Angiosperms - angiospermes

A classification of seed plants whose seeds are covered by protective organs, as opposed to gymnosperms. They are trees or herbs, with the xylem of the vascular bundles in the stem containing vessel elements, and companion cells found in the phloem elements. Leaves can be simple, compound, or have a variety of forms depending on the species. Flowers are unisexual or bisexual, with the sepals, petals, stamens (male flowers), and pistils (female petals) arranged from bottom to top. Perianths come in a variety of shapes, including those in which the sepals and petals are not distinct (Magnoliaceae), and those in which the sepals are conspicuous like petals and there are no petals at all (Ranunculaceae). The stamens differentiate through meiosis into anthers, which produce pollen, and filaments, which act as stalks. The pistil, also called the carpel, is an organ homologous to the leaf and consists of three parts: the stigma, which pollinates, the ovary, which produces the ovule, and the style located between them. The ovule consists of the nucellus, which is surrounded by one or two integuments, and the embryo sac, which is derived from embryo sac cells that are produced by meiosis of embryo sac mother cells that differentiate from the nucellar tissue. The embryo sac cells divide three more times, usually to become an embryo sac with eight nuclei and seven cells.

Angiosperms have a unique form of double fertilization. When pollen is fertilized by wind or insects at the stigma of the pistil, the pollen tube extends to the ovule and produces one pollen tube nucleus and two reproductive nuclei. One of the two reproductive nuclei fertilizes an egg cell to form an embryo, and the other combines with the two polar nuclei to form endosperm to supply the nutrients necessary for the embryo to grow. This is double fertilization. In many plants, such as persimmon and rice, which have well-developed endosperms, the cotyledons that develop inside the seeds go dormant for a while and then grow out and emerge above ground. However, in the case of plants of the Fabaceae and Fagaceae families, which have endospermless seeds, the cotyledons that develop well inside the seeds become the source of nutrients necessary for germination instead of the lost endosperm, so it is not the cotyledons but the common leaves that grow out above ground first. Angiosperms are a taxonomic group that flourished remarkably in the Cenozoic era, and are broadly divided into dicotyledons and monocotyledons, with an estimated 220,000 to 300,000 extant species.

[Akiko Sugiyama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

種子植物のなかで種子が保護器官に覆われるものをいい、裸子植物に対する分類群である。樹木または草本で、茎の維管束の木部は道管要素をもち、篩部(しぶ)要素には伴細胞がみられる。葉は単葉、複葉など、種属によってさまざまな形態を示す。花は単性花または両性花で、下から萼片(がくへん)、花弁、雄蕊(ゆうずい)（雄しべ）、雌蕊（雌しべ）の順に配列する。花被(かひ)には、萼片と花弁が区別されないもの（モクレン科）、萼片が花弁のように目だち、花弁のないもの（キンポウゲ科）など、さまざまな形がある。雄蕊は、減数分裂によって花粉を生じる葯(やく)と、柄(え)としての花糸に分化する。雌蕊は心皮(しんぴ)ともいい、葉と相同の器官で、受粉を行う柱頭、胚珠(はいしゅ)を生じる子房、これらの間に位置する花柱の三部から構成される。胚珠は1、2枚の珠皮に包まれた珠心と胚嚢(はいのう)からなるが、胚嚢は珠心組織から分化した胚嚢母細胞の減数分裂によって生じた胚嚢細胞に由来する。胚嚢細胞はさらに3回分裂を行い、普通、八核、七細胞をもつ胚嚢となる。

　被子植物は特有の受精形式である重複受精を行う。花粉が風や昆虫などによって雌蕊の柱頭で受粉すると、花粉管を胚珠へ伸ばしながら1個の花粉管核、2個の生殖核を生じる。2個の生殖核のうち1個は卵細胞と受精して胚をつくり、他は極核2個と合体して胚の成長に必要な養分供給を行うための胚乳をつくる。これが重複受精である。胚乳のよく発達したカキやイネなど多くの植物では、種子の中で発達した子葉は一時休眠したのち、そのまま伸びて地上に現れる。しかし、無胚乳種子であるマメ科やブナ科の場合には、消失した胚乳のかわりに、種子の中でよく発達した子葉が発芽に必要な養分の供給源となるため、最初に地上に伸びてくるのは子葉ではなく、普通葉である。被子植物は新生代に著しく繁栄した分類群であり、双子葉植物と単子葉植物に大別され、現存種は約22万から30万種とされる。

［杉山明子］

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