BRYOPHYTES
Bryophytes include the various mosses and liverworts.
These are non-vascular embryophytes, characterised by the
presence of an independent gametophyte and parasitic
sporophyte.
Habitat
Bryophytes commonly grow in moist, shaded areas in hills.
These are also called amphibians of the plant kingdom
because, these can live in soil, but are dependent on water
for sexual reproduction.
Some bryophytes grow in diverse habitats, such as
(i) Aquatic, e.g. Riccia, Ricciocarpus, Riella
(ii) Epiphyte, e.g. Radula, Dendroceros
(iii) Saprophyte, e.g. Buxbaumia, Cryptothallus
(iv) Dry habitats, e.g. Polytrichum
(v) Deserts, e.g., Tortula desertorum
(vi) Dry rocks, e.g., Porella
Thallus ()rganisation
The plant body of bryophytes is more differentiated than
that of algae. It is thallus-like and prostrate erect
attached to the substratum by unicellular or multicellular
rhizoids. Root, stem and leaves are not present on thallus.
Sex Organs
The main plant body is haploid that produces gamers
hence it is called gametophyte. The sex organs in bryophyte
are multicellular, the male sex organs in bryophytes is called
antheridium (which produces biflagellate antherozoids) and
the flask-shaped female sex organ is called archegonium
(produces a single egg).
Reproduction
It is a sexual type of reproduction. The antherozoids art
released into water where they come in contact with
archegonium.
Fertilisation
It occurs inside the archegonium. The egg secretes a
chemical which attracts spermatozoids. Sperms require a
thin film of water for swimming and reaching the dehisced
archegonium. One sperm fuses with an egg and produces a
diploid zygote. Zygotes do not undergo reduction division
immediately instead, they produce a multicellular bodv
called sporophyte.
Dependent Sporophyte
The sporophyte is not free-living, but attached to the
photosynthetic gametophyte deriving nourishment from it
Some cells of the sporophyte undergo reduction division
(meiosis) to produce haploid spores (which germinate to
produce gametophyte).
Sporogonium
Che sporophyte of bryophytes is called sporogonium
It is mainly dependent and meant for producing spores
Bryophytes have heteromorphic alternation of generation. The
gametophyte may bc produced directly or a first formed
juvenile stage called protonema.
Bryophytes in general are of little economic importance. But,
several species have some uses.
(i) Some mosses provide food for herbaceous mammals,
birds and other animals.
(ii) Species of Sphagnum (a moss), provides peat that have
long been used as fuel. It has the capacity to hold water
as packing material for transshipment of living material.
(iii) Mosses along with lichens are the first organisms to
colonise rocks. Hence, these help in biological
succession. They decompose rocks making the substrate
suitable for the growth of higher plants.
(iv) Mosses form dense mats on the soil. They reduce the
impact of falling raindrops and thus prevent soil
erosion.
(v) Marchantia has medicinal properties to cure lungs and
liver infections. It also has anti-tumour properties.
Types of Bryophytes
The bryophytes are generally classified into two main groups
i. Liverworts
Liverworts (Hepaticopsida) usually grow in moist, shady places
such as banks of streams, marshy ground, damp soil, bark of
trees and deep in the woods.
(a) Thallus The plant body of a liverwort is thalloid,
e.g., Marchantia,. The thallus is dorsoventral and
closely appressed to the substrate. The leafy members
have tiny leaf-like appendages in two rows on the
stem-like structure.
(b) Rhizoids These are unicellular. These may be of two types
in some liverworts, i.e., smooth-walled and tuberculate.
(c) Reproduction It occurs both by asexual and sexual means.
Asexual Reproduction It occurs 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 parent body and
germinate to form new individuals.
Sexual Reproduction It is accomplished by formation
of male and female sex organs either on the same or on
different thalli. The sporophyte develops from zygote
which continues to grow on gametophyte. It is
differentiated into a foot, seta and capsule. After
meiosis, spores(n) are produced within the capsule.
These
spores germinate
to form
free-living
gametophytes, e.g., Riccia, Marchantia, Pellia, Porella,
etc.
ii. Mosses
Mosses (Bryopsida) grow in dense mats over moist
shady places, especially during rainy seasons. Some
mosses grow in desert bogs and streams.
(a) Plant Body The predominant stage of life cycle
of a moss is the gametophyte which consists of
two stages. The first stage is the protonema stage
which develops directly from a spore. It is a
creeping, green, branched and frequently
filamentous stage, the second stage is the leafy
stage which develops from the secondary
protonema as a lateral bud. They consist of
upright, slender axes bearing spirally arranged
leaves. This stage bears sex cells.
(b) Rhizoids These are long, multicellular branched
structures with oblique septa. They take part in
fixation and absorption of water. However,
surface conduction through capillarity is an
important mechanism of water supply to aerial
parts.
(c) Reproduction This can occur both by vegetative and
sexual means.
Vegetative Reproduction It occurs by fragmentation
and by budding in the secondary protonema from
exposed rhizoids and other parts (like gemmae, buds
and tubers) of plant body.
Sexual Reproduction Sex organs, antheridia (male) and
archegonia (female) are produced at the apex of the
leafy surface. Male organs appear cup-shaped, while
female organs are bud-like. After fertilisation, the
zygote develops into a sporophyte, consisting of foot,
seta and capsule (containing spores).
(d) Sporophyte The sporophyte in mosses is more
elaborate than that in liverworts. It derives its nutrition
from gametophyte for its development. Spores are
formed after meiosis in sporophyte. The mosses have
an elaborate mechanism of spore dispersal, e.g., Funaria,
Polytrichum and Sphagnum.