THE BRYOPHYTES

Like the rest of land plants, the Bryophytes are Embryophytes (plants that produce an embryo) and they have traditionally been viewed as a distinct lineage from other land plants. They are not considered to have given rise to the vascular plants but they probably were the earliest land plants (Qui & Palmer, 1999). Like the rest of the land plants, they evolved from green algal ancestors, closely related to the Charophytes. They are a group of simple land plants, well-adapted to moist habitats.

With it varied landscape and habitats, it is not surprising that about one third of the world's Bryophyte species are found in Tropical America, with high levels of endemism (Gradstein et al., 2001).

Like other land plants, the Bryophytes:-

have multicellular sex organs, i.e. the gametes are enclosed by a sterile jacket of cells
are parenchymatous, not filamentous
retain the zygote within the female sex organ and allow it to develop into an embryo there
have cutin (a cuticle) on the plant and spores

Bryophytes, in contrast,

have no lignin usually
are small, low-lying, (generally) moisture-loving plants
have no roots, only filamentous rhizoids

sporophyte (2n)

gametophyte (n)

THE ONLY LAND PLANTS WITH A DOMINANT GAMETOPHYTE!

The sporophyte is parasitic on the gametophyte. This stems from the embryo being retained in the female sex organ of the gametophyte.

What is a bryophyte anyway?
   Get a quick introduction from the New York Botanical Garden


   Which moss covers 1% of the earth's surface?

There are 3 groups of Bryophytes;-

	Mosses (~10,000 species)
	Liverworts Leafy liverworts (4,000-6,000 species) - predominately tropical and poorly covered in most texts Thallose liverworts (~3,500 species) - these are further sub-divided into simple and complex thalloids
	Hornworts (not covered in this course)
These are generally viewed as three monophyletic lineages emerging from the very earliest land plants.

Click here for another introduction to mosses and liverworts


Moss - radially symmetrical rhizoids multicellular leaves with or without a midrib	Thallose liverwort - bilaterally symmetrical rhizoids unicellular	Leafy liverwort - bilaterally symmetrical rhizoids unicellular leaves without a midrib

Liverworts can also be distinguished from mosses by the possession of oil bodies, unique organelles. Oil droplets in mosses and other land plants are just naked blobs of fat but liverwort oil bodies are single membrane-bound organelles and contain essential oils (terpenoids).

Liverwort leaf cells each containing two to five (grey) oil bodies (as well as numerous chloroplasts).
Photo © Steven L. Jessup, Southern Oregon University.

Role of Bryophytes

Many are pioneer plants, growing on bare rock and contributing to soil development.
In bogs and mountain forests they form a thick carpet, reducing erosion.
In forest ecosystems they act like a sponge retaining and slowly releasing water
They provide habitat for other plants and small animals as well as microorganisms like N₂-fixing blue-green bacteria
Lacking a cuticle and transport tissue they readily absorb whatever is around them and can serve as bioindicators of pollution and environmental degradation

Bryophyte Physiology

These plants all require moist conditions for at least part of their life cycle. If really wet, they may grow as epiphytes (plants that grow on other plants).

Many mosses are quite resistant to desiccation. They can lose water in the dry season, become dormant and then imbibe water or spring back to life when it rains. Similarly, many temperate mosses can survive freezing and thawing without damage.

Being prostrate, Bryophytes have much of their surface in contact with the substratum and readily absorb moisture this way. Water often is drawn along the surface of these plants by capillarity and this external water movement is important. In certain mosses, specialised transport cells, hydroids and leptoids, analogous to the xylem and phloem of vascular plants, are found at the centre of the stem.

Gases simply diffuse across the plant surface but liverworts also have special pores which are permanently open for gas exchange. Certain mosses also have stomata on their capsules (sporophytes).

Surface view (L) and V.S. (R) liverwort air pore

Thallose liverworts are quite distinctive. They are flattened with a dichotomously branching thallus.

This group is absent from drier Caribbean islands like Barbados.

Mosses and leafy liverworts can be confused.

Leaves of leafy liverworts never have a mid-rib (unlike those of most mosses).
Mosses have multicellular rhizoids vs. the unicellular rhizoids of liverworts
The capsules are quite different, as we will see
Moss leaves are of equal size and spirally arranged while the main leaves of liverworts are arranged in one plane on either side of the stem with a third row of smaller leaves on the underside of the stem.
Moss leaves are never lobed
Oil bodies occur in the leaves of 90% of liverworts, but are absent from moss leaves.

See if you would recognise these plants as liverworts...

Click here to see a beautiful pink moss from Sweden.

Vegetative reproduction

Mosses and liverworts have two means of vegetative reproduction;-

fragmentation - pieces of the gametophyte breaking off (the sole means of dispersal in the Arctic!)
gemmae - specialised propagules produced mitotically.

In some liverworts, gemmae are produced
in cup-like structures called gemmae cups
(labelled b, at left).

Sexual Reproduction

The gametophytes (which are the stage of the life cycle we recognise as the moss or liverwort) bear male sex organs termed antheridia (antheridium sing.) and female sex organs termed archegonia (archegonium sing.).

	Flask-like archegonium (A) with single egg cell (B) at base of canal. Biodisc photomicrograph
	Club-shaped antheridium (A) with mass of antherozoids (a) being discharged. A single biflagellate antherozoid or sperm cell (b) is also shown.

In some Bryophytes, male and female sex organs are borne on separate gametophytes. Such species are said to be dioecious. In contrast, when a single plant carries both male and female sex organs the species is said to be monoecious.

The sex organs are typically borne in clusters, often surrounded by sterile hairs. These may also be borne in a head on a stalk in some species. At maturity, the antheridium bursts releasing the sperm cells or antherozoids. These can only swim a few cm so that if the archegonia are not adjacent, the sperm rely on raindrops to "splash-launch" them to a suitable location.

The sperm cells are very compact with the nucleus wrapped in a minimum of cytoplasm. Some chemical exuded by the egg cell attracts the sperm which swims down the neck of the archegonium and fertilises the egg. The resulting zygote is the start of the sporophyte generation. This develops within the archegonium and remains parasitic on this for its entire life (except a few moss sporophytes which develop photosynthetic capacity.)

Click here for NBII Moss & Liverwort links

Click here for Bryophyte images.
And here for more Moss images

Click here for Cryptogam of the Month at the Swedish Musuem of Natural History.

For an in-depth review of Bryophyte evolution consult
Shaw, J & Renzaglia, K (20040 "Phylogeny and diversification of Bryophytes" Amer. J. Botany 91: 1557-1581.

We have only looked at part of the Bryophyte life cycle.
Click the button to move on to asexual reproduction in this group.