HETEROSPORY AND SEED HABITAT IN PTERIDOPHYTES WITH RESPECT TO SELAGINELLA

Hetrospory is the condition that interprets the production of spore of two different sizes and two different development pattern. The two different size are smaller spores also known as microspores and the larger spores is also known as megaspores. Hetrospory can be observed in some of the pteridophytes such as Selaginella, marsilia etc.

They have differential developmental patterns because the microscope germinate to produce male gametoohyte or microgametophyte that bear male sex organs called antheridia, and the megaspores germinate to produce female gametophyte or megagametoohyte that bear female sex organs called archegonia.

The two kinds of spore are produced in two kinds of sporangia. The microscope are produced in microsporangia and the megaspores are produced in the megasporangia. The microspore are produced in large numbers and are comparatively smaller than megasporangia which are produced leeser number and larger in size.

IMPORTANCE OF HETROSPORY 

1. The most important aspects of hetrospory is that it is an expression of  sex determining process of plant. It has brought about along with it's onset, the sex determining capacity from the gametophyte to sporoohyte. In all the homosporous individual, sex can be determined in their gametophytic phase, during the formation of antheridia and archegonia. But in the hetrosporous individual sex can be determined in their sporophytic phase during sporogenesis i.e., during the formation of microspore and megaspores.
2. Hetrospory is the most important evolutionary development in pteridophytes because it has ultimately led to seed development. It is rather a pre-requisite to seed habit. Hetrospory is brought about number of changes in the characterstics of spore development which is the pioneer character of seed habit in higher plants.

Hetrospory in Selaginella

Selaginella, no doubts, illustates an example of hetrosporous pteridophytes that approach seed habit because of the following notable characterstics. 

1. It is hetrosporous
2. The megaspores start germination with in the megasporangia and their time of release from the megasporangia varies with species.
3. The number of megaspores in S.rupestris and S.monospora and reduced to one.
4. In S.ruoestris the megaspores is never shed and fertilization and development of the embryo take place while the megaspores still with in the megasporangium, which retains it's connection with 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 seed because of the following shortcoming.

1. They have no protective structure like the integuments surrounding their megasporangia. 
2. The permanent retention of megaspores within the megasporangia is not established.
3. Histological union between the megaspores and the megasporangium is absent.
4. Lack of resting period after the development of embryo.