HETEROSPORY AND SEED HABITAT IN PTERIDOPHYTES WITH RESPECT TO SELAGINELLA
Heterospory is the condition that interprets the
production of spores of two different sizes and two different development
patterns. The two different sizes are smaller
spores also known as microspores
and the larger spores also known as megaspores. Hetrrospory can be observed
in some of the pteridophytes such as Selaginella,
Marsilea, etc.
They have differential developmental patterns because the
microspores germinate to produce male
gametophytes or microgametophytes that bear male sex organs called antheridia,
and the megaspores germinate to produce female
gametophytes or megagametophytes that bear female sex organs called archegonia.
The two kinds of spores are produced in two kinds of
sporangia. The microspores are produced in microsporangia
and the megaspores in megasporangia.
The microspores are produced in large numbers and are comparatively smaller
than megasporengia which are produced in lesser numbers and larger in size.
Importance of Heterospory
(a) The most important aspects of heterospory is that it
is an expression of sex determining process of the plant. It has brought about
along with its onset, the sex determining capacity from the gametophyte to
sporophyte. In all the homosporous individuals, sex can be determined in their
gametophytic phase, during the formation of antheridia and archegonia. But in
the heterosporous individuals sex can be determined in their sporophytic phase
during sporogenesis i.e., during the formation of microspores and megaspores.
(b) Heterospory is the most important evolutionary
development in pteridophytes because it has ultimately led to seed
developments. It is rather a pre-requisite to seed habit. Heterospory has
brought about a number of changes in the characteristic of spore development
which is the pioneer characters of seed habits in higher plants.
Heterospory in Selaginella
Selaginella, no doubt,
illustrates an example of heterosporous pteridophyte that approach seed habit
because of the following notable characteristics –
(a) It is heterosporous.
(b) The megaspore starts germination within the
megasporangia and their time of release from the megasporangia varies with
species.
(c) The number of megaspores in S.rupestris and S. monospora
is reduced to one.
(d) In S. rupestris
the megaspore is never shed and fertilization and development of the embryo
takes place while the megaspore is still within the megasporangium, which
retains its connection with the parent plant. This condition can be linked with
the vivipary in some angiosperms.
Considering the above points we can reach to the
conclusion that had reached to the level of seed habit but fail to develop
seeds because of the following shortcomings –
(a) They have no protective structures like the
integuments surrounding their megasporangia.
(b) The permanent retention of megaspores within the
megasporangia is not established.
(c) Histological union between the megaspore and the
megasporangium is absent.
(d)
Lack of resting period after the development of embryo.
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