beginnings

What are bryophytes?

Bryology, as defined by Webster’s dictionary, is a branch of botany that deals with the study of bryophytes.  The term “bryophyte” originated from the Greek language and refers to “plants that swell upon hydration” (Vanderpoorten & Goffinet 2009).  This ability to swell when wetted certainly hints at the capability of these organisms to resume their former stature following desiccation, a key feature in the physiological ecology of bryophytes.  Frequently viewed as “poor relatives” of the more flashy angiosperms (Goffinet & Shaw 2009) or as the “lower plants” (Proctor 2000), these characterizations no doubt allude to their early evolutionary emergence. 

Bryophytes are often lumped into a loose collection of organisms referred to as cryptogams.  The cryptogams (kryptos, meaning hidden and gameein, meaning to marry) are a disparate assemblage of organisms such as algae, lichens, fungi, and ferns, most of which are at best remotely related to the other members of the artificial group.  As such, this term is largely obsolete in contemporary plant systematics, although it does illustrate the overlooked status of these organisms. 

As the earliest land plants, bryophytes successfully moved from an aquatic existence into a terrestrial environment and have evolved for millions of years independently of the vascular plants (Crum 2001).  In contrast to the vascular plants, bryophytes possess attributes which effectively set them apart from their more advanced descendants.  As non-vascular plants, bryophytes exhibit differences in morphology and physiology, in addition to reproductive and dispersal mechanisms, when compared to vascular plants.  Characteristics such as a dominant gametophyte generation, non-lignified stems and leaves, and repeated desiccation tolerance are not shared by most vascular plants, nor the purported ancestor of the bryophytes, the charophycean algae.  Having assumed a poikilohydric lifestyle, bryophytes are inherently responsive to changes in environmental conditions and have evolved complex mechanisms which allow them to tolerate repeated desiccation without apparent structural or physiological damage (Vanderpoorten & Goffinet 2009; Proctor 2011).  At the same time, these attributes impose significant liabilities for life in an often challenging terrestrial environment and has resulted in the evolution of variants with rather narrowly defined habitat niches, and frequently relatively limited geographic distributions (Gignac 2001).  Bryophytes utilize these attributes as mechanisms to effectively exploit numerous microhabitats in their terrestrial environment.

Bryophytes as a group consist of between 15,000 (Gradstein et al. 2001) and 25,000 (Crum 2001) species worldwide and are the second largest group of plants.  Bryophytes, in the traditional sense, include the Bryophyta (mosses), Marchantiophyta (liverworts), and Anthocerotophyta (hornworts).  All of these groups are non-vascular plants with similar habits, and which frequently occupy the same habitats.  Although regularly consolidated into a single group, based on an overall lack of complexity and similar life cycle strategies, these groups have very little in common (Steere 1969) and likely reflect differing ancestral lineages (Goffinet 2000).  As non-vascular plants, bryophytes are generally thought of as the first land plants.  Mishler et al. (1994) concluded the Charophyceae, one of four basal algal lineages, are the ancestor of all extant land plants to include the bryophytes and tracheophytes.  However, the placement of the bryophyte groups within this basal lineage has not yet reached a final consensus. Summarizing relationships among the bryophytes relative to their ancestors (charophytes) and their descendants (tracheophytes), Goffinet (2000) suggests the convergence of hypotheses place the hornworts as the most basal group, with the mosses and liverworts in a monophyletic group sister to the tracheophytes.