Short answers, pulled from the story.
A sporophyte is a diploid multicellular organism that produces asexual spores during the life cycle of plants and algae. This stage alternates with a multicellular haploid gametophyte phase to complete the reproductive cycle.
Several plant groups independently evolved heterospory during the Devonian period. These developments led to endospory where gametophytes grow inside spore walls as seen in seed ferns.
Meiosis reduces chromosome numbers by half during spore production through reduction division. This process provides direct DNA repair capabilities for dealing with oxidative damage in germline reproductive tissues.
Bryophytes like mosses maintain a dominant gametophyte phase on which the adult sporophyte depends for nutrition. Most algae also maintain dominant gametophyte generations though some species show morphologically similar stages.
A zygote forms when a haploid egg cell meets a haploid sperm to restart the life cycle. The resulting zygote develops into the next generation while retained within the pre-ovule in seed ferns.