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 sporopollenin on the spores and may have a cuticle-like layer on the gametophyte

 

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 Royal Botanic Garden, Edinburgh

   
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

 

Note that the division of liverworts into thallose and leafy liverworts is not a fundamental evolutionary divide but these distinctive gametophyte forms are spread over many sub-groups of the liverworts. Details of this can be found in Shaw et a.l (2011) Amer. J. Bot. 98: 352-369.

 

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 transport tissue and a true cuticle 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.

 

Vegetative reproduction

Mosses and liverworts have two means of vegetative reproduction;-

  1. fragmentation - pieces of the gametophyte breaking off  (the sole means of dispersal in the Arctic!)
  2. 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 Australian Bryophytes

Check out this great Moss animation on YouTube

Have a look at these beautiful photographs of Bryophytes at the British Bryological Society site.

For an in-depth review of Bryophyte evolution consult 
Shaw, J & Renzaglia, K (2004)"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. 

   .  

C. M. Sean Carrington 1997
Last modified 4 November, 2013