What Is The Role Of Heterospory And Megaspore In The Evolution Of Seed?

The major steps associated with the evolution of seed habit are as follows:
The evolution of heterospory.

Primitive vascular land plants produced spores which were all morphologically and structurally alike, a condition called homospore. All groups of land plants up to Pteridophytes (lower vascular plants such as club mosses, horsetails) are homosporous.During the early phase of evolution some plant groups started producing morphologically, structurally and functionally two different types of spores, the smaller ones called microspores and the larger ones known as megaspores.

The microspores produced inside Microsporangia germinated to form male gametophyte or the micro gametophyte, whereas the megaspores germinated to form female gametophyte or mega gametophyte.

Retention and Germination of Megaspore with in the Megasporangium
During the usual reproductive cycle in the heterosporous vascular land plants, the megaspores used to be shed and dispersed soon after their formation in order to germinate into female gametophyte. However in some plants (e.g., Selaginella) the megaspore was not allowed to escape from megasporangium after its formation. Its release from megasporangium was either delayed or the megaspore(s) was/were permanently retained within the megasporangium. Here, within the confines of the megasporangium wall, the megaspore germinated to form egg containing female gametophytes.

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