Gymnosperms represent a broad range of seed-bearing plants that differ from flowering plants in not having seeds enclosed within carpels. Gymnosperms are probably monophyletic, having arisen among ‘progymnosperm’ stock in the Devonian. Key adaptations involving retention of the megagametophyte within a protective coating to form a seed on the parent sporophyte, dissemination of the microgametophyte in durable pollen, production of complex root systems, and extensive development of secondary xylem in the stem, provided gymnosperms with opportunities to exploit upland and drier habitats in the late Paleozoic that had not been occupied by earlier free-sporing plant groups. Gymnosperms diversified in the late Paleozoic and were prime contributors of organic matter to some of the world's largest coal deposits, which accumulated at that time. After suffering widespread extinctions at the close of the Permian, gymnosperms re-radiated in the Triassic and dominated the global floras until the mid-Cretaceous, after which they were progressively supplanted by angiosperms. Gymnosperms are represented in the modern flora by over 1000 species, but their diversity and distribution have contracted greatly since the Mesozoic. They still dominate large tracts of the Northern Hemisphere boreal forests, and are locally dominant in more restricted vegetation types in other parts of the world. Extant gymnosperms provide a range of food, industrial and medical resources to modern society.