(flowering plants) A class of vascular plants, all characteristically bearing seeds within enclosing car-pellary structures. The sporophyte is the dominant generation and is either herbaceous or woody, the woody habit being considered more primitive. The reproductive axis and its associated, often brightly coloured, sepals and petals, is called a flower. The gametophyte is reduced to the female embryo sac and the male pollen grain. The pollen does not germinate directly on the ovule, as in the *
Gymnospermae , but on a specialized extension of the carpel, the stigma. The male gametes, unlike certain gym-nosperm gametes, are never flagellate. Double fertilization to form a zygote and a diploid endosperm nucleus is characteristic.
Secondary vascular tissue is usually but not always present. The xylem contains vessels, except in certain primitive woody forms, and the phloem has distinct companion cells associated with the sieve tube elements. Angiosperms are the most advanced, most abundant, and most widely distributed vascular plants. The group contains some 250 000 species and is subdivided, on the basis of the number of cotyledons in the embryo, into the *
Monocotyledonae and the *
Dicotyledonae . Beyond these groups further subdivision into superorders and orders is based mainly on the structure of the flower and especially on the form, number, and arrangement of the stamens and carpels. Different classifications recognize various numbers of orders and the names and contents of these often differ widely between various authorities.
From fossil pollen evidence it would seem the angiosperms appeared at the beginning of the Cretaceous. They had replaced the gymnosperms as the dominant vegetation by the second half of the Cretaceous period. This may have been due in part to the relatively rapid life cycle of angiosperms in which seed set occurs days or weeks after flowering whereas in gymnosperms the period between pollination and seed set is at least a year. The radial symmetry of angio-sperm seeds suggests the group may have evolved from the Pteridospermales. However numerous groups, including the Coniferales, Gnetales, Bennettitales, and Cycadales, have been postulated as angiosperm ancestors. These have all been discounted for various reasons and the origin of the angiosperms remains obscure. See also
Magnoliophyta .
