Download TEXT Bryophyta is a group of the simplest and primitive plants of

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Bryophyta is a group of the simplest and primitive
plants of class Embryophyta. These plants occupy a
position between the green thallophytes (algae) and
pteridphytes. Bryophytes are those embryo-bearing land
plants
that
are
non-vascular.
They
have
tissue
differentiation and enclosed reproductive system but they
lack vascular tissues. They have neither flower nor
produce seeds and reproduce through spores. They are
also called the amphibians of the plant world due to their
preference for moist habitat and dependence on water for
fertilization, despite being land plants.
Bryophytes
are
generally
small,
but
form
a
significant component of vegetation in cooler and moist
northern and southern latitudes, and extremely humid
climates of both temperate and tropical regions. Almost
majority of the bryophytes are moisture-loving, mostly
terrestrial, growing in shaded places, moist rocks, bases
of trees and similar moist places; or a few inhabit aquatic
habitats,
e.g.,
Riccia
fluitans,
Ricciacarpus
Fontinalis antypyretica and Riella sp.
natans,
A number of
bryophytes grow in bogs, such as species of Sphagnum,
Leucobryam, Sphalozea etc. Others grow in exposed
sites,
and
can
endure
drought,
e.g.,
Polytrichum
juniperinum and Tortula viralis.
General Characters of Bryophytes
1. Bryophytes are generally small-statured land-plants,
possessing chlorophyll-a and -b, starch, cellulosic
walls, and sometimes possess a cuticle.
2. These
are
mostly
autotrophic;
a
few
are
saprophytes, e. g., Cryptothalus merabilis is a
saprophyte which grows in swamps. It is completely
devoid of chlorophyll and lives at the expense of its
mycorrhizal fungus.
3. The main plant body of bryophytes is gametophyte.
It is more conspicuous, long- lived, independent,
green and freely branched. On the other hand, the
sporophyte is short-lived and completely dependent
on the gametophyte.
4. Gametophytic and sporophytic phases are present in
the life cycle of bryophytes. Both these phases are
morphologically distinct (i. e., heteromorphic).
5. In the liverworts such as Riccia and Marchantia,
the gametophyte is prostrate and thalloid, but in
the mosses, the gametophytic plant body is erect
and
differentiated
into
stem
and
lateral
appendages (Fig.)
6. The true roots are absent in bryophytes and the
function of anchorage and absorption is performed
by filamentous structures known as rhizoids. The
rhizoids
may
be
unicellular
and
unbranched
(liverworts and hornworts), or multicellular and
branched (mosses). Rhizoids are analogous to the
roots of higher plants.
7. The plant body is composed of parenchymatous
cells and lacks differentiated xylem and phloem characteristic of true vascular plants.
8. The plan body of bryophytes is devoid of filaments
as seen in many algae, but rather of true
parenchyma derived by three dimensional growth,
usually from apical meristem.
9. Being
embryophytes,
they
have
multicellular
sporangia and gametangia: reproductive cells are
always surrounded by one or several layers of
sterile cells.
10. Bryophytes,
being
non-vascular
plants,
can
never grow too large; their small and simple
structure provides great selective advantage in
certain habitats. The tiny parenchymatous bodies
of mosses and liverworts permit them to thrive in
microhabitats, such as stone-walls, fences and
bear rock, microhabitats that have too little water
or soil for larger vascular plants.
11. The
bryophytes
reproduce
by
vegetative,
asexual and sexual methods.
12. Vegetative multiplication in bryophytes takes
place by:
a. Fragmentation: Progressive death and decay
of older portions of thallus at the posterior end
reaching a dichotomy, and younger branches at
anterior end by apical growth separating into
new individuals. In this way, an increasing
number of plants are produced.
b. Adventitious branches: In thallose liverworts
and hornworts, often there is formation of
adventitious branches from the underside midrib
which on breaking form new plants.
c. Innovations: These are few axillary branches
which grow vigorously and after separation from
mother plant grow into new individuals, e.g.
Sphagnum.
d. Gemmae: These are special vegetative organs,
and in a more restricted sense refer to a
propagative organ of definite form. The gemmae
characteristically originate from single cell, but
may be unicellular, bicellular or multicellular at
the time of its release.
13.
Typical spore formation in bryophytes is lacking.
But they show the formation of gametophyte directly
from the cells of sporophyte, other than a spore, a
phenomenon called as apospory. The mosses have
great power of regeneration and the wounding of
unspecialized cells of various parts of sporophyte
induces the production of green filament, the latter
bears a new crop of leafy gametophores.
14.
The sexual reproduction in bryophytes is of
oogamous type. The sex organs are multicellular and
considerably
more
complicated
than
those
of
thallophytes. The male sex organs are known as
antheridia;
the
latter
are
stalked,
globose
or
somewhat elliptic. They have an outer sterile one-cell
thick jacket, which surrounds a solid mass of fertile
cells, the androcytes. Each androcyte eventually
metamorphosis
into
a
motile,
biflaggelate
antherozoid.
15.
The female sex organ known as archegonium is
a flask-shaped structure having basal swollen ventor
and somewhat more slender and elongated upper
part, the neck. The ventor and neck are surrounded
by a jacket of sterile cells. Four cover cells are
located at the top of the neck. Each archegonium
contains a single egg cell which is located in the
ventor. A short stalk attaches the archegonium to
gametophyte.
16.
Water is necessary for fertilization. Shortly
before egg cell is mature, the cover cells separate.
At the same time, the cells in the centre of neck
dissolve so that an open canal connects the ventor
with
moisture
outside
the
archegonium.
Free
swimming
sperms
substances
formed
move
by
the
towards
chemical
archegonium.
This
response to chemical stimuli is known as chemotactic
response.
Several
sperms
may
enter
the
archegonium but only one fertilizes the egg to form
diploid zygote within ventor. The zygote does not
have any resting period and it divides immediately
after fertilization. The first division of zygote is
always transverse and outer cell gives rise to
embryo. Thus, they show exoscopic mode of embryo
development.
17.
The embryo develops within the ventor of the
archegonium
and
gives
rise
to
sporophyte
or
sporogonium. The sporophyte is a simple structure
without rhizoids, stem or leaves. It is completely
dependent on gametophyte.
18.
The
sporophyte
is
a
projecting
structure
differentiating into foot, seta and capsule.
19.
The sporogenous cells present in capsule form
haploid spores (meiospores) after reduction division.
All spores are similar in shape and size. The spores
are non-motile and disseminate exclusively by wind.
Under
favorable
conditions,
spores
either
form
filamentous germ tube which divides to give rise to
young gametophyte (Riccia, Marchantia) or form a
protonema, as in mosses. The protonema bears
many buds which develop into erect gametophores.
20.
In bryophytes, growth of gametophore is by
means of a single apical cell rather than by a
meristematic tissue. Regardless of where bryophyte
grows, all bryophytes require water for fertilization
and dispersal.
Importance of Bryophytes
Bryophytes form an important part of vegetation in
cold temperate regions of world, including Himalayas.
Different species of bryophytes are widely used as
medicine, construction material, insect repellant, packing
materials and smoke filters. Some are also used in
horticulture because of their high water holding capacity.
Gardeners use moss- wrapped baskets and pots for
cultivating flowers. Mosses are used as soil-additives.
Some insects and birds use mosses for building shelter.
Many bryophytes are first to colonize open and often
nutrient poor sites where no other plant is capable to
grow; for instance, they grow on bare rocks and on
recently deposited volcanic ash. Thus, they act as
pioneers of vegetation.
Certain mosses have preference for special kind of
substrate for their growth. Therefore, some of them may
serve as the indicators of mineral deposits, pH condition
of substrate and also of particular seed plant community
and pollution levels.
Classification of Bryophytes
The term bryophytes was first time used by R. Braun
(1864), who included algae, fungi, lichens and mosses in
this group. In later systems of classification, however,
algae, fungi and lichens were placed in a separate
division thallophyta, and liverworts and mosses together
in bryophytes.
Eichler, way back in 1813, had recognized two classes in
division bryophytes;
i.
Hepaticae (liverworts)
ii.
Musci (mosses)
Takhtajan (1953) recognized three classes in bryophytes:
i.
Hepaticae (liverworts)
ii.
Anthocerotae (hornworts)
iii.
Musci (mosses)
Rothmaler (1957) changed nomenclature of three classes
of bryophytes as under:
i. Hepaticae  Hepatocopsida
ii. Anthocerotae  Anthocerotopsida
iii. Musci  Bryopsida
At present, majority of the workers recognize three
divisions for those plants previously grouped together as
bryophytes under the Kingdom Plantae. These are:
Division: Hepatophyta (liverworts)
Division: Anthocerotophyta (hornworts)
Division: Bryophyta (mosses)
Diagnostic features of the Divisions
I.
Hepatophyta (liverworts)
The name Hepatophyta has been derived from Latin word
‘hepato’, which means liver. Hence, members of this
class are commonly known as liverworts. The name
liverwort is very old having been used in the 9th century.
It was probably applied to these plants because of their
resemblances to liver and the belief that plant resembling
human organs would cure diseases of organs they
resemble.
The diagnostic features of Hepatophyta include:
1. The gametophyte which forms main plant body is
independent, dorsi-ventral and thalloid or foliose.
2. In thalloid forms, plant body is prostrate, lobed and
dichotomously branched.
3. The ventral surface of thallus bears many unicellular,
unbranched, smooth-walled and tuberculate rhizoids
and scales.
4. In foliose forms, central axis bears leaves in two or
three rows. Leaves are without midrib.
5. Anatomically, thallus is either simple or complex and
is devoid of mechanical tissue.
6. Photosynthetic cells contain numerous chloroplasts
without pyrenoids.
7. Vegetative multiplication takes place by death and
decay of other parts of thallus and by adventitious
branches, gemmae or tubers.
8. The
members
of
Hepatophyta
are
homothallic
(monoecious, e.g. Riccia) or heterothallic (dioecious,
e.g. Marchantia).
9. The sex organs occupy dorsal or terminal position on
thallus. They develop from single superficial initial.
10.
Sporophyte is simple, represented by capsule
only (e.g. Riccia), or differentiated into foot, seta and
capsule (e.g. Marchantia).
11.
The sporogenous cells, called as archesporium,
develop from the endothecium. The sporogenous
tissue either forms only spore mother cells (e.g.
Riccia), or becomes differentiated into fertile spore
mother cells and sterile elater mother cells (e.g.
Marchantia, Pelia).
12.
The capsule does not have sterile columella. The
only sterile cells are elaters. The spores give rise to
gametophyte on germination.
II. Anthocerotophyta (hornworts)
It is a small group of bryophytes, widely distributed in
both temperate and tropical regions of world. The
diagnostic features of anthocerotophyta are as under:
1. Gametophyte
is
flat,
dorso-ventrally
lobed
and
thalloid.
2. The thallus is attached to substratum with the help
of smooth-walled rhizoids. Tuberculate rhizoids and
scales are absent.
3. Air
pores
and
air
chambers
are
absent
but
intercellular mucilage cavities are present which
open on ventral surface by slit-like pores.
4. Sex organs are embedded in thallus. Antheridia
develop on dorsal surface from hypodermal cells.
They
occur
singly
or
in
groups
in
antheridial
chambers.
5. Archegonia develop from dorsal epidermal cells.
6. Sporophyte is differentiated into a long cylindrical
capsule, small meristematic seta and a bulbous foot.
7. The capsule wall is 4-6 layers and the epidermis has
stomata.
8. Elaters do not have thickening bands and are called
pseudo-elaters.
III. Bryophyta (mosses)
The mosses are the largest group of bryophytes,
which occur in almost all parts of the world. They are
abundant at altitudes ranging from 1200 to 2500 meters,
and some species occur up to 6500 meters.
The
characteristic features of
differentiate
vegetative
them
from
features
of
other
the
the
mosses
bryophytes
are
gametophyte,
that
the
early
development and structure of mature sex organs and
that of sporophyte. These features are explained as
under:
1. The gametophyte has two growth stages.
a. Protonema stage: It is the juvenile stage
represented by prostrate, creeping, green and
branched filamentous structure; it develops from
the spore and is only a transitory vegetative
stage.
b. Leafy
stage
or
gametophores:
an
erect
cylindrical shoot with persistent leaves and sex
organs.
2. The main axis of gametophore may be, branched or
unbranched. The branches always arise below the
leaves.
3. The leaves are simple, minute, sessile and usually
with a distinct midrib. They are only one cell in
thickness.
4. The rhizoids are multicellular, branched and have
oblique septa.
5. The axis is differentiated into a central conducting
tissue and a peripheral cortex.
6. Vegetative
multiplication
takes
place
by
fragmentation, stolons, branching of protonema,
special leafy shoots, gemmae and persistent apices.
7. The sex organs are borne in groups at the apices of
the
branches.
The
plants
are
are
club-shaped,
dioecious
or
monoecious.
8. The
antheridia
narrow
and
elongated. The antheridial jacket is single layered.
9. The male gametes are biflagellate and spirally-coiled.
10.
The
archegonia
are
stalked
with
a
much
elongated neck and a massive ventor.
11.
The sporophyte is usually differentiated into
foot, seta and capsule. The elongated seta raises the
capsule much above the gametophyte to facilitate
the dispersal of spores.
12.
The capsule usually has a peristome which helps
in dispersal of spores.
13.
On germination, the spores first form protonema
which in turn develops into leafy gametophore.