Magnoliophyta

(botany) The angiosperms, a division of vascular seed plants having the ovules enclosed in an ovary and well-developed vessels in the xylem.

A division of seed plants consisting of about 250,000 species, which form the bulk and most conspicuous element of the land plants. Often called flowering plants or angiosperms, they have several unique characteristics, the most prominent of which are their reproductive structure, flowers, and covered seeds. The other obvious woody land plants are the gymnosperms, which have cones instead of flowers and have naked seeds. Another trait distinguishing the angiosperms is the presence of double fertilization, which results in the production of stored food (starch or oils) within their seeds. See also Flower.

Angiosperms range from some of the smallest plants known to large forest trees, and they occur in all habitats, including the oceans, where they are only a minor element in most marine ecosystems. Some are capable of growing directly on rock surfaces as well as on the limbs of trees. The angiosperms are usually considered to be the most highly evolved division of the subkingdom Embryobionta. Their highly specialized and relatively efficient conducting tissues, combined with the protection of their ovules in an ovary, give them a competitive advantage over most other groups of land plants in most regions. See also Embryobionta.

The angiosperms may be characterized as vascular plants with roots, stems, and leaves, usually with well-developed vessels in the xylem and with companion cells in the phloem. The central cylinder has leaf gaps or scattered vascular bundles; the ovules are enclosed in an ovary; and the female gametophyte is reduced to a few-nucleate embryo sac without an archegonium. The male gametophyte is reduced to a tiny pollen grain that gives rise to a pollen tube containing a tube nucleus and two sperms; one sperm fuses with the egg in the embryo sac to form a zygote, and the other fuses with two nuclei of the embryo sac to form a triple fusion nucleus that is typically the forerunner of the endosperm of the seed. See also Leaf; Phloem; Pollen; Reproduction (plant); Root (botany); Seed; Stem; Xylem.

Among plants with alternation of sporophyte and gametophyte generations, the angiosperms represent the most extreme stage in reduction of the gametophyte, which in effect is reduced to a mere stage in the reproduction of the sporophyte. The pollen grain, with its associated pollen tube, and the embryo sac represent the male and female gametophyte generations; the endosperm is a new structure not referable to either generation; and the remainder of the plant throughout its life cycle is the sporophyte. Many angiosperms can also propagate asexually by means of creeping stems or roots or by other specialized vegetative structures such as bulbils.

It is obvious to biologists that the angiosperms must have evolved from gymnosperms, but beyond this the facts are obscure. They appear in the fossilrecord early in the Cretaceous Period as obvious angiosperms, without any hintof a connection to any particular group of gymnosperms. Many believe that amongthe gymnosperms the seed ferns provide the most likely ancestors. See also Paleobotany; Pinophyta.

The Magnoliophyta consist of two large groups that have not been formallynamed: the eudicots and the magnoliids. The eudicots are characterized by flowers that are highlyorganized in terms of the number and orientation of parts whereas themagnoliids have many parts without any particular fixed patterns among theparts—except for the monocots, in which the most developed groups, likethe eudicots, exhibit developed flowers with highly organized patterns. See also Liliopsida; Magnoliopsida.