Annual plants - Annual plants

A herbaceous plant that completes its life cycle from seed to seed within one year. In deserts, etc., some plants germinate immediately after rainfall and flower and fruit in about 10 days before dying, but usually they have a growth period of one to eight months. There are three types of annuals: summer type, winter type, and ephemeral (short day) type, but all of them are monadulterous and flower and fruit only once before dying. They are often found in places that are subject to unsuitable natural conditions or frequent human disturbance, such as deserts, cultivated fields, and roadsides, but they are not particularly adapted to dryness, disturbance, or trampling, and overcome unsuitable growth conditions by shortening their life cycle. In other words, the individual has a high degree of plasticity (the ability to change shape due to external pressure), and can complete seed production in a very short period of time regardless of the size of the plant or the length of the growth period. It is considered to be the most advanced stage of plant life.

[Masahiko Osawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

種子から種子までの生活を、1年以内で終える草本植物をいう。砂漠などでは、降雨直後に発芽し、10日ほどで開花結実し枯死する植物もあるが、普通は1か月から8か月の生育期間をもつ。夏型、冬型、エフェメラル（短日）型の各一年生が区別されるが、いずれも1回繁殖型で、1回だけの開花結実で枯死する。生育地は砂漠、耕地、路傍など、不適な自然条件や頻繁な人為的攪乱(かくらん)などを受ける所に多いが、乾燥、攪乱、踏みつけなどに特別に適応しているわけではなく、不適な生育条件を生活環を短く終えることで克服している。すなわち、個体の可塑性（外圧で形が変わる性質）は大きく、植物体の大きさや生育期間の長短などと無関係に、ごく短期間で種子生産を完了できる。植物の生活型としては、もっとも進んだ段階にあると考えられる。

［大澤雅彦］

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