See the moss cycle in fig. Dispersal phase is the spore, Dominant phase is the gametophyte.
Dispersal is a mechanism where by offspring mature at a location away from the parental plant.
Notice the location of the sporangia on the sporophyte. Phloem for seasonal two way flow of water from leaves to roots in summer and from roots to leaves in spring. Presence of lignin a substance which re-enforces cell walls for better support. Once plants became abundant, the competition for light began as light became a limited resource. Competition increases the strength of natural selection.