Gymnosperms - Gymnosperms

This refers to seed plants that do not have protective organs in their seeds, and is a taxonomic group that is distinct from angiosperms. All are trees, with xylem consisting of tracheids and no companion cells in the phloem elements. Leaves vary from large compound leaves to small needle-like leaves and scale leaves. Flowers are unisexual, with pollen sacs and ovules forming male and female flowers, respectively. They usually lack perianths. In cone-bearing pinecone species, the ovules are covered by hard perianths that look like perianths, but these perianths are thought to be organs homologous to branches and are therefore not considered to be the same as the perianths of angiosperms, which are homologous to leaves. The ovules are not enclosed by carpels but are exposed, and usually consist of a nucellus enclosed in a single integument. The embryo sac mother cells differentiated in the nucellus produce embryo sac cells by meiosis, which then undergo repeated somatic cell division to become a multicellular embryo sac that stores a large amount of starch. This corresponds to the endosperm of angiosperms, and its formation before fertilization is a phenomenon unique to gymnosperms. Two to several archegonia are produced from the endosperm tissue close to the micropyle (a small hole at the tip of the ovule). When pollen is fertilized by the wind, it is taken into the pollen chamber between the integument and the nucellus inside the ovule, where it remains for several months until fertilization, producing sperm and sperm cells. Fertilization occurs when endosperm formation is complete inside the ovule. The number of cotyledons produced after germination varies, with ginkgoes having two and pinecones having six to twelve. Gymnosperms flourished in the Mesozoic era and many fossils remain. They are broadly divided into three groups: cycads, which have large compound leaves; conifers, which have cones; and ephedra, which have two integuments; and there are approximately 800 extant species.

[Akiko Sugiyama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

種子植物のなかで種子に保護器官のないものをいい、被子植物に対する分類群である。すべて樹木であり、木部は仮道管からなり、師部(しぶ)要素には伴細胞がみられない。葉は大形の複葉から小形の針状葉、鱗片(りんぺん)葉などさまざまである。花は単性花で、花粉嚢(のう)や胚珠(はいしゅ)がそれぞれ集合して雄花や雌花をつくる。普通、花被(かひ)はない。球果をつける松柏類(しょうはくるい)では、胚珠が堅い包鱗に覆われて花被のようにみえるが、この包鱗は枝に相同な器官と考えられるため、葉と相同である被子植物の花被と同じとはみなさない。胚珠は心皮に包まれないで裸出し、普通、1枚の珠皮に包まれた珠心からなる。珠心で分化した胚嚢母細胞が減数分裂によって胚嚢細胞をつくり、さらに体細胞分裂を繰り返して、多量のデンプンを蓄えた多細胞の胚嚢となる。これは被子植物の胚乳に相当する部分で、受精前に形成されるのが裸子植物に特有な現象である。珠孔（胚珠の先端にある小さな穴）に近い胚乳組織からは2～数個の造卵器がつくられる。花粉は風によって受粉すると、胚珠内の珠皮と珠心のすきまにある花粉室へ取り込まれ、受精するまでの数か月間はここにとどまり、精子や精細胞をつくる。胚珠内で胚乳形成が完了すると受精が行われる。発芽後生じる子葉の数はさまざまで、イチョウは2個、松柏類は6～12個である。裸子植物は中生代に繁栄した植物で化石が多い。大形の複葉をもつソテツ類、球果をつける松柏類、珠皮が2枚のマオウ類の3群に大別され、現存種は約800種である。

［杉山明子］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例