21.2 Bryophytes  (Page 3/2)

Introduction

It was to Hofmeister, working as a young man, an amateur and enthusiast, in the early morning hours of summer months, before business, at Leipzig in the years before 1851, that the vision first appeared of a common type of Life-Cycle, running through Mosses and Ferns to Gymnosperms and Flowering Plants, linking the whole series in one scheme of reproduction and life-history.

The commonality of the plant life cycle unites the Plant Kingdom, indicating that it appeared very early in the evolution of plants. The Bryophytes are the group of plants that are the closest extant relative of those early terrestrial plants. The first bryophytes (liverworts) most likely appeared in the Ordovician period, about 450 million years ago. Because of the lack of lignin and other resistant structures, the likelihood of bryophytes forming fossils is rather small. Some spores protected by sporopollenin have survived and are attributed to early bryophytes. By the Silurian period, however, vascular plants had spread through the continents. This compelling fact is used as evidence that non-vascular plants must have preceded the Silurian period.

More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They constitute the major flora of inhospitable environments like the tundra, where their small size and tolerance to desiccation offer distinct advantages. They generally lack lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more accurate, bryophytes are commonly called non-vascular plants.

In a bryophyte, all the conspicuous vegetative organs—including the photosynthetic leaf-like structures, the thallus, stem, and the rhizoid that anchors the plant to its substrate—belong to the haploid organism or gametophyte. The sporophyte is barely noticeable. The gametes formed by bryophytes swim with a flagellum, as do gametes in a few of the vascular plants. The sporangium—the multicellular sexual reproductive structure—is present in bryophytes and absent in the majority of algae. The bryophyte embryo also remains attached to the parent plant, which protects and nourishes it. This is a characteristic of land plants.

The Bryophyte life cycle follows the pattern of alternation of generations as shown in [link] . The most familiar structure is the haploid gametophyte, which germinates from a haploid spore. Cells similar to an apical meristem actively divide and give rise to the photosynthetic stem and leaf-like structures. Sperm and egg producing structures form on separate or the same stems. Sperm swim along the bryophte and unite with the egg inside the egg-producing structure (archegonium). The zygote, protected by this structure, divides and grows into a sporophyte, still attached to the gametophyte. The sporophyte forms spores by meiosis; these disperse and will form new gametophytes.