Introduction to Bryophytes

Introduction

Bryophytes (mosses, hornworts, and liverworts) occur in ecosystems across the globe, with their diversity highest in tropical and subtropical regions. British Columbia has the greatest bryophyte diversity in Canada, making it an excellent place to study them. Bryophytes are also commonly called nonvascular plants because they lack xylem and phloem, the conductive tissues found in vascular plants (tracheophytes). Tracheophytes, which include familiar plants such as ferns, conifers, and flowering plants, have these vascular tissues to transport water and nutrients efficiently. Bryophytes and tracheophytes are both monophyletic, meaning each group consists of all the descendants of a common ancestor. Together, bryophytes and tracheophytes are called embryophytes. As the name suggests, these plants have an embryonic stage in their life cycle, in contrast to their closest relatives, the green algae.

All embryophytes have a life cycle that alternates between sporophyte and gametophyte generations. In vascular plants, the sporophyte is the dominant, conspicuous stage—the grasses, flowering trees, ferns, and conifers we admire are all sporophytes. This generation produces spores that develop into the usually inconspicuous gametophyte stage, which is often hidden from view. (Though people affected by hayfever may find the male gametophytes of seed plants very noticeable—pollen is the male gametophyte of conifers and flowering plants.)

Leucolepis acanthoneuron (moss with gametophyte and sporophyte stages)

In bryophytes, by contrast, the gametophyte is usually the conspicuous stage, forming the green mats and tufts we see. Unlike tracheophytes, the bryophyte gametophyte is composed either of stem and leaf-like structures or a flattened, ribbon-like thallus. The leaves of bryophytes are structurally very different from those of tracheophytes; they are generally only one cell layer thick. The sporophyte of bryophytes depends on the gametophyte for water and nutrients and, in most cases, is also conspicuous (as in the image of Leucolepis acanthoneuron to the left). Unlike the indeterminate, multisporangiate sporophytes of tracheophytes, bryophyte sporophytes are unisporangiate and determinate; once spores are shed from the often elaborate sporangium, the sporophyte is finished.

Plagiochila porelloides (a leafy liverwort)

Despite their simple structure, bryophytes exhibit remarkable diversity in growth form and habitat. Some can withstand extended periods of desiccation, while others are aquatic. They grow on virtually every substrate: rocks, soil, tree bark, decaying wood, and even cars and other synthetic materials. Because of their small size and limited human use, bryophytes have remained largely overlooked. Mosses, liverworts, and hornworts represent some of the most ancient lineages of plants and can provide valuable insights into early plant evolution. While they may appear to be small versions of their vascular relatives, bryophytes have life strategies that follow a very different path, and many of their unique qualities reflect this. What a fascinating group of organisms!

Bryophytes include three lineages: