belong to a primitive group of plants. They are usually green,
small and among the simpliest land plants (few are aquatic).
They produce no flowers or fruits and most of them have no inner
vessels for water or nutrient
transfer. They reproduce with spores and can
also generate new plants from segments of cut stems or leaves.
have no roots, but they have thin (one-cell thick!) rhizoids,
useful for adhesion to substrate
and water absorbance. Most species have very little tolerance
to drought and are restricted in moist and protected areas.
They are often observed on rocks, ground, fallen trunks and
on trees, but they prefer cold and wet forests, swamps and riverbanks.
However, several other species exist, growing in extreme weather
conditions, such as the Arctic Cycle, Antarctic or deserts.
They also thrive on sand dunes, where they play an important
role in stabilizing the sand.
The number of bryophyte species
is estimated at 14,000-15,000.
are another primitive group of
organisms. They result from a peculiar symbiosis
of two species belonging in separate kingdoms, functioning as
one biological unit. This alliance takes place from a fungus
and an alga. Algae participating in this formation belong to
either blue-algae or green-algae and participating
fungi belong to either discomycetes or basidiomycetes.
During this symbiosis, the fungus is fed by organic matter produced
from alga's green cell, while the alga absorbs inorganic matter
from the fungus.
lichen categories exist, according to their structure:
are found on rocks, soil, trees or artificial structures, in
intact environments. They can live in various ecosystems all-over
the world, even in inhospitable deserts, in Arctic and Antarctic
territories. They are considered along with bryophytes, pioneer
species in certain biotopes, because they are often the first
organisms to invade in newly exposed rock or surface. A lichen
may absorb some salts from the substrates it grows on, but it
generally feeds through photosynthesis performed by the alga
cell. This means that tree lichens are not parasites, they just
use trees as their home. However, the lichens growing on rocks
may excrete chemicals that accelerate rock's corrosion, creating
are important in chinese medicine and in modern times are widely
used for the production of antibiotics. Also, the essential
oils they contain are the basis for producing perfumes and alcoholic
drinks. Several are nutritient and some N African tribes smoke
a lichen mixture. They are also used in maquettes and rock gardens,
and for dye by American Indians. They are most useful though
as air pollution indicators, because they are extremely sensitive
to high concentrations of air pollutants. They have no defense
mechanisms to pollutants as more evolved plants do, thus when
they disappear from an area, they inform us for air pollution
in that area.
During 1960's, approximately 50 lichen species were identified