Why is the female gametophyte of selaginella considered as precursor of seed habit

Dear student,

Selaginella and Salvinia are heterosporous. They produce microspores and macrospores. These spores germinate and give rise to male and female gametophyte and retained in the sporophyte for variable periods. The development of zygotes into young embryos takes place within the female gametophytes. This event is the precursor to the seed habit. Hence these species are considered to represent a significant step towards evolution of seed habit.
 
Regards