In mosses, the plant you see is the half-chromosome generation
Every land plant alternates between two generations, one with a single set of chromosomes and one with two. Trees and ferns put the double-set phase in charge. Mosses, liverworts and hornworts do the reverse: the green cushion you notice is the single-set stage, and the spore stalk is its dependent offspring.
Bryophytes comprise about 20,000 species of small, non-flowering, seedless land plants. They lack true vascular tissue stiffened with lignin, although some mosses have specialised water-conducting cells. Because they do not rely on roots to draw nutrients from soil, they colonise bare rock and soil, and they turn up from Arctic cold to hot deserts and from sea level to alpine heights. The name joins Greek words for tree moss and plant.
Their life cycle is unusual among today's plants. The long-lived plant, the gametophyte, produces eggs and swimming sperm; the sperm must travel through water to reach an egg, sometimes on a different plant, and small arthropods can help carry them. The fertilised egg grows into a sporophyte that stays attached to its parent and depends on it for food. Each sporophyte is an unbranched stalk topped by a single capsule releasing spores, usually on the wind. The three groups even lengthen that stalk differently: mosses add cells from a zone just below the capsule, hornworts from the base, and liverworts simply by cells swelling.
Whether bryophytes form one natural group has flip-flopped. Wilhelm Schimper grouped all three in 1879. By 2010 most systematists thought they were not a single lineage. A 2014 study argued the earlier DNA analyses were skewed by composition bias and that protein sequences favoured one lineage after all, and most nuclear and chloroplast studies since agree, though mitochondrial data still dissent. Liverworts and mosses are now strongly supported as sister groups, called Setaphyta.
If the single-lineage view is right, the elaborate branching sporophytes of vascular plants evolved separately from the simple bryophyte version, and stomata arose only once before being lost by liverworts. Some botanists suggest the real dividing line is not plumbing but sporophyte shape: bryophytes carry one spore capsule, while all other land plants bear branched sporophytes with many.
Source: Bryophyte
