It is a temperate brown alga belonging to the group known as kelps. These plants form dense underwater forests near the shore in temperate regions. The sporophyte produces motile haploid spores which settle and grow into the next generation, the gametophyte. The gametophyte is a tiny microscopic plant. The sporophyte and gametophyte generations look quite different and for this reason Laminaria is said to show heteromorphic alternation of generations .
Reproduction is oogamous with sperm swimming towards the large, non-motile eggs.
If you prefer a tropical example of an alga with a heteromorphic alternation of generations, the siphonous green alga Derbesia is a good example. We now know they are 2 stages of one plant. The sporophyte produces multiflagellate spores by meiosis which settle to form the baloon-shaped gametophyte. Gametes are discharged from these which fuse to produce the filamentous sporophyte. Gametes are produced on special branches called receptacles.