The heterosporous pteridophytes show certain characteristics,which are precursors to the seed habit in gymnosperms. Explain.

(N/A) $\rightarrow$ Heterospory: This refers to the production of two distinct types of spores: small microspores and large megaspores. These spores develop into male and female gametophytes,respectively.
$\rightarrow$ The female gametophyte is retained on the parent sporophyte for variable periods. The development of the zygote into young embryos takes place within the female gametophytes retained on the parent sporophyte.
$\rightarrow$ This event is a precursor to the seed habit,which is considered an important step in evolution. In heterosporous pteridophytes like $Selaginella$ and $Salvinia$,the retention and germination of the megaspore within the megasporangium and the subsequent development of the embryo are the key features that lead to the seed habit observed in gymnosperms.