This is a plate from Phycologia Barbadensis published in 1908. It is a collection of paintings of Barbadian seaweeds made in the 1890's by a French visitor Anna Vickers. Seaweed morphology has even been classified from an ecological, functional perspective relative to herbivory, wave action etc. In the sea, which is such a stable environment, spores are a means of dispersal not a resting stage. What this means is that there is more than one free-living stage of the organism.
Most plants have two recognisable phases - the sporophyte and the gametophyte .
Let's look at some examples of this. Ulva lactuca - Sea Lettuce Ulva has a membranous thallus and attaches to rocks by a holdfast. The sporophyte produces motile haploid spores which settle and grow into the next generation, the gametophyte. This produces anisogamete s which fuse to form a zygote which grows into the sporophyte generation. The sporophyte and gametophyte generations look exactly alike.