What are bryophytes?

Evolutionary origin

Bryophytes belong to the embryophytes, which include all land plants. Evidence from structural, biochemical, and molecular data supports the view that bryophytes and all other plants share a common ancestor in the green algae (Shaw et al. 2011). Bryophytes are considered the closest modern relatives of those ancestors, which likely colonized land about 470–515 million years ago (mya) (Morris et al. 2018, Vanderpoorten and Goffinet 2009). Today’s bryophytes diverged before the development of vascular plants (= tracheophytes) (Morris et al. 2018; illustrated phylogeny can be found here). There is no agreement on whether bryophytes as a group are mono- (originating from a common ancestor) or paraphyletic (originating from several separate ancestral lines) (Budke et al 2018; Morris et al. 2018; Shaw et al. 2011). They mark both the transition to land and the origin of vascular plants and due to their reproductive cycle, which is similar to the ancestral embryophytes, they allow us a unique view into the early evolution of land plants (Shaw et al. 2011, Vanderpoorten and Goffinet 2009). Today’s bryophytes can be found in most and very varied habitats from arid to rainforests and grow on all kinds of surfaces.

What characterises bryophytes?

Except for one are all bryophytes autotroph with chlorophyll. They can vary in size from just over a millimeter to well over a meter long, or large cushions for a single moss individual.

Bryophytes differ from other land plants by having a dominant gametophyte and a simple, reduced sporophyte, which is dependent on the gametophyte. In contrast, land plants other than bryophytes have a dominant sporophyte, which develops out of the male (pollen) and female (embryo) gametophytes. Unlike flowering plants, which reproduce by seeds, bryophytes reproduce by spores. They lack roots and do not show the same efficient vascular internal conducting system with xylem and phloem, which is the conductive tissues of vascular plants. Therefore, they are also commonly referred to as nonvascular plants. Instead, bryophytes have rhizoids – hair-like structures mainly filling the function of anchoring organisms to the substratum, but also facilitating the absorption of minerals and water by simple capillary action.

Some bryophytes (e.g. Polytrichales), however, are known for a primitive conducting tissue enabling them to transport nutrients within the shoot. Generally, water and nutrient uptake is done through the gametophyte surfaces that are above the ground. Lignin and real stomata for regulating water loss as found in vascular plants, are missing in bryophytes.

Bryophytes - a collective term

For now, the English name bryophytes, which has no taxonomic status but is a collective term, consist of three quite distinct phyla of relatively unspecialized plants. Recently, it has been suggested to raise a fourth group to phylum rank, but this has not been widely accepted yet (Glime 2007). The 3 widely recognized phyla are:  

No representatives in the Arctic, about 215 species worldwide. Plants resemble a more or less flattened sheet (called thallus) and sporophytes develop within the gametophyte, protruding as horn-like structures, giving the hornworts its name. Fun-fact: hornworts have typically only 1 chloroplast per cell. (More info can be found here)

There are estimated to be between 5000 and 7500 species world-wide. Liverworts can either have a thallose plant body, or be leafy, similar to mosses. Like mosses, liverworts form sporophytes on the surface of the gametophyte. Differences between liverworts and mosses are presented in the table below (more info can be found here).

As often in taxonomy, several phylogenies are proposed for liverworts. Here we present one of them, where the liverworts are sub-divided into two classes (Goffinet & Shaw 2009): a) Marchantiopsida and b) Jungermanniopsida. The latter makes up approximate 90% of all known arctic liverworts. Marchantiopsida is characterized by species with a thallus, a flat, branching, ribbon-shaped body with lobes. While leaves, roots, and stems are missing, the thallus itself shows an intriguing differentiation in upper and lower tissue, with rhizoids on the lower surface and pores on the upper surface, which permit gas exchange. The order Jungermanniopsida, often called leafy liverworts, typically has leaves arranged in two lateral rows and a third row of smaller leaves present on the ventral side of the stem flattened to the substrate.

There are about 9000-13000 moss species world-wide. Mosses have a leafy plant body (stems with leaves). In difference to the other bryophytes, a branching protonema develops from a single spore. From the protonema, multiple leafy stems grow upward and the organism can form mats or cushions.
The Bryophyta are split into several classes. There are different opinions and the number of classes varies. Glime (2007) operates with 6 classes: Andreaeopsida, Andreaeobryopsida, Bryopsida (95% of described species), Polytrichopsida, Sphagnopsida (peat moss, by some considered own phylum) and Takakiopsida. Takaliopsida were until now not found in the Arctic. (Other suggestions e.g. Vitt et al. 2014 and Cox et al. 2010)

The hornworts are not represented in the Arctic, while liverworts and mosses are commonly distributed. For reference and extended intro see Budke et al. 2018.
Which bryophyte is it?” can help you to find out which bryophyte the “mossy thing” in front of you is.

From left to right: Representatives of the mosses (Polytricum juniperinum), leafy liverwort (Arnellia fennica), and thallose liverworts (Marchantia polymorpha)

How are bryophytes build?

Here are schematic drawings of the general build of mosses and liverworts with names of the main parts. If you want to identify bryophytes, those are terms you should be familiar with.

How do you distinguish between liverworts and mosses? (morphological comparison)

 

Mosses

Liverworts

Protonema

  • filamentous
  • many buds
  • many species with gemma
  • mostly globose or thallose
  • no gemma
  • 1 bud

Gametophyte shape

leafy shoot

leafy shoot or thallus

Leaves

  • usually spirally arranged
  • in most species one/few cell layers thick, often with a midrib/nerve
  • undivided
  • simple, small oil bodies (special leave organelles) or none
  • thallose or foliose
  • if foliose: in 2 to 3 rows, ventral/dorsal row normally with smaller and deviating leaves
  • flattened with a dorsal and ventral side
  • one cell-layer thick, without any nerve/midrib
  • often with incisions, divided into 2 or more lobe
  • often with complex  oil bodies
  • if thallose: can be highly differentiated

Rhizoids

multicellular

unicellular

Setae (stalk of the sporophyte)

  • stiff
  • opaque
  • long-lived
  • photosynthetic
  • soft
  • transparent
  • collapse after spores are shed

Sporophyte capsules

  • open by operculum
  • surrounded by 4–64 “teeth”
  • with stomata
  • split along lines, leaving four parts that bend out
  • no teeth
  • no stomata

Gemma (small dispersal unit)

common on leaves, stems, rhizoids or protonema

common on leaves

If you are trying to sort mosses from liverworts in the field or lab, take a look at the beginners-guide here or here.