Magnified, Labeled, and Panoramic Views

Bryophytes (bryon = moss) are terrestrial plants most often found on moist soil or growing on the surface of other plants. Some tolerate cold or dry conditions and therefore grow in arctic or arid habitats. Many characteristics of the group are shared with other green land plants. Shared characteristics include: 10 chlorophyll’s a & a as photosynthetic pigments; 2) true starch as a food reserve; 3) cellulosic cell walls; 4) eukaryotic cell structure with Charophyte-like nuclear behavior; 5) multicellular sexual reproductive structures (Fig.1, Fig.2); 6) sperm with whiplash, Charophyte-like flagella; and 7) an alternation of multicellular generations with retention and protection of young developing sporophytes as "embryos." Bryophytes differ from other green land plants in two fundamental ways. First, the haploid "gametophyte" phase of the life cycle is the dominant phase. The "sporophyte" phase is small and never outgrows the gametophyte. Mature sporophytes are anchored to gemetophytes by a simple structure called a "foot." Spores are produced by meiosis in a capsule that is usually elevated by an unbranched stalk (Fig.3). Secondly, internal vascular tissue systems, if present, are not lignified. Vascular systems supply food and water to internal cells of large organisms, analogous to the way cardiovascular systems in animals work. The lignified walls of water conducting cells of true vascular plants provide structural support. As a result, true vascular plants are generally much larger than their "non-vascular", bryophyte cousins.

Bryophytes can be divided into at least three distinct groups. Most (9500 species) are mosses. Many more (6000 species) are liverworts. A few (100 species) are hornworts. Identifying differences in sporophyte structure and growth is the easiest way to distinguish between the three groups. The horn-like sporophyte of hornworts has a zone of actively dividing cells – a meristem – in a region between foot and capsule (Fig.4). In liverworts, there is no localized region of meristematic activity. Intermixed with spores of both hornworts and liverworts are sterile cells called elaters (Fig.5). These cells play a role in spore nutrition or dispersal. Development of moss sporophytes involves the activity of apical meristems, These meristems are active first at the bottom and then at the top end of the plant. No elaters are produced in mosses. Instead, many species use teeth-like structures surrounding a capsule opening to help distribute spores (Fig.6). Others use a pressure discharge mechanism for spore dispersal. Surface pores called stomates that function in gas exchange appear on moss and hornwort sporophytes, but not on liverworts.

Bryophyte gametophytes are more variable in form. The hornwort gametophyte is a dorsiventral, thalloid plant attached to its substrate with long slender cells called rhizoids. Cells of the thallus each usually have a single chloroplast. Each chloroplast contains a protein body called a pyrenoid. Pyrenoids are found in chloroplasts of some algal cells but in no other land plant group. Liverwort gametophytes can be either thalloid or leafy. Leafy forms generally have two flattened rows of leaves arranged on stems that lie flat along a substrate. Thalloid and leafy forms sometimes produce asexual buds called gemmae, found in splash cup on some species (Fig.7). Unicellular rhizoids attach thalli or leafy forms of liverworts to their substrates. Moss gametophytes generally begin life as filaments (Fig.8) but then differentiate into upright stems with a radial display of leaves. Stems of moss gametophytes are attached to the substrate with multicellular rhizoids. Leaves are usually quite simple, often only a cell layer thick. In a few moss species, leaves have surface projections called photosynthetic lamellae (Fig.9) as well as internal conducting tissue. In peat moss leaves, large dead cells are interspersed with smaller photosynthetic cells (Fig.10). This gives peat moss an important water absorbent quality. Massive amounts of peat accumulate in bogs due to conditions there that decrease the rate of decomposition. Peat moss deposits are harvested commercially for use as a soil conditioner. Destruction of boggy wetlands due to over harvesting can be of ecological concern.

Bryophytes other than peat moss are also ecologically important. Many are pioneer organisms that invade and stabilize exposed soil or rocks. Because of differential species sensitivity to pollution, bryophytes can also be useful in monitoring air pollution. Most species grow readily in culture, a feature that facilitates their use in research.

Although somewhat limited, the fossil record suggests that bryophytes evolved 400 million years ago. Molecular data and morphological features further suggest that the group evolved from a green algal ancestor that underwent extensive adaptive change for life on land. Included in this group are the earliest living land plants derived from the same algal ancestor that subsequently gave rise to other early land plants. Interrelationships between the three groups of bryophytes are still a matter of debate.