Vascular bundle

Japanese: 維管束 - いかんそく

The term often simply refers to vascular tissue, which is the main pathway for the transport of materials within the plant body and contains specialized tissue for this purpose, but it can also refer to a bundle formed by the combination of xylem and phloem, and is also called a vessel bundle. Generally, the structure of a vascular bundle is shown in primary vascular tissue. Vascular bundles are given special names depending on their location within the plant body; those in leaf blades are called leaf veins, and those in stems that are in the process of separating into leaves and branches are called leaf scars and branch scars, respectively. Vascular bundles are composite tissues made up of several types of tissue, and contain cells with thick, hard cell walls, so they also play a role in mechanically supporting the plant body. In plant anatomy, the structure of vascular bundles is treated as a particularly important trait. The arrangement of vascular bundles within plant organs and the arrangement of xylem and phloem within a vascular bundle are diverse, but can be divided into several types. The thickness and number of vascular bundles that run through organs vary, from a single stem with only one to as many as 20,000 vascular bundles.

German botanist Sachs divided the tissues of higher plants into three tissue systems: epidermal, basal, and vascular. Vascular tissue is surrounded by basal tissue, but in plants with many vascular bundles in their organs, the vascular bundles join and separate to form a complex vascular system. Because of the physiological and systematic importance of vascular bundles, plants that have them are collectively called vascular plants, which include ferns, gymnosperms, and angiosperms. Plants such as mosses and algae do not have vascular bundles and are called non-vascular plants or avascular plants. However, some mosses have structures involved in the transport of water called ducts, and some, such as the brown algae kelp, have differentiated tissues similar to the sieve tubes of angiosperms.

[Eimasa Nishino]

Vascular bundle structure

Vascular bundles consist of xylem, which transports water absorbed from the roots, and phloem, which transports and transfers nutrients and metabolic products produced by leaves and other parts of the plant. Xylem is composed of vessel cells, tracheids, fibers, and parenchyma cells, and the passageways for water are vessel cells and tracheids. Angiosperms have vessels, which are hollow tubes formed by a series of vessel cells, while gymnosperms and ferns have tracheids. However, even in angiosperms, thin vascular bundles such as those at the ends of leaf veins often become tracheids, and some gymnosperms and ferns have vessels. The phloem is composed of sieve tube cells, sieve cells, companion cells, fibers, and parenchyma cells, and the passageways for nutrients are sieve tube cells and sieve cells. Angiosperms have sieve tubes, which are a series of sieve tube cells, and companion cells adjacent to the sieve tube cells. Gymnosperms and ferns have sieve cells.

[Eimasa Nishino]

Development of Vascular Bundles

The axial growth of stems and roots in vascular plants occurs when cells proliferate in the apical meristem at the tip of the stem or root and join the stem or root tissue. Undifferentiated cells gradually differentiate into the protodermis, basic meristem, and procambium, which then mature into epidermal tissue, ground tissue, and vascular tissue, respectively. In leaves, the various tissues are similarly formed during the growth of the leaf primordium. The primary vascular tissue that is generated in this way consists of the primary xylem and the primary phloem, and may become a single vascular bundle in an organ or multiple vascular bundles separated by the ground tissue. The differentiation of xylem and phloem from the procambium occurs in a certain direction, and within the xylem, first the protoxylem differentiates, followed by the metaxylem, and the structures of the tracheids and vessel cells are different. The phloem is also divided into protophloem and metaphloem. After the cells of the procambium differentiate into vascular tissue, there are open vascular bundles in which the intravascular cambium differentiates at the boundary between the xylem and phloem, and closed vascular bundles in which the intravascular cambium does not differentiate. The former are mainly found in the stems and roots of gymnosperms and dicotyledons, while the latter are mainly found in the leaves of dicotyledons and the stems and roots of ferns and monocotyledons. When the cambium differentiates into the areas between the vascular bundles, it connects in a ring, and the cells grow by repeated tangential divisions to form secondary vascular tissue, called secondary xylem on the inside and secondary phloem on the outside. In some cases, the vascular cambium differentiates away from the primary vascular tissue.

[Eimasa Nishino]

Types of Vascular Bundles

Depending on the arrangement of the xylem and phloem, vascular bundles can be classified as parallel, compound parallel, or enclosed vascular bundles. Parallel vascular bundles are most common in the stems and leaves of gymnosperms and angiosperms, where the xylem and phloem are in contact on one side, with the xylem on the inside and the phloem on the outside in stems, and the xylem on the upper side and the phloem on the lower side in leaves. The stems of plants such as Cucurbitaceae and Solanaceae have compound parallel vascular bundles with phloem on the inside and outside of the xylem, but in some cases the inner phloem is quite separated from the xylem. Encircled vascular bundles have a structure in which one of the xylem and phloem is surrounded by the other. Outer phloem enclosed vascular bundles, in which the xylem is surrounded by phloem, are often seen in the stems and leaves of ferns, and are rarely seen in angiosperms. The xylem-encircling vascular bundle, in which the phloem is surrounded by xylem, is commonly seen in the underground stems of monocotyledons, and is also seen in the aboveground stems of monocotyledons that have formed secondary vascular tissue. In all roots, xylem and phloem are arranged alternately in a ring as independent vascular bundles, and because of this arrangement, they are sometimes called ray vascular bundles.

[Eimasa Nishino]

Arrangement of Vascular Bundles

The arrangement of vascular bundles in stems varies depending on the taxonomic group. In dicotyledons and gymnosperms, they are usually arranged in a ring around the stem, while in monocotyledons, they are irregularly arranged. When the leaf scar makes a long detour through the pith or cortex, it is called a pith streak or a cortical streak, respectively. Ferns are diverse, ranging from one vascular bundle to many, and some have xylem and phloem arranged alternately like all roots. When the arrangement of vascular bundles differs between stems and roots, the arrangement changes at the transition between the stem and root, which corresponds to the part centered on the hypocotyl of the sprout. The movement of vascular bundles, especially in the axial direction, is called the vascular bundle run. The streak theory divides the area containing the vascular bundles of stems and roots into the central streak, and discusses the types and arrangements of vascular bundles and their mutual relationships. The leaf blade is usually flat, and the vascular bundles are arranged in a plane, but the arrangement of the main vascular bundles is broadly classified as bifurcated, reticulate, or parallel venation. The petiole also shows a planar arrangement, but in some cases, as in the stem, it shows a three-dimensional, complex arrangement.

[Eimasa Nishino]

Diagram of vascular bundle structure
©Shogakukan ">

Diagram of vascular bundle structure

Dicotyledonous root (cross section)
© Satoshi Shimazoe

Dicotyledonous root (cross section)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

植物の体内で物質の輸送の主要な通路となり、そのための特殊な組織を含む維管束組織を単にさすことが多いが、木部(もくぶ)と篩部(しぶ)とが組み合わさり1本の束となった状態のものをさし、管束ともいう。一般に第一次維管束組織において維管束の構造を示す。維管束は植物体内での場所によって特別の名称でよばれ、葉身(ようしん)の中にあるものは葉脈、茎の中にあるが葉や枝のほうへ分離していく途中のものをそれぞれ葉跡(ようせき)、枝跡(しせき)という。維管束は数種類の組織からなる複合組織で、厚くて硬い細胞壁をもつ細胞を含むことから、植物体を機械的に支持する役割もあわせもつ。植物解剖学では維管束に関する構造はとくに重要な形質として扱われる。植物の器官の中での維管束の配列や、1本の維管束の中での木部と篩部との配列は多様であるが、いくつかの型に分けられる。器官の中を貫いている維管束の太さや数はさまざまで、たとえば1本の茎の中に1本しかないものから、2万本もの維管束をもつものまである。

　ドイツの植物学者ザックスは高等植物の組織を表皮系、基本系、維管束系の三つの組織系に大別した。維管束組織は基本組織に囲まれているが、器官の中に多数の維管束をもつ植物体全体では、維管束の互いの離合により、複雑な維管束系を形成する。維管束の生理的、系統的な重要性から、これをもつ植物群をまとめて維管束植物とよび、このなかにはシダ植物、裸子植物、被子植物が含まれる。コケ植物、藻類などの植物群は維管束をもたず、非維管束植物または無維管束植物とよばれる。しかし、コケ植物のなかには道束(どうそく)とよばれる水液の輸送に関与する構造をもつものがあり、褐藻類のコンブのように、被子植物の篩管と同様の組織を分化させているものもある。

［西野栄正］

維管束の構造

維管束は根から吸収された水液を輸送する木部と、葉などでつくられた養分や代謝産物の輸送および転送を行う篩部とからなる。木部は道管細胞、仮道管、繊維、柔細胞によって構成され、水液の通路は道管細胞や仮道管である。被子植物では道管細胞が連なって中空の管となった道管をもち、裸子植物やシダ植物は仮道管をもつ。しかし、被子植物でも葉脈の末端など細い維管束では仮道管となることが多く、裸子植物やシダ植物でも道管をもつものがある。篩部は篩管細胞、篩細胞、伴(ばん)細胞、繊維、柔細胞によって構成され、養分の通路は篩管細胞や篩細胞である。被子植物は篩管細胞が連なった篩管と、篩管細胞に隣接した伴細胞をもつ。裸子植物とシダ植物は篩細胞をもつ。

［西野栄正］

維管束の発生

維管束植物の茎や根が軸方向へ成長していくのは、それらの先端にある頂端分裂組織で増殖した細胞が、茎や根の組織に加わることによっておこる。未分化な細胞はしだいに原表皮、基本分裂組織、前形成層に分化、さらにそれぞれ表皮組織、基本組織、維管束組織へと成熟する。葉でも葉原基(ようげんき)の成長の間に同様に各組織が形成される。このように発生したのが第一次維管束組織で、第一次木部と第一次篩部とからなり、器官の中の1本の維管束となる場合や、基本組織によって分断された多数の維管束となる場合がある。前形成層からの木部と篩部の分化はそれぞれ一定の方向におこり、木部の中ではまず原生木部が、ついで後生木部が分化し、仮道管や道管細胞の構造が異なる。篩部も原生篩部と後生篩部とに分けられる。前形成層の細胞が維管束組織に分化したあと、木部と篩部の境界部で維管束内形成層が分化する開放維管束と、それが分化しない閉鎖維管束とがあり、前者はおもに裸子植物と双子葉類の茎や根に、後者はおもに双子葉類の葉とシダ植物や単子葉類の茎や根にみられる。形成層が維管束間の部分にも分化すると環状につながり、その細胞が接線分裂を繰り返して増殖し、内側には第二次木部、外側には第二次篩部という第二次維管束組織を形成する。維管束形成層が第一次維管束組織とは離れた場所で分化する場合もある。

［西野栄正］

維管束の種類

維管束は木部と篩部の配置によって、並立(へいりつ)、複並立、包囲の各維管束に分けられる。並立維管束は裸子植物と被子植物の茎や葉にもっとも普通で、木部と篩部とが一面で接し、茎では内側に木部、外側に篩部が、葉では上側に木部、下側に篩部が位置する。ウリ科やナス科植物などの茎には木部の内外に篩部のある複並立維管束があるが、内側の篩部が木部とかなり離れている場合もある。包囲維管束は木部と篩部のうち、一方が他方に囲まれた構造をもつ。木部が篩部に囲まれた外篩(がいし)包囲維管束はシダ植物の茎や葉に多くみられ、被子植物にもまれにみられる。篩部が木部に囲まれる外木(がいもく)包囲維管束は単子葉類の地下茎でよくみられ、第二次維管束組織をつくった単子葉類の地上茎でもみられる。すべての根では木部と篩部とが交互に、それぞれ独立した維管束として環状に配列しているが、この配列から、それぞれを放射維管束とよぶことがある。

［西野栄正］

維管束の配列

茎の中の維管束は分類群によって異なる配列を示す。双子葉類や裸子植物では、普通、茎の周辺部に環状に並び、単子葉類では不規則な配列を示す。葉跡が髄や皮層の中を長く迂回(うかい)する場合には、とくにこれらをそれぞれ髄走条(ずいそうじょう)、皮層条とよぶ。シダ植物では1本の維管束をもつものから多数の維管束をもつものまであって多様で、またすべての根のように木部と篩部とが交互に並ぶものもある。茎と根とで維管束の配列が異なる場合は、茎と根の移行部で配列が変化し、芽生えの胚軸(はいじく)を中心とした部分がそこにあたる。維管束のとくに軸方向の推移を維管束走向という。茎と根の維管束を含む領域を中心柱として区分し、維管束の種類や配列と相互の関係を論じるのが中心柱説である。葉身は普通扁平(へんぺい)で、維管束も平面的に配列するが、主要な維管束の配列から二又(ふたまた)脈、網状脈、平行脈に大別される。葉柄(ようへい)内でも平面的な配列を示すが、茎の場合のように立体的で複雑な配列を示す場合も少なくない。

［西野栄正］

維管束の組織図

双子葉植物の根（断面図）

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