morphology

As with any "biological entity" it is sometimes difficult to determine the group to which a particular organism belongs.  Bryophytes are no exception.  With a little knowledge, and some practice, this problem can be overcome.  So, in a very simplistic sense the following characteristics can be used to differentiate groups within the bryophytes.

How do I tell the difference between the various bryophyte groups?

The three bryophyte lineages differ from one another in a number of characteristics, to the extent these can be used as field indicators for differentiating the groups from one another (Vanderpoorten & Goffinet 2009).  One morphological character shared by all three bryophyte groups is the dominance of the gametophyte generation.  The bryophyte gametophyte is typically perennial, free-living, and photosynthetic; while the sporophyte is short-lived and completes its entire life attached to the maternal gametophyte (Goffinet & Shaw 2009).  Although not necessarily “parasitic” the sporophyte is nonetheless near-completely dependant on the gametophyte for its nutritional needs (Crum 2001; Crandall-Stotler & Bartholomew-Began 2007).  This is the opposite of the tracheophytes where the sporophyte generation is dominant.  Although there is much diversity in the observed form of the bryophyte gametophyte, there are two basic types or habits: leafy and thalloid. 

Moss Gametophyte

Mosses typically possess leaves arranged alternately on a horizontal to erect stem; however, there is considerable variation in the orientation and arrangement of these leaves.  Leaves can range from imbricate (closely overlapping) to somewhat distant (with space between leaves).  In some genera the leaves are arranged distichously (in the same plane) instead of the typical spirally imbricate appearance.  Most moss leaves have 1 (or sometimes 2-3) costa(e) (nerves or veins to borrow from vascular plant terminology) that is positioned down the center of the leaf, originating at the base and extending toward the apex.  The number and length of the costa(ae) are often important characteristics for determining the correct identification of species.  Most moss leaves are undivided (without lobes) although the margins may possess a few to numerous serrations or teeth. The Bryophyta all possess leaves, of varying morphologies, and do not exhibit a thalloid growth form (Crum 2001). Gametophytes in the mosses consist of branched or unbranched stems with alternately arranged leaves that are typically spirally imbricate, or distichous as in the genera Fissidens and Schistostega.  Moss leaves are generally undivided and typically have one to several costa, and laminal cells that range from round or variously quadrately isodiametric, to short or long linear.  Many species of mosses have leaves that are one cell thick, others have multiple layers.  Regardless of the number of layers, leaf cells are exposed directly to the environment on both the abaxial and adaxial surfaces.

Liverwort Gametophyte

The Marchantiophyta exhibit both forms divided loosely into two classes; the Marchantiopsida or thalloid liverworts, and the Jungermanniopsida or leafy liverworts, with the leafy liverworts representing at least 85% of the species in the Marchantiophyta (Schuster 1984; Crandall-Stotler & Stotler 2000; Shaw & Goffinet 2000). 

The leaves of leafy liverworts are in two or three rows with the largest leaves alternately arranged and distichous, and if present, reduced underleaves occur on the ventral side of the stem.  The plants appear to have a distinct upper and lower surface.  The leaves may or may not have variously shaped lobules.  The unistratose leaves are ecostate and have cells that range from quadrate to variously multi-angled, often with one to numerous oil bodies and chloroplasts in each cell.  Thalloid gametophyte growth forms in the liverworts are loosely subdivided into simple and complex forms.  The complex thalloids typically have multiple layers of cells, dorsal air-pores, air chambers, and a ventral storage zone.  Complex thalloids are often opaque and have a definite fleshy appearance.  Simple thalloids consist of an unspecialized thallus with a thickened midrib and two lateral wings (Crandall-Stotler et al. 2009). Thus, simple thalloids although often multilayered, are thinner and have a more delicate translucent appearance. 

Hornwort Gametophyte

The Anthocerotophyta gametophytes are always thalloid (Crum 2001), typically with a single large chloroplast in each cell, and without oil bodies.  Thalli are typically in rosettes with or without lobed or divided margins.  Hornworts also house endosymbiotic colonies of the cyanobacteria Nostoc in globular or channeled clusters (Vanderpoorten and Goffinet 2009).

Moss Sporophyte

Although the gametophytic generation is dominant, there are marked differences in sporophyte morphologies between the three groups.  In brief, mosses possess an unbranched stalk (seta) which supports a single sporangium that is always terminal. Spores develop within a spore sac which surrounds a columella inside the capsule.  The distal end of the developing capsule typically has an operculum and this is partially or wholly covered by, at least initially, a calyptra derived from modification of the archegonium that is torn away during the elevation/extension of the seta.  In some mosses a series of peristome teeth, and in some cases an annulus, is present between the urn and the operculum.  These teeth, of differing number and morphologies, are hygroscopic and aid in spore dispersal (Crum 2001; Crandall-Stotler & Bartholomew-Began 2007; Vanderpoorten & Goffinet 2009).

Liverwort Sporophyte

Liverwort sporophytes are typically elevated on a seta (with some exceptions such as the genus Riccia which are setaless and have sporophytes imbedded in the thallus) with a single terminal sporangium that encloses a mass of spores and elaters (elongated, coiled cells that aid in spore dispersal).  A columella is not present.  Spore dispersal is facilitated by the splitting of the capsule along four medial lines to release the spores and elaters in a single event (Vanderpoorten & Goffinet 2009).

Hornwort Sporophyte

In the hornworts the sporophyte is long and linear.  The sporophyte is composed entirely of the sporangium except for the basal foot and a meristematic zone just above the foot that gives rise to sporangial tissue (no seta is produced); the sporangium (capsule) elongates from its base.  A columella extends through the center of the sporangium.  Spores and pseudoelaters are dispersed by the longitudinal splitting of the capsule which releases the spores gradually, from the base to the apex, throughout the growing season (Renzaglia et al. 2009; Vanderpoorten & Goffinet 2009).