Bryophytes: Link of Protists and True Plants*

The moss and its kind are the precursors of the Plant Kingdom 

Dr Abe V Rotor

Moss (Musci)

 

 Hornwort (Anthoceros), Liverwort (Marchantia) 

Bryophytes are the intermediate forms of life between the Algae (Kingdom Protista) and the Tracheophytes (Vascular) or true plants. Bryophytes bridge the evolution of life in the Plant Kingdom.

Anyone who has seen “Honey, I Shrunk the Kids," or the second travel of Gulliver in Brodningnag, could easily place himself into imagery where small things are very big.

A blade of grass becomes a perfect slide, an ant becomes a pony for going places, a raindrop can knock one down hard.

Now imagine the lowly moss to be as large as a tree. A liverwort becomes a large green carpet shaped like a liver. A hornwort has pinnacles in Gothic style. When you are microscopic in size, everything you see around you becomes large.

Bryophytes are the link between the two kingdoms of the protists, and the true (or vascular) plants. They are early forms of plants, which botanists believe to have stopped evolving. Thus, they are today what they were millions of years ago. They are, indeed, living fossils.

Observe a piece of rock covered with bryophytes. It appears like a miniature forest under the magnifying glass. It is dense and every space is occupied by structures that look like trunks and leaves. On closer look, however, these structures are not true organs, because they lack vascular tissues, which are found in higher plants. The tissues are needed for water and food to flow to keep the plant alive.

Thick mats of  moss and lichen (alga and fungus living together in symbiosis) are commonly found growing on the trunk of old trees.

 Alternation of Generations

The moss has a unique two-in-one life cycle. Botanists describe the gametophyte as either male or female plant, while the sporophyte is one containing the total number of chromosomes. The former carries only half the number of chromosomes (haploid). When the sporophyte plant matures, it produces spores, which will germinate and develop into gametophytes. When the gametophytes mature, they form both eggs and sperms that fuse together to form a zygote. The zygote grows into another sporophyte that will carry the next generation. This alternation of generation is the key to the survival of bryophytes even under harsh conditions.

Bryophytes are Nature’s Soil Builder

When the plants are uprooted, one will find soil underneath. This means that bryophytes grow on rock by digesting it first with acid. The softened rock yields to the roots and releases elements needed for growth and development. As the plants die, their organic debris is mixed in with the rock particles and form into soil.

Since bryophytes are short-lived and seasonal, the soil deposit becomes thicker by each generation, with the plant borders extending to form new frontiers. Soon entire walls and rocks become covered like a green carpet. As the bryophyte community expands to reach its peak and climax, more and more organisms become dependent on it. Millipedes find it an ideal place for a home, while providing their nutrition. Insects frequent the place as a hunting ground for their prey. Frogs, however, stay near the byrophytes to stalk the insects.

Bryophytes Create a Microclimate

A carpet of mosses on the wall or rock feels soft to the touch. It is thick and spongy. When it rains, the carpet absorbs and stores the water. At night and early in the morning, dew precipitates and is absorbed by the moss, creating a microclimate in the surrounding area that is favorable to other plants.

With the passage of time, new plants grow out from the middle of the carpet. This is the beginning of the second part of plant invasion, courtesy of the ferns. The plants are large and diverse, the forerunners of vascular plants which once dominated the Carboniferous forest (PHOTO), even before the dinosaurs roamed the earth.

Ferns actually form a canopy above the moss carpet, and as they do, they block the sun, wrest for space and compete for water. Fern roots wedge the open cracks in the rock, sending boulders down together with their tenants. While it is catastrophe to the pioneering plants, it is advantage to others. Nature works its way following a formula aimed at dynamic balance or homeostasis.

Soon the bryophytes do not only lose their dominance to the ferns, they have lost the place. Their job is over because the rock is gone.

“What good is rock when it loses the essence from which life rises?”

So thus the fern continue to change the landscape. When nature writes “finis” to the lowly moss, larger plants, like trees, come around, and soon the place becomes a forest. And life goes on. 
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Phylogeny of the Bryophytes 

Land plants	Liverworts Mosses Hornworts Polysporangiophytes "Protracheophytes", such as Horneophyton or Aglaophyton Tracheophytes or Vascular plants

*Reference: The Living with Nature Handbook by AV Rotor (UST Publishing House, 2004)