Cork - Cork (English spelling)

It is the outermost protective tissue of the trunk, branches, and roots of plants that are growing larger, and is also formed in parts of the plant that have been injured or where leaves have fallen. The secondary phloem, which is formed by cell division outward from the cambium, is gradually pushed outward and loses its function as a conductive tissue. In this area, a new cork cambium, which has the ability to divide again, is formed. This forms cork tissue on the outside and the cork cortex on the inside through cell division, becoming the periderm. The cells of cork tissue are regularly arranged and are hollow dead cells that lack protoplasm. The cell walls are made of a thick layer of a polymer of fatty acids called suberin, which makes them difficult for water and air to pass through. The accumulation of suberin on the cell walls is called corkification. When cork tissue is formed, the secondary phloem tissue outside it no longer receives a supply of water and dies.

Cork oaks, which belong to the Fagaceae family and are found in Spain and southern Europe, have the property of accumulating large amounts of this cork tissue, which is commonly known as cork and is used in its various forms. Cork is lightweight and has excellent thermal and sound insulation properties, electrical insulation, elasticity, and chemical resistance, and is still used in many ways today, even though a variety of plastic products have been developed. The Japanese variety Quercus serrata, which is in the same genus, also has a thick cork layer, but the quality is far inferior. Cork was also used by the 17th century British scientist Robert Hooke, who discovered that the bodies of living organisms are made up of cells, and he observed dead cells using this material.

[Mitsuo Suzuki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

肥大成長をする植物の幹、枝、根のいちばん外側にある保護組織で、植物体が傷を受けた部分や葉の落ちた跡にも形成される。形成層から外側に細胞分裂してできた二次篩部(しぶ)は順次外側に押し出されてゆき、通道組織としての機能を失ってゆく。この部分にはふたたび細胞分裂の能力をもったコルク形成層が新たにできる。これは、その外方にコルク組織を、内方にコルク皮層を細胞分裂によって形成し、周皮となる。コルク組織の細胞は規則正しい配列を示し、原形質を欠いた中空の死細胞で、細胞壁はスベリンとよばれる脂肪酸の重合体の厚い層からなり、水や空気を通しにくい。また細胞壁にスベリンが堆積(たいせき)することをコルク化という。コルク組織がつくられると、それより外方にある二次篩部の組織は水分などの供給が受けられなくなり死んでゆく。

　スペインや南ヨーロッパに分布するブナ科のコルクガシなどではこのコルク組織が多量に蓄積する性質があり、これは一般にコルクとよばれ利用される。コルクは断熱、防音、電気的絶縁性、弾力性、耐薬品性に優れ、軽く、さまざまなプラスチック製品が開発された現在でも多方面に利用される。日本産のものでは同じ属のアベマキもコルク層が厚くなるが、品質ははるかに劣る。また、生物の体が細胞からできていることを発見した17世紀、イギリスの科学者ロバート・フックが用いた材料がコルクで、彼は死んだ細胞を観察したことになる。

［鈴木三男］

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