Mosses (Phylum Bryophyta) are distributed worldwide and occupy a wide range of habitats. These habitats include exposed rock surfaces, shaded coniferous forests, and bogs. Mosses can be distinguished from liverworts (Phylum Marchantiophyta) and hornworts (Phylum Anthocerotophyta) by several characteristics of both the gametophytic and sporophytic generations.
The gametophyte is the dominant and most conspicuous generation in mosses. Similar to tracheophytes (vascular plants), mosses possess leaf-like structures arranged radially around a stem. These leaves are typically unistratose (one cell layer thick) and function as the primary photosynthetic organs of the plant, containing numerous chloroplasts. Unlike liverworts, moss leaves lack complex oil bodies, which are characteristic organelles of liverwort cells. In addition to photosynthesis, the leaves contribute to the absorption of water and dissolved nutrients across their surfaces.
Rhizoids—unicellular, hairlike structures—anchor the gametophyte to the substrate. Although they may help retain moisture around the plant, they do not play a significant role in water or nutrient uptake.
Mosses may be classified as either acrocarpous or pleurocarpous based on the growth form of the gametophyte and the position of the sporophyte. Acrocarpous mosses grow upright in tufts or cushions, and the sporophyte is produced terminally at the apex of the main shoot, where the archegonia are formed. In contrast, pleurocarpous mosses grow laterally, forming spreading mats, and produce sporophytes on short, specialized lateral branches rather than at the tip of the main shoot.
The conducting tissues of Bryophyta are not lignified; therefore, mosses are not considered tracheophytes. However, many mosses possess specialized conducting cells called hydroids, which facilitate water transport. Some species also contain leptoids, which are involved in nutrient conduction. Hydroids and leptoids may occur in the stem and, in some species, within the leaves.
The sporophyte generation in mosses is generally more structurally complex than that of liverworts and hornworts and plays an important role in classification. Sporophyte growth is determinate. It consists of three main parts: the foot, seta (stalk), and sporangium (capsule). The foot anchors the sporophyte to the gametophyte and absorbs nutrients. The seta elongates to elevate the sporangium, enhancing spore dispersal. This elongation typically occurs before the spores reach full maturity.
Calyptra on the tips of developing sporophyte
The calyptra is a protective covering derived from the venter of the archegonium (the portion that surrounds the egg). It initially encloses the developing sporangium. As the seta elongates—generally before full sporangium development—the calyptra is torn at its base and carried upward like a cap atop the developing sporophyte.
Spore release occurs when the capsule is mature and typically dry. At maturity, an operculum at the apex of the sporangium falls away. In many species, peristome teeth are present beneath the operculum; these structures regulate and facilitate controlled spore dispersal. The arrangement and morphology of the peristome teeth are important taxonomic features.
Additionally, stomata are commonly present on the capsule wall. These stomata facilitate gas exchange and contribute to capsule drying during spore maturation. Stomata are also characteristic of the sporophytes of tracheophytes.
MOSS CLASSES: