BRYOPHYTA
Bryophytes includes the various mosses and
liverwort that are found growing in moist, shady area in the hills. Bryophytes
are the first land plant. It is believed that, they originated from aquatic
plants and they come on land through water. Because some bryophytes have
characters similar to aquatic plants (eg. presence of air canal and formation
of motile, flagellated male gametes.)
Bryophytes are known as amphibians of the plant kingdom, because these plants can live in soil but are dependent on water for fertilization.
Bryophytes are not considered as the successful land plants because vascular tissue is absent and they need water for fertilization. Due to the absence of vascular tissue bryophytes cannot grow very tall. The process of water conduction in bryophytes takes place with the help of parenchyma. Parenchyma is a living tissue.
The plant body of bryophytes is more differentiated than that of algae. It is thallus like prostrate or erect and attached to substratum by unicellular or multicellular rhizoids. They lack true root, stem
and leaf. They may possess root like, stem like and leaf like structures.
Bryophytes are sciophytes, i.e. they prefer to grow in moist (wet) and shady places.
The main plant body of bryophyte is haploid. It
produces gametes, hence is called a gametophyte.
Sex organs are formed on gametophyte. Sex organs are multicellular and jacketed in bryophytes. Male sex organ is called antheridium. They produces biflagellated antherzoids. The female sex organ is
called archegonium. Archegonium is flask shaped and produce a single egg.
The antherozoids are released in water where they come in contact with archegonium. In Bryophyta, fertilization is performed by zoidogamy i.e. male gamete swims into water to reach the female gametes and fertilizes it.
As a result of the fertilization, a diploid zygote is formed. Zygote does not undergo reduction division (meiosis) immediately. They produce a multicellular body called sporophyte.
The sporophyte of Bryophyta is not free living but attached to photosynthetic gametophyte and derive nourishment from it. It is
made of foot, seta and capsule, so it is known as sporogonium.
Some of cells present in capsule of sporophyte function as spore mother cells and undergo reduction divison to produce haploid
spores. These spores germinate to produce gametophytes.
The germination of spores is direct or indirect. In Liverworts & Hornworts, the germination of spore is direct, i.e. each spore
forms a gametophyte after germination means each spore forms one thallus.
But the germination of spores in Mosses is indirect. In mosses, a multicellular filament is formed after the germination of spore. This filament is known as protonema. Now lateral buds are formed on
protonema. Each bud develops and forms a gametophyte plant. Indirect germination is best for survival. Mosses are gregarious in nature because they appear in group.
Bryophyta is divided into two classes-:
(A)
Hepaticopsida
(B)
Bryopsida
HEPATICOPSIDA – LIVERWORTS
Bryophytes include in this class have shape like
liver (e.g. Marchantia or flat like Riccia) so they are known as liverworts.
The
liverworts usually grow in moist, shady habitat, such as banks of streams, marshy
ground, damp soil, bark of tree and deep in woods.
Plant body
is dorsiventral thalloid and closely appressed to the substrate. Rhizoids and
scales are present on thallus. Rhizoids are unicellular and unbranched. Scales
are multicellular.
The leafy
members (eg Porella) have tiny leaf like appendages in two rows on the stem
like structures.
Asexual
(vegetative) reproduction in liverworts takes place by fragmentation of thalli,
or by the formation of specialised structures called gemmae (sing. gemma).
Gemmae are green, multicellular, asexual buds, which develop in small
receptacles called gemma cups located on the thalli. The gemmae become detached
from the parental body and germinate to form new individuals. Eg. Marchantia
The
sporophyte of Liverworts is differentiated in foot, seta and capsule.
(Exception - In Riccia sporophyte is made up of only capsule).
The
sporophyte of Liverworts is completely dependent on gametophyte i.e. it is
dependent on gametophyte for food, water and habitat.
BRYOPSIDA -
MOSSES
The
predomiant phase of the life cycle of a moss is the gametophyte which consistst
of two stages. The first stage is the protonema stage which directly develops
from a spore.
Vegetative reproduction in mosses is by
fragmentation and budding in the
During sexual reproduction, sex organs are produced
at the apex of the leafy shoots. After fertilization, zygote develope into
sporophyte. The sporophyte in mosses is more elaborated (developed) than that
in liverworts. The sporophyte of moss is divided into foot, seta and
capsule.
Bryophytes in general are of little economic
importance but some mosses provide food for herbaceous mammals, birds and other
animals. Species of Sphagnum, a moss, provide peat that have long been used as
fuel, and because of their capacity to hold water as packing material for
trans-shipment of living material. Mosses along with lichens are the first
organisms to colonise rocks and hence, are of great ecological importance. They
decompose rocks making the substrate suitable for the growth of higher plants. Since
mosses form dense mats on the soil, they reduce the impact of falling rain and
prevent soil erosion.
PTERIDOPHYTA
Pteridophytes are known as reptiles of plant
kingdom.
Pteridophytes are also called as vascular
cryptogames. Evolutionary Pteridophytes are first terrestrial plant possess
vascular tissues i.e. xylem and phloem. In pteridophytes, vessels in xylem and
companion cells in phloem are absent.
Pteridophytes are used for medicinal purpose and as
soil binders. They are also frequently grown as ornamentals.
Pteridophytes are more adapted terrestrial plants
as compared to bryophytes. Because - Vascular tissue is present in
pteridophytes.
They have roots.
Pteridophytes are not completely successful
terrestrial plants because they need water for fertilization, so
pteridophytes grow in cool, shady and moist places.
Main plant body is sporophyte which is completely
differentiated into root, stem and leaves.
These organ possess well differentiated vascular
tissue. The primary root remains alive for short period. After some time
it is replaced by adventitious roots.
Stem is erect or prostrate. When the stem is
underground, it is known as rhizome. On the basis of leaves, pteridophytes are
of two types -
First in which stem is smaller, while leaves are
large (Megaphylls). They are known as megaphyllous pteridophytes.
eg. Ferns
Second, in which stem is large and leaves are
smaller (Microphylls). They are called as microphyllous pteridophytes.
eg. Selaginella
LIFE CYCLE OF PTERIDOPHYTES :-
Plant is sporophyte. i.e. diploid and they
reproduce by spore formation.
In majority of the pteridophytes all the spores are
of similar kinds, such plants are called homosporous.
Exception - Some pteridophytes produce two kinds of
spores, macro and microspore, are known as heterosporous. e.g. Selaginella,
Salvinia
Formation of spores takes place in sporangia.
Sporangia are formed at the abaxial surface of leaves.
The sporophyte bears sporangia that are subtended
by leaf like appandeges called sporophyll. Normal photosynthetic leaves are
called trophophylls (vegetative leaves). Sporangia are present in groups, these
groups are called sorus (Plural-sori). Sori are found on sporophylls.
In some cases sporophyll may form distinct compact
structure called strobilli or cone.
The spore germinates to give rise inconspicuous,
small but multicellular, free living mostly photosynthetic, thalloid
gametophyte called prothallus. These gametophyte require cool, damp, shady
places to grow. Because of this specific restricted requirement and the need
for water for fertilisation, the spread of living pteridophytes is limited and
restricted to narrow geographical regions.
Occurence of two types of spores, small
(Microspores) and large (macrospore = Megaspore) is heterospory. A very few
genera are heterosporous in pteridophytes e.g. Selaginella, Salvinia, Azolla,
Marsilea
The megaspore and microspores germinate and give
rise to female and male gametophytes, respectively.
In these plants microspores form male gametophyte, male sex organ and male gametes which come outside the microsporangium but the megaspore forms female gametophyte, female sex organ and female gamete inside the megasporangium on the parental sporophyte. The female gametophyte in these plants are retained on the parent sporophyte for variable period.
After the fertilisation the development of zygote
into young embryo takes place within the female gametophyte. Embryo essentially comes outside the megasporangium, so seed could not be formed in Pteridophyta but the heterospory is found in them which is a very important step in evolution of seed habit in higher plants.
The developement of male and female gametophytes in
these plants takes place inside the microsporangium and megasporangium, respectively, hence the gametophytes are not completely independent and are not very much developed so they are generally not completely regarded as prothallus.
Pteridophyta is divided into four class
( (B) Lycopsida
- Lycopodium, Slaginella
( (C) Sphenopsida
– Equisetum
(D) Pteropsida
– Pteris, Dryopteris , Adiantum , Salvinia
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