Petals

Also called petals. The leaves that make up the corolla are usually larger and thinner than the sepals. They are colored by various pigments, but usually do not contain chlorophyll. Anatomically, they are simple, with bifurcated veins and most without a clear midvein. Some have nectar glands.

There are two ways in which petals are derived. The first is when the tepals differentiate into the sepals and petals, and many of these show a transitional form between the sepals and the sepals. This is seen in the Magnoliaceae family. The second is when the outer stamens stop producing pollen and become petals, and many of these show a transitional form between the stamens and the sepals. This is seen in the Ranunculaceae family. In double and semi-double horticultural plants, the stamens are petals. In rare cases, such as Eupomatia, which is classified as a Magnoliales, the inner stamens are petals. When the stamens stop producing pollen but do not become petal-like, but are transformed into small spherical, club-shaped, whip-shaped, or scale-shaped structures, they are called pseudo-stamens. Many pseudo-stamens become nectar-secreting organs.

[Michio Tamura]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

花びらともいう。花冠を構成している花葉(かよう)のことで、萼片(がくへん)に比べると普通は大きくて薄い。種々の色素を含んで色づくが、普通はクロロフィルを含まない。解剖的には簡単で、脈は二叉(にさ)分岐をし、明らかな中脈はもたないものが多い。蜜腺(みつせん)をもつものもある。

　花弁の由来には二つの場合がある。第一は花被片(かひへん)が萼片(がくへん)と花弁に分化した場合で、萼片との間に移行型を示すものが多い。モクレン科などにみられる。第二は外部の雄しべが花粉形成をやめて弁化したもので、雄しべとの間に移行型を示すものが多い。キンポウゲ科などにみられる。園芸植物の八重咲きや半八重咲きでは雄しべが弁化している。まれにモクレン目に分類されるエウポマチアEupomatiaのように、内部の雄しべが弁化するものもある。雄しべが花粉形成をやめても花弁状にならず、小形の球形、棍棒(こんぼう)状、鞭(むち)状、鱗片(りんぺん)状などの構造に変形したものを仮雄蕊(かゆうずい)という。仮雄蕊は蜜分泌の器官となるものが多い。

［田村道夫］

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