dependent upon the gametophyte (Fig. 8.1). Gametophytes are monoecious, dioecious or occasionally heteroecious, depending on the species. In mosses, antheridia and archegonia develop in groups at the apices of leafy shoots in acrocarpous species, or of reduced lateral branches in pleurocarps. Groups of gametangia with their surrounding bracts are termed inflorescences, and are bisexual, or more commonly exclusively male (perigonia) or female (perichaetia). Female bracts in leafy liverworts fuse to form tubular perianths that eventually surround the developing sporophytes.

Each antheridium liberates many biflagellate sperm, and a single, nonmotile ovum develops in each archegonium. Where fertilisation occurs, repeated mitotic division of the zygote and its derivatives gives rise to the sporophyte, normally one per perichaetium. The mature sporophyte comprises a haustorial foot embedded in the gametophyte, and a slender stalk, the seta, on which the sporangium, or capsule, is borne several centimetres above the gametophyte. Meiosis occurs during sporogenesis, and several hundred thousand spores commonly develop synchronously within each capsule. Bryophytes with sporophytes are traditionally, if loosely, described as fruiting.