HETEROSPORY AND SEED HABITAT IN PTERIDOPHYTES WITH RESPECT TO SELAGINELLA

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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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Comments

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