Nilsonia - Nilsonia

These are the leaves of a plant thought to belong to the gymnosperm cycad family, which appeared in the Permian Period of the Paleozoic Era, flourished in the Mesozoic Era, and became extinct at the end of that period. The leaves are diverse, including linear, obovate, and spatula-shaped, and may be pinnate, divided into numerous lobes at almost right angles or wide angles to the central axis. The leaf blade is attached to the upper surface of the central axis and may cover the upper surface of the axis, and the veins emerge from the central axis at almost right angles and run parallel to the leaf margin without branching. Permian and Triassic leaves are divided into numerous lobes, but the number of lobes gradually decreases, and Cretaceous leaves tend to be simple leaves.

Although many details of the stem and reproductive organs are unknown, Beania , which is considered to be the female reproductive organ of cycads, and Andorostrobus , which is considered to be the male reproductive organ, are sometimes found together with Nilssonia. It is also known that the leaves of Nilssonia nipponensis , found in the Early Cretaceous of Ishikawa Prefecture, grow on short branches of a vine-like stem. In Japan, it is found in abundance in the Triassic of Yamaguchi Prefecture and Okayama Prefecture, the Jurassic of Yamaguchi Prefecture, Nagano Prefecture, and Fukushima Prefecture, and the Cretaceous of Wakayama Prefecture, Kochi Prefecture, Fukui Prefecture, Ishikawa Prefecture, and Hokkaido.

[Kazuo Asama and Harufumi Nishida]

"Evolutionary Biology Research Institute Library 4: The History of Gymnosperms - Unraveling the Memories of Gondwana," edited by Makoto Nishida and jointly planned by the Institute of Evolutionary Biology and the Agricultural Resource Center of Tokyo University of Agriculture (1999, Shinzansha)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

裸子植物ソテツ類に属するとみられる植物の葉で、古生代ペルム紀（二畳紀）に出現し、中生代に栄えてその末期に絶滅した。葉は線形、倒卵形、へら状など多様で、中軸にほぼ直角または広角をなす多数の裂片に分かれて羽状を示すことがある。葉身は中軸の上面につき、軸の上面を覆い隠すことがあり、葉脈は中軸からほぼ直角に出て、分岐することなく並行に葉縁に向かう。ペルム系、三畳系のものは多数の裂片に分かれているが、しだいに裂片の数が減少し、白亜系のものは単葉となる傾向が強い。

　茎や生殖器官の詳細は不明な点が多いが、ソテツ類の雌の生殖器官とされるベアニアBeania、雄の生殖器官とされるアンドロストロブスAndorostrobusがニルソニアと併産することがある。また、石川県の前期白亜系から産出するニルソニア・ニッポネンシスNilssonia nipponensisの葉は、つる状の茎に生ずる短枝上に生じたことがわかっている。日本では山口県、岡山県の三畳系、山口県、長野県、福島県のジュラ系、和歌山県、高知県、福井県、石川県、北海道などの白亜系から多産する。

［浅間一男・西田治文］

『西田誠編、進化生物学研究所・東京農業大学農業資料室共同企画『進化生研ライブラリー4　裸子植物のあゆみ――ゴンドワナの記憶をひもとく』（1999・信山社）』

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