Classification Part 2
This
group of bryophytes differs in many respects and is slightly advanced
than bryopsida and Hepaticopsida. The gametophyte is highly lobed and
irregular in outline.except for a little early stage of development, the
sporophyte is not dependent upon gametophyte for nourishment and
protection. Antheridia and archegonia are partially sunken in the
gametophytic tissue. The sporophyte exhibit many advanced characters due
to which it can thrive better on alnd as compared to other groups. The
sporophyte has stomata and chloroplasts in the epidermis and can thus
photosynthesize its own food rather than obtaining it from ghametophyte.
It also has a waxy cuticle to check excessive loss of water
(desiccation). Furthermore. It also has a waxy cuticle to check
excessive loss of water (desiccation). Furthermore, at the junction of
foot and spore producing region there is a band of meristematic tissue.
This tissue keeps on adding cells towards the spore-producing region
during the formation, maturation and dispersal of spores from the
opposite end. Due to the fast growth rate of this meristematic
tissue the sporophyte keeps on increasing in length for an indefinite
period of time. Due to these chareacters the sporophyte continues to
survive as such even after the death and decay of the gametophyte. One
good example of anthoceropsida is anthoceros which is also found in the
hilly areas of Pakistan (Fig. 9.9).
(a)
|
(b)
|
Fig. 9.9 Anthoceros, a hornwort (a) Gametophyte with attached horn-shaped sporophyte (b) V.S. of sporophyte.
In
the life history of liverworts, mosses and hornworts there are two
distinct multicellular phases or generations. These generations are
haploid gametophyte and diploid sporophyte,
which regularly alternate with each other. The gametophyte is the
dominant generation because it is more conspicuous. It produces gametes
called spermatozoids or antherozoids and eggs, therefore called gamete-producign generation. A haploid spermatozoid fuses with a haploid egg to produce diploid oospore.
The oospore does not produce the gametophyte directly but produces a totally different plant called sporophyte. The sporophyte in bryophytes is a less conspicuous generation, which is usually differentiated into foot, seta and capsule (also called sporogonium).
Spores develop within the capsule by reduction division (meiosis) from
spore mother cells. The sporophyte produces spores and is, therefore,
called spore producing generation. The spore on germination does not
develop into a sporophyte but gives rise to the gametophyte. Thus in the
life-history of a bryophytic plant, the two generations, the
gametophyte and the sporophyte, regularly alternate with each other. The
phenomenon of alternation of gametophyte and sporophyte in the life
history of a plant is called alternation of generations (Fig. 9.10).
It
should be noted that the gametophyte or haploid stage begins with
spores and ends at gameters, whereas the sporophyte begins with oospore
and ends at spore mother cell.
The significance of alternation of generations
During
the formation of spores from spore mother cells by meiotic division
reshuffling of genes occurs. As a consequence, a great variety of spores
with different genetic make-up are produced. These spores in turn
produce gametophytes with different genetic combinations. The
gametophytes with better genetic make-up will have a better chance for
survival in the environment where they occur. On the other hand, the
gametophytes with less advantageous characteristics will be eliminated.
There is no reshuffling of genes during gametogenesis in the gametophyte
as gametes are produced after mitosis.
The
oospore developing after fertilization now has a new genetic make-up as
compared to the parent. This genetic variation passes to the new
sporophyte which on maturity once again produces further genetic
recombination which are transferred to the gametophyte. In this natural
process the sporophyte thus rovide a large amount of genetic variability
and nature selects the best genetic combinations. In the long run, this
will allow the populations to become increasingly better adapted to
their environment.
0 Comments:
Post a Comment
Subscribe to Post Comments [Atom]
<< Home