Division Bryophyta
The bryophyta are poorly adapted to live on land and are mainly confined to damp shady places (Fig. 9.1)
Fig. 9.1 A moss bug, lacking rigid supporting tissue, bryophytes are low-profile plants they are most common in damp habitats.
These plants are devoid of specialized conducting (xylem and phloem)
and strengthening tissues. only the process of diffusion and osmosis
helps in the transportation of water and minerals as well as in
transportation of prepared food and other substances. The plant body is
with a proper cuticle, or has 2 very thin one. The water is absorbed by
the general surface of the plant. The bryophyta are said to be the amphibians of the plant world because they cannot live away from water. They need water for reproduction (Fig. 9.2).
The bryophytes are non-vascular flowerless plants. These plants show a regular alternation of heteromorphy (morphologically different) generation. They have a dominant independent free living gametophyte.
This may be tabloid as in many liverworts or is differentiated into
structures resembling to stem, leaves and absorbing and anchoring
organs, rhizoids, as in mosses and some liverworts. The gametophyte produces a sprophyte,
which is a less conspicuous generation, partially or totally dependent
upon the gametophyte for its nutrition. The sporophyte generally
consists of foot, seta and capsule.
The sporophyte is diploid (2n) which produces in sporangia one kind of haploid spores (i.e. it is homosporous)
by meiosis. The spores germinate and give rise to gametophyte which is
also haploid. Multicellular male and female exe organs i.e. antheridia and archegonia
respectively, are born on gametophyte either on same or different
plants. These sex organs are multicellular and protected by a sterile
covering of cells (Fig. 9.3).
Gametes are produced by mitosis. Male gameters produced within antheridia are called antherozoids; antherozoids are motile and always produced in large number . female gametes formed within archegonia are termed as eggs.
A single egg is formed in each archegonium. Fertilization takes place
in water. Antherozoids (n) are attracted towards archegonia (n)
chemotactically. A single antherozoid fuses with an egg(n) thus
accomplishing fertilization which results in the formation of the
diploid zygote (2n). the zygote is retained within the female sex organ
(archegonium) for some time. After a resting period the zygote develops
through mitotic divisions into a diploid embryo. The embryo ultimately develops into a sporophyte which is also diloid.
The entire development of sporophyte thus takes place within the
gametophyte plant body. Even which the sporophyte is fully developed it
remains attaché d to the gametophyte for nourishment and protection
because it does not contain chloroplasts and is unable to perform
photosynthesis. There is an alternation of generations
in the life cycle of bryophytes i.e. multicellular haploid gametophytic
(gamete producing) generation (Fig. 9.6). it is a very important
phenomenon, which provides continuous genetic variabilities and
selection for the best genetic make up for survival and adaptation in
the changing environment (s)(as explained in a later section).
An view of the above mentioned discussion, bryophyta can therefore be
defined more precisely as plants with the distinguishing characters as
follows:“Vascular system absent; gametophyte dominant; saprophyte attached of gametophyte; homosporous.”
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