Tracheid - English spelling: tracheid

A type of cell in the xylem of vascular plants that serves as a pathway for water, and is the main pathway in ferns and gymnosperms. As the cell matures, it becomes elongated and the cell wall thickens and lignifies due to the accumulation of cellulose and lignin, and as the cell matures, it loses protoplasm and becomes a dead cell. There are thin sections in the cell wall that consist only of the intercellular layer, and these are called pits. In tracheids, the cell wall at the edge of the pit becomes thick and protrudes into the pit, so they are specifically called marginal pits. The outline of a marginal pit is an elongated oval to a circle, and only circles are seen in conifers. This marginal pit is a pathway for water, and tracheids have two functions: mechanical support provided by the thick cell wall, and water transport. The tracheids of ferns and other plants are connected vertically, and bordered pits are gathered in the wall where each cell meets above and below, forming a state similar to the perforation plate of a vessel, which is specifically called a vessel-like tracheid. In some ferns, such as bracken, the intercellular layer in this part has disappeared, forming a perforation, making it a vessel rather than a tracheid. Thus, the difference between a tracheid and a vessel is whether there is an intercellular layer in the bordered pit part or whether it has disappeared and become a complete hole, and the latter allows water to flow more easily, so it can be said that the tracheid is a more primitive form of conductive tissue. Dicotyledonous plants also have tracheids, but the water flow is mainly carried out by the vessels, and the mechanical support function is performed by the xylem fibers, so the tracheid shows an intermediate form between the two. This is specifically called a fibrous tracheid.

[Mitsuo Suzuki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

維管束植物の木部にあって水分の通路となる細胞の一種で、シダ類と裸子植物の主要な通道要素である。細胞の成熟につれて細長くなり、細胞壁はセルロースやリグニンの堆積(たいせき)により厚く木化し、完成と同時に原形質を失って死細胞となる。細胞壁にはところどころ壁が細胞間層だけからなる薄い部分があり壁孔(へきこう)とよぶが、仮道管では壁孔の縁(へり)の細胞壁が厚くなり壁孔側にせり出しているのでとくに有縁壁孔という。有縁壁孔の輪郭は細長い楕円(だえん)から円形で、とくに針葉樹では円形のみがみられる。この有縁壁孔が水分の通路で、仮道管は厚い細胞壁による機械的支持機能と水分流通の二つの働きをもっている。シダ類などの仮道管は上下に連なっており、各細胞が上下に接する壁の部分に有縁壁孔が集まって道管の穿孔板(せんこうばん)に似た状態になり、これをとくに道管状仮道管とよんでいる。ワラビなど一部のシダ類ではこの部分の細胞間層が消失して穿孔となっており、仮道管ではなくて道管である。このように仮道管と道管の違いは、有縁壁孔の部分の細胞間層があるか、それが消失して完全な孔(あな)となっているかであり、水分の流通のしやすさは後者が勝っており、通道組織としては仮道管のほうが原始的形態といえる。また双子葉植物にも仮道管はあるが、水分の流通は道管がもっぱらにし、機械的支持機能は木部繊維が行い、仮道管はこの両者の中間的形態を示す。これをとくに繊維状仮道管とよぶ。

［鈴木三男］

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