The species in the class Sphagnopsida, commonly known as peat moss, exhibit morphology that is quite distinct from other classes of the phylum Bryophyta. These unique features contribute to the economic importance of this group. Sphagnum is widely used in horticulture and soil management to increase water retention, and it has historically been employed to dress wounds, helping prevent microbial infections.

Class Sphagnopsida comprises approximately 300–500 species, with the highest diversity found in boreal and arctic ecosystems of the Northern Hemisphere.  In British Columbia, Sphagnum‑dominated peatlands (bogs) occur throughout the province but are most abundant in northern regions and on the coastal lowlands. Large peatland complexes such as Burns Bog in the Lower Mainland and Rithet’s Bog on southern Vancouver Island are well‑known examples of ombrotrophic peatlands in the south. Peatlands also occur more widely on the flat, poorly drained landscapes of northern and central BC, as well as scattered across areas such as the Cariboo and the South Coast, where they contribute importantly to water regulation, carbon storage, and habitat diversity.

Modern taxonomy recognizes Sphagnopsida as a single order, Sphagnales, which is subdivided into three families:

• Flatbergiaceae — containing two species in the genus Flatbergium.

• Ambuchananiaceae — containing two species: Eosphagnum rigescens and Ambuchanania leucobryoides.

• Sphagnaceae — the most species-rich family, consisting entirely of the genus Sphagnum, which accounts for the majority of peat-moss species (Shaw et al., 2016).

In addition, a fossil order, Protosphagnales, is recognized in paleobotanical classifications, though it includes only extinct species.

Gametophyte:

Sphagnum is a truly beautiful and remarkable bryophyte. It serves as the foundation of bog ecosystems, acting as an ecosystem engineer by modifying its environment—acidifying soils, retaining water, and creating the unique conditions that support a wide range of specialized plants and animals. Beyond peatlands, Sphagnum can also be found along slow-moving streams and damp forest floors in coniferous forests, such as those in Pacific Spirit Park and Stanley Park in Vancouver.

Unlike the filamentous protonema of other mosses, Sphagnum has thalloid protonema with rhizoids.  Rhizoids only occur in the early stages of this plant:

The gametophyte of Sphagnum has two types of branches; pendant and divergent. The pendant branches hang down along the stem, which aid in the capillary movement of water, while the divergent branches stick out from the stem at about a 90 degree angle. A fascicle comprises of both the pendent and divergent branches that emerge from the same point on the stem. The capitulum, which is a cluster of young developing fascicles, is located at the apex of the main shoot. As the stem continues to elongate, new clusters of branches form at the apex, thus maintaining the capitulum. Furthermore, the protonemal stage of Sphagnum is very brief and filamentous, and is quickly followed by a thalloid stage. The species exhibit rhizoids only when the gametophyte is young.

When looking at Sphagnum, one must remember to observe both the leaves of the stem and those of the branches because the leaf shape and cell arrangement will differ. These observations are extremely important when trying to key Sphagnum species.

The leaves of the class Sphagnopsida are one cell thick (unistratose), arranged spirally and lack costa. Furthermore, the leaf tissue are characterized by being comprised of two cell types; chlorophyllose cells and hyaline (colorless) cells. The chlorophyllose cells, as the name suggests, possess chloroplasts and they form a network pattern in the leaves. The hyaline cells are dead at maturity and have one or more pores, which function to retain water and allows access to the environment. The structure of the hyaline cells is strengthened by fibrils, which are thickenings of the cell walls. The arrangement of the chlorophyllose cells as well as the variations in the pore distribution seen in the hyaline cells provide useful information when classifying species in this class.

 

A cross section of a branch leaf can  be very useful in identifying a Sphagnum species. When observed carefully, one can see there is variation in how chlorophyllose cells are exposed.

In some species the chlorophyllose cells are completely enclosed, while in others they can either be exposed more broadly on the concave surface (inner/adaxial), the convex surface (outer/abaxial) or equally exposed on both surfaces.

Furthermore, the inner cell walls of the hyaline cells can further aid in the identification. Seen here in this picture is a branch leaf cross section of Sphagnum papillosum, demonstrating the papillae on the cell walls adjacent to the cholorophyllose cells.

 

 

 

 

Stem (left) and branch (right) leaves of the same plant often look very different as you can see in the image below (stained with crystal violet):

Cross-sections, as illustrated in the image below, of the main stem (left, also just called just stem) and branch stem (right) of the same plant often also look very different:

The central part of the stem has a cylinder that is called a “wood”. The outer layer of the stem, also referred to as the cortex, is also composed of hyaline cells, which are called cortical cells. In some species, the hyaline cells may have pores and fibrils present.

Species in the class Sphagnopsida do not have hydroids or leptoids.

 

 

The perichaetial branches of the Sphagnum species are initially at the apex of the main stem, where the cluster of branches are found. Per each perichaetium, one can find multiple archegonia, however paraphyses are absent.

 

The perigonial/antheridial branches, which are modified divergent branches, can easily be identified because the tips are often darker in colour, often red or orange.

 

 

A single antheridium occurs in the axil of each perigonial leaf. Paraphyses are absent in these branches. The antheridia themselves are nearly globose in shape and are borne on short stalks, elevating them slightly above the leaf axil for effective release of sperm.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sporophyte:

The sporangia of Sphagnum share some features with those of other mosses, such as Bryopsida and Polytrichopsida, in that they possess an operculum—a cap-like structure that is shed prior to spore release. However, unlike these other moss classes, Sphagnum lacks peristome teeth.   Its spores are expelled all at once rather than gradually. Another distinctive feature is that the sporangia in Sphagnopsida are elevated by a pseudopodium—a gametophyte-derived stalk—rather than by a true seta as in most other mosses. Similar to liverworts, this stalk does not elongate until the sporangium has reached maturity. The sporangia themselves are generally globose (round) in shape and are often short-lived, present only long enough to release their spores before detaching and falling from the plant.

When a sporangium reaches maturity, the columella (the central column within the capsule, globose in Sphagnum) begins to degrade. The sporangial wall then starts to constrict, which builds internal pressure until the operculum is shed explosively, dispersing the spores all at once. This unique mechanism allows Sphagnum to disperse its spores efficiently over short distances, which is important for colonizing nearby wet substrates in bogs and forests.

In addition to sexual reproduction via spores, some Sphagnum species can reproduce asexually through fragmentation, where portions of the gametophyte break off and develop into new plants. Unlike many liverworts, Sphagnum does not produce gemmae, so fragmentation is its primary means of asexual propagation.

 

 

 

 

Examples of species that belong to the class Sphagnopsida:

• Sphagnum capillifolium
• Sphagnum pacificum
• Sphagnum palustre
• Sphagnum papillosum
• Sphagnum squarrosum

Roadmap of Sphagnum structure to help with Sphagnum Identification – pdf

Key to Common Sphagnum Species in Lower Mainland (British Columbia) – pdf