A class of the * Tracheophyta or * Spermatophyta containing about 700 species of chiefly arborescent seed-bearing plants. These are divided into some 67 genera and 12 families. Gymnosperms differ from the angiosperms in having naked seeds with no enclosing carpellary structure. Double fertilization does not occur and thus no true endosperm is developed. The ga-metophyte generation is not as reduced as in angiosperms, but neither is it autotrophic. The female gametophyte typically consists of at least 500 cells and distinct archegonia are produced in all genera except Welwitschia and Gnetum. Secondary vascular tissue develops in all members and consists of tracheids and sieve cells. Companion cells are lacking and vessels are absent except in Gnetum. The class contains the extinct orders * Pteridospermales , * Caytoniales , * Bennettitales , * Cordaitales , and * Pentoxylales and the extant orders * Cycadales , * Ginkgoales , * Coniferales , * Taxales , and * Gnetales. Gymnosperms have a particularly rich fossil record and were abundant in Carboniferous times, contributing largely to the formation of coal deposits. They originated during the Devonian period and were the dominant vegetation during the Jurassic and early Cretaceous. Towards the end of the Cretaceous their dominant position was taken over by the angiosperms. See also Pinophyta. |
