Hornworts, liverworts, and mosses - commonly referred to as bryophytes -
are considered to be a pivotal group in our understanding of the origin of
land plants because they are believed to be among the earliest diverging
lineages of land plants. Mosses, liverworts and hornworts are found
throughout the world in a variety of habitats, from the harsh environs of
Antarctica to the lush conditions of the tropical rainforests. Bryophytes
are unique among land plants in that they possess an alternation of
generations, which involves a dominant, free-living, haploid gametophyte
alternating with a reduced, generally dependent, diploid sporophyte.
Bryophytes are small, herbaceous plants that grow closely packed together
in mats or cushions on rocks, soil, or as epiphytes on the trunks and
leaves of forest trees. Bryophytes are remarkably diverse for their small
size and are well-adapted to moist habitats and flourish particularly well
in moist, humid forests like the fog forests of the Pacific northwest or
the montane rain forests of the southern hemisphere.
Significance of bryophytes|
Bryophytes have a significant role in contributing to nutrient cycles,
providing seed-beds for the larger plants of the community, and form
microhabitats for insects and an entire array of microorganisms.
Bryophytes are also very effective rainfall interceptors, and the
overwhelming abundance of epiphytic liverworts in "cloud" or "mossy" forest
zones is considered an important factor in eliminating the deteriorating
effect of heavy rains, including adding to hill stability and helping to
prevent soil erosion. The chemical compounds of some liverworts are also
particularly interesting because they have important biological activities,
for example, against certain cancer cell lines, anti-bacterial properties,
anti-microbial, anti-fungal, and muscle relaxing activity.
Over the last decade, recent advances in DNA sequencing technology and
analytical approaches to phylogenetic reconstruction, including the use of
ultra-structural, morphological and anatomical data, have enabled
unprecedented progress toward our understanding of plant evolution. A
growing consensus suggests that the bryophytes possibly represent three
separate evolutionary lineages, which are today recognized as mosses
(phylum Bryophyta), liverworts (phylum Marchantiophyta) and hornworts
The greatest species diversity in bryophytes is found in the mosses, with
estimates of the number of species ranging from 10,000 to 15,000.
Higher-level classification of the mosses remains unresolved with
considerable difference of opinion on the names of the major groups.
However, generally four major groups or classes are recognised. These include: Sphagnopsida (peat or Sphagnum mosses), Andreaeopsida (rock or
lantern mosses), Polytrichopsida (nematodontous mosses), and the Bryopsida
(arthrodontous mosses). The Sphagnum mosses are one of the most
ecologically and economically important groups of bryophytes. The class
Bryopsida accounts for the largest and most diverse groups within the
mosses with over 100 families.
The estimated number of liverwort species range from 6000 to 8000.
Traditionally, liverworts have been subdivided into two major groups or
classes based, partially, on growth form. The class Marchantiopsida,
includes the well-known genera Marchantia, Monoclea, Lunularia, and Riccia,
and has a complex thalloid organisation. The class Jungermanniopsida
represents an estimated 85% of liverwort species and shows an enormous
amount of morphological, anatomical and ecological diversity; plants with
leafy shoot systems are the most common growth form in this class, e.g.,
Frullania, Jubulopsis, Cololejeunea, and Radula.
Hornworts get their name from their long, horn-shaped sporophytes and are
the smallest group of bryophytes with only approximately 100 species.
Hornworts resemble some liverworts in having simple, unspecialized thalloid
gametophytes, but they differ in many other characters. Hornworts differ
from all other land plants in having only one large, algal-like chloroplast
in each thallus cell.