Pipe - Dokan

A type of tissue that controls the passage of water in the xylem of plants, and is found mainly in angiosperms. The individual cells that make up the vessels are called vessel elements, and are long cylindrical or polygonal columns along the long axis of the plant body. The diameter and length of vessel elements vary, with thick ones being short and thin ones being long. Thick ones can be as large as 500 micrometers in diameter, while thin ones are only a few tens of micrometers. A vessel is a long, thin tube formed by connecting many of these vessel elements vertically. At the joints between the upper and lower elements, the cell wall disappears, like when a bamboo node is removed, and a perforation is formed. There are various shapes of perforations, including a single large hole called a simple perforation, a ladder-shaped or mesh-shaped perforation in which the cell wall remains and is divided into many holes called a step perforation, and a mesh-shaped perforation.

The first xylem vessels are formed in the elongating parts of stems and roots near their growth points. These protoxylem vessels have thin side walls with circular or spiral thickenings on the inside, and are called ring-patterned or spiral-patterned vessels. Later, when elongation stops, the entire side walls thicken, forming metaxylem or secondary xylem vessels that also function as mechanical support. These have bordered pits on the side walls, and based on their shape and arrangement, they are classified as ladder-like stair-patterned vessels, net-like reticulated vessels, and pitted vessels with dense circular or polygonal pits.

Vessels are very similar to tracheids in both shape and function, but they are much more efficient than tracheids because water circulates through holes where the cell walls have disappeared. Therefore, it is thought that xylems evolved from tracheids, and lower ferns and gymnosperms only have tracheids. However, there are exceptions to this, and while the fern bracken and gymnosperm ephedra have vessels, angiosperms such as Japanese quince and water lily lack vessels. The latter are specifically called nonvascular plants, and are thought to be the most primitive type of angiosperm.

[Mitsuo Suzuki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

植物の木部にある水分の通道をつかさどる組織の一種で、おもに被子植物にみられる。道管を構成する個々の細胞は道管要素とよばれ、植物体の長軸方向に長い円柱状、または多角柱状をなしている。道管要素の直径と長さはさまざまで、一般に太いものでは短く、細いものでは長い。太いものは直径500マイクロメートルにもなるのに対して、細いものは数十マイクロメートルである。この道管要素が上下に多数つながって細長い管となったものが道管で、上下の要素のつなぎ目では、タケの節(ふし)を抜いたように細胞壁が消失し、穿孔(せんこう)となっている。穿孔にはさまざまな形があり、大きく一つの穴が開いている単穿孔、梯子(はしご)状や網目状に細胞壁が残って多数の穴に分かれている階段穿孔、網状穿孔がある。

　最初の道管は、茎や根の成長点近くの伸長成長をしている部分にできる。この原生木部の道管は側壁が薄く、その内側に環状または螺旋(らせん)状の肥厚をもち、環紋道管、螺旋紋道管とよばれる。その後、伸長成長がやむと側壁全体が厚く肥厚して、機械的支持機能をも備えた後生木部や二次木部の道管がつくられる。これらは側壁に有縁壁孔をもち、その形や配列の仕方から、梯子状にみえる階紋道管、網目状の網紋道管、円形ないし多角形の壁孔が密にある孔紋道管などに分けられる。

　道管は、形のうえからも、また機能のうえからも仮道管によく似ているが、道管は、細胞壁が消失した穿孔を通して水分の流通が行われるために、仮道管に比べて効率がはるかによい。したがって、道管は仮道管が進化してできたものと考えられ、下等なシダ植物や裸子植物では仮道管のみしかもっていない。しかし、これにも例外はあり、シダ植物のワラビや裸子植物のマオウ類は道管をもつ一方、被子植物のヤマグルマ、スイセイジュなどは道管を欠く。後者はとくに無道管植物とよばれ、被子植物のうちでも原始的な仲間であると考えられている。

［鈴木三男］

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