These male gametes swim towards the female gametes, which are held in the archegonium on the female plants. Fertilisation takes place and the offspring grows upwards, forming the seta ('stem') and eventually the spore-bearing capsule. This sporophyte looks like it is part of the original plant, but is actually parasitic on it, so when you see a spore capsule forming like the one below, it isn't one plant with green leaves and a spore capsule, it's the mother (leaves) and child (spore-producing sporophyte).
|Emerging spore capsule on Polytrichum moss|
|Old capsules of Polytrichum commune|
The fact that the male gametes swim towards the females gives us one reason that mosses flourish in damp places.