6. Bryophytes
“ Amphibians Of The Plant Kingdom”
Plants grow in two well defined habitats.
These are the water and the land.
The plants which grow are called the aquatics and the
others terrestrial.
The best examples of aquatic plants are the algae and of
land dwellers the seed plants (spermatophytes).
Between these two extremes of habitats is a transitional
zone.
It is represented by swamps and the areas where water and
land meet. It may well be called the amphibious zone.
7. Bryophyta ( In Greek Bryon - moss; phyton - plant), a
division of kingdom Plantaecomprises of mosses, Hornworts
and Liverworts.
They are groups of green plants which occupy a position
between the thallophytes (Algae) and the vascular
cryptogams (Pteridophytes).
Bryophytes produce embryos but lack seeds and vascular
tissues.
They are the most simple and primitive group of
Embryophyta.
They are said to be the first land plants or non-vascular
land plants (Atracheata).
Presence of swimming antherozoids is an evidence of their
aquatic ancestory.
8.
9. Origin of Bryophytes
Nothing definite is known about the origin of Bryophytes
because of the very little fossil record. There are two views
regarding the origin of Bryophytes.
These are
(i) Algal hypothesis of the origin of Bryophytes.
(ii) Pteridophytean hypothesis of the origin of Bryophytes.
10. (i) Algal Hypothesis of Origin
There is no fossil evidence of origin of Bryophytes from algae
but Bryophytes resemble with algae in characters like-
amphibious nature, presence of flagellated antherozoids and
necessity of water for fertilization.
This hypothesis was supported by Lignier (1903), Bower
(1908), Fritsch (1945) and Smith (1955) etc.
According to Fritsch (1945) and Smith (1955) Bryophytes
have been originated from the heterotrichous green algae
belonging to the order Chaetophorales for e.g., Fritschiella,
Coleochaete and Draparnaldiopsis.
11. (ii) Pteridophytean Hypothesis of Origin
According to this hypothesis Bryophytes are descendent of
Pteridophytes.
They are evolved from Pteridophytes by progressive
simplification or reduction.
This hypothesis is based on certain characters like-presence
of type of stomata on the sporogonium of Anthoceros and
apophysis of mosses similar to the vascular land plants,
similarly in the sporophytes of some Bryophytes (e.g.,
Anthoceros, Sphagnum, Andreaea) with some members of
Psilophytales of Pteridophytes (e.g., Rhynia, Hormophyton
etc.)
12. Distribution of Bryophytes
Bryophytes are represented by 960 genera and 24,000
species.
They are cosmopolitan in distribution and are found
growing both in the temperate and tropical regions of the
world at an altitude of 4000-8000 feet.
In India, Bryophytes are quite abundant in both Nilgiri
hills and Himalayas; Kullu, Manali, Shimla, Darjeeling,
Dalhousie and Garhwal are some of the hilly regions which
also have a luxuriant growth of Bryophytes.
Eastern Himalayas have the richest in bryophytic flora.
13. A few species of Riccia, Marchantia and Funaria occur in
the plains of U.P., M.P. Rajasthan, Gujarat and South
India.
In hills they grow during the summer or rainy season.
Winter is the rest period.
In the plains the rest period is summer, whereas active
growth takes place during the winter and the rainy season.
Some Bryophytes have also been recorded from different
geological eras e.g., Muscites yallourensis (Coenozoic era),
Intia vermicularies, Marchantia spp. (Palaeozoic era) etc.
14. Habitat of Bryophytes
Bryophytes grow densely in moist and shady places and
form thick carpets or mats on damp soils, rocks, bark of trees
especially during rainy season.
Majority of the species are terrestrial but a few species grow
in fresh water (aquatic) e.g., Riccia fluitans, Ricciocarpos
natans, Riella etc.
Bryophytes are not found in sea but some mosses are found
growing in the crevices of rocks and are being regularly
bathed by sea water e.g., Grimmia maritima.
15. Some Bryophytes also grow in diverse habitats e.g.,
Sphagnum-grows in bogs, Dendroceros-epiphytic, Radula
protensa. Crossomitrium -epiphyllous, Polytrichum
juniperinum-xerophytic, Tortula muralis-on old walls.
Tortula desertorum in deserts, Porella platyphylla-on dry
rocks, Buxbaumia aphylla (moss), Cryptothallus mirabilis
(liverwort) are saprophytic.
17. The gametophyte is a stage in the life cycle that is
found in all plants and certain species of algae.
A multicellular haploid generation known as
Gametophyte.
This process includes both multicellular diploid
generation known as Sporophyte.
The word Diploid refers to two sets of chromosomes
in the cells, and normally written as ‘2n’.
Haploid to only one set of chromosomes in the cells
and written as ‘n’.
18. Gametophyte
1.Plant body
The bryophytes are a small group of most primitive land
dwellers . A few of them, however are strictly aquatic.
The plant body is undoubtedly more differentiated than that
of a complex alga.
It is compact and better protected against desiccation.
It grows prostrate on the ground and is thallus like .
It is attached to the substratum by delicate, unicellular hair
like organs called rhizoids.
19. The majority of the bryophytes however have the plant
differentiated into stem and leaves.
The leafy gametophyte of the liverworts is dorsiventral but in
mosses it is erect.
20. The erect, leafy moss gametophyte has a stem like central axis
which bears leaf like appendages.
It is fixed to substratum by means of branched multicellular
rhizoids apparently resembling the roots.
The rhizoids arise from the older, basal part of the stem.
The thallus like plant body of bryophytes bears the gametes ,
for this reason it is called the gametophytes plant .
It is concerned with sexual reproduction amd consitutes the
most conspicous nutritionally independent phase in the life
cycle.
21.
22.
23. Reproduction
Bryophytes show a marked advance over the
thallophytes in the method of sexual reproduction .
Without exception it is highly oogamous in the whole
group.
The gametes are produced in complex sex organs.
They have attained a degree of complexity far above
that of the thallophytes.
In the thallophytes the sex organs are formed directly
out of the protoplasts of the sex cells.
24. The bryophytes on the other hand have multicellular,
jacketed sex organs.
Each sex organs consists of outer, protective wall of
sterile cells surrounding the cell or group of cell which
produce the gametes.
The male sex organ is still called the antheridium.
the female is known as the archegonium.
Both kinds of sex organs may be developed on the same
individual (monoecious) or on distinct plants
(dioecious).
25. Antheridium
The antheridium is a
multicellular object
ellipsoidal or club-shaped
sometimes spherical in form.
It is borne on short stalk which
attaches it to the
gametophyte tissue .
Often it is embbeded in the
gametophyte tissue.
The body of the antheridium
has a wall of single layer of
sterile cells .
26. It is surrounds a mass of small squarish or cubical cells
called the androcytes.
The androcytes produce the biflagellate male gametes
called the sperms .
They are motile structure .
Each sperm usually consists of a minute slender,
spirally curved body furnished with two long
terminal, whiplash type flagella.
27.
28.
29. Archegonium
The female sex organ of the bryophytes is a remarkable
structure.
It is called the archegoium
It appears for the first time in the liverworts and
mosses and continues in the pteridophytes.
In this respect the liverworts and mosses seem to be
akin to the pteridophytes than to the algae to which
they resemble in their thallus like plant body.
30. The archegonium is a flask shaped
organ .
The slender, elongated upper portion is
called the neck and the lower sac-
like, swollen portion, the venter.
The venter is attached to and often
deeply embedded in the parent plant
tissue.
The neck has a wall of single layer of
sterile cells called the neck canal
cells.
31.
32.
33. The neck is usually projecting
or freely exposed so as to
be accessible to sperms.
The venter also has a wall of
sterile cells one or more cell
layers in thickness
The venter wall encloses two
cells.
they are the larger egg cell or
the ovum and the smaller
venter canal cell just above
it .
34. Fertilization
It occurs when the sex organs are mature.
Moisture is essential for the maturing of the sex organs and
also for the movements of the sperms to the archegonia.
The mature antheridium ruptures at its apex liberating the
sperms. At the same time the axial row of neck canal cells
including the ventral cell in the mature archegonium
disorganise.
The tip of the archegonium also opens.
the narrow canal opening to the exterior is formed .
It acts as a passage way to the ovum in the venter.
35. The liberated sperms swimming in a thin film of water reach
the archegonia.
They enter through the open neck and swim down the canals
of the archegonia.
Reaching the venter one of them, probably the first one to
reach there, penetrates the ovum.
It fuses with the nucleus of the ovum to accomplish
fertilization.
with the act of fertilization the gametophyte generation end
and the sporophyte generation starts.
The gametes are the last structures of the gametophyte
generation.
36. SPOROPHYTE
With fertilization starts the second phase in the life cycle of
the bryophytes.
It is called the sporophyte.
The pioneer structure of this phase is the zygote.
Zygote
The fertilized egg or ovum secretes a cellulose wall around it
and is called the zygote.
The zygote has a fusion or diploid nucleus which contains
chromatin material of both the male and female gametes.
The zygote marks the beginning of the sporophyte generation
in the life cycle.
37. It is neither independent of the parent gametophytes plant nor
passes into the resting condition.
The development of the zygote into the embryo occurs within
the venter of the archegonium which protects the egg, the
zygote and the embryo against the varagies of the external
environment.
B. Embryo
Within venter of the archegonium the zygote undegoes
segmentation and develops without a resting period into a
muticellular, undifferentiated structure called the embryo.
38. It obtains its nourishment directly from the thallus or the
parent gametophyte to which it is organically attached.
The zygote of the bryophyteshas thus a better chance for
growth as compared with the zygote of the thallophytes
which is always independent.
c. Sporogonium
The embryo by further segmentation and differentiation finally
develops into a full fleged sporophyte individual.
It is called the sporogonium.
The sporogonium in bryophyte is leafless and rootless.
Throughout its life it remains attached to the gametophyte
plant.
39. It is thus spatially separated from the soil by the gametophyte
tissues.
Generally it has a limited life span.
In some bryophytes it remains embedded in the gametophyte
tissue.
Generally it is projecting and consists of three parts
1. The foot
2. The seta
3. The capsule
The foot is embbeded in the tissue of the parent gametophyte.
It absorbs nutrition for the sporogonium.
40. The rest of the sporogonium is free and projecting to promote
easy dispersal of spores.
The seta conducts the food absorbed by the foot to the capsule.
The terminal capsule is where the development of the spores
take place which are non-motile and wind disseminated.
The spores of bryophytes are highly specialised cells.
They are differentiated from the diploid spore mother cells by
meiosis.
They are thus haploid structures and are known as the
meiospores or gonospores.
Morphologically the spores produced in the capsule in all the
bryophytes are homosporous.
