The early Paleozoic Era was an exciting period in Earth’s history, marked by the colonization and diversification of terrestrial organisms, including the ancestral lineages of extant embryophytes. The bryophytes, which are now important components of virtually all terrestrial ecosystems, were among the earliest of land plants.
Traditionally, “bryophytes” include the mosses, liverworts, and hornworts. Together, these groups comprise some 15000–20000 species and as such, are, combined, more diverse than the nonflowering vascular plants. The three bryophyte groups share a similar life cycle in which the gametophyte is perennial and dominant in terms of both size and longevity and the sporophyte is unbranched, monosporangiate, and completes its entire development attached to the maternal gametophyte. The sporophyte generation begins as a fertilized egg and eventually produces spores via meiosis in a terminal sporangium, after which, especially in the mosses and liverworts, it ceases to be photosynthetic, dries, and senesces. The sporophytes of many moss species mature over somewhat more than a year but do not continue to grow after spore production and are for the most part essentially annual.
Liverwort sporophytes are especially short-lived and generally persist for weeks to months (from fertilization to senescence). The sporophytes of hornworts are potentially indeterminate in life span, but in nature they also appear to rarely persist for more than a year. Although some bryophytes produce spores that develop into genetically determined male or female gametophytes and are sometimes described as heterosporous, they are not heterosporous in the sense of many vascular plants because the male and female spores are produced by the same sporangium rather than in different micro- and megasporangia. Bryophyte species that form male and female spores that are differentiated in size are better described as anisosporous, but in most species with unisexual gametophytes the spores are indistinguishable.
Liverwort sporophytes are especially short-lived and generally persist for weeks to months (from fertilization to senescence). The sporophytes of hornworts are potentially indeterminate in life span, but in nature they also appear to rarely persist for more than a year. Although some bryophytes produce spores that develop into genetically determined male or female gametophytes and are sometimes described as heterosporous, they are not heterosporous in the sense of many vascular plants because the male and female spores are produced by the same sporangium rather than in different micro- and megasporangia. Bryophyte species that form male and female spores that are differentiated in size are better described as anisosporous, but in most species with unisexual gametophytes the spores are indistinguishable.
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