Structural Characteristics of Stems, Roots and Leaves | 茎、根和叶的结构特征

📚 Structural Characteristics of Stems, Roots and Leaves | 茎、根和叶的结构特征

In A-Level Biology, understanding the internal anatomy of flowering plants (angiosperms) is fundamental to explaining how plants obtain, transport and utilise water and nutrients. This article systematically examines the structural features of dicotyledonous stems, roots and leaves, along with comparisons to monocotyledonous plants, as required by the CIE syllabus.

在 A-Level 生物学中，理解开花植物（被子植物）的内部解剖结构是解释植物如何获取、运输和利用水分与养分的基础。本文系统地考察双子叶植物茎、根和叶的结构特征，并与单子叶植物进行对比，完全符合 CIE 考纲要求。

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1. General Organisation of Plant Tissues | 植物组织的基本结构

All plant organs are composed of three tissue systems: the dermal tissue system (epidermis), the ground tissue system (cortex and pith) and the vascular tissue system (xylem and phloem). These systems are arranged differently in each organ to suit its specific function.

所有植物器官都由三种组织系统构成：表皮组织系统（表皮）、基本组织系统（皮层和髓）以及维管组织系统（木质部和韧皮部）。这三种系统在每个器官中的排列方式不同，以适应其特定功能。

The three main vegetative organs — stem, root and leaf — form a continuous transport network. The root absorbs water and mineral ions, the stem supports the plant and conducts substances between root and leaves, and the leaf carries out photosynthesis and transpiration.

三大营养器官——茎、根和叶——形成一个连续的运输网络。根吸收水分和无机盐，茎支撑植物并在根与叶之间传导物质，叶则进行光合作用和蒸腾作用。

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2. Structure of the Dicotyledonous Stem | 双子叶植物茎的结构

In transverse section, a dicotyledonous stem is organised into distinct concentric layers. From outside to inside, these are the epidermis, the cortex, the vascular bundles and the central pith.

在横切面上，双子叶植物茎呈明显的同心圆排列。从外到内依次为表皮、皮层、维管束和中央的髓。

Epidermis | 表皮

The epidermis is the outermost single layer of cells. It is covered by a waxy cuticle that reduces water loss, and it contains stomata and trichomes (plant hairs). The cuticle also provides a barrier against pathogens.

表皮是最外层的单层细胞，覆盖有蜡质角质层以减少水分蒸发，并包含气孔和毛状体（植物毛）。角质层还能作为抵御病原体的屏障。

Cortex | 皮层

The cortex lies beneath the epidermis and is made up of collenchyma and parenchyma cells. Collenchyma cells have thickened corners and provide mechanical support; parenchyma cells store starch and can be involved in photosynthesis if they contain chloroplasts.

皮层位于表皮之下，由厚角组织和薄壁组织细胞构成。厚角组织细胞角部增厚，提供机械支撑；薄壁组织细胞储存淀粉，若含有叶绿体还可进行光合作用。

Vascular Bundles | 维管束

The vascular bundles of a dicot stem are arranged in a ring around the pith. Each bundle is described as open and collateral, meaning it contains both xylem and phloem, with a cambium layer between them. The xylem is located towards the inside, and the phloem towards the outside, of each bundle.

双子叶植物茎的维管束围绕髓排列成环状。每个维管束被称为开放型并生维管束，即同时含有木质部和韧皮部，两者之间还有形成层。在每个维管束中，木质部位于内侧，韧皮部位于外侧。

Key features of the vascular bundle include:

维管束的关键特征包括：

• Xylem vessels and tracheids have thickened, lignified walls and are dead at maturity, functioning in water and mineral transport and providing support.

  木质部导管和管胞具有加厚的木质化细胞壁，成熟时细胞死亡，负责水分和无机盐运输并提供支撑。

• Phloem consists of sieve tubes and companion cells; sieve tubes lack nuclei and rely on companion cells for metabolic support, functioning in sucrose and amino acid transport.

  韧皮部由筛管和伴胞构成；筛管无细胞核，依赖伴胞提供代谢支持，负责蔗糖和氨基酸运输。

• The vascular cambium, between xylem and phloem, is a lateral meristem that produces secondary xylem and phloem in secondary growth.

  位于木质部和韧皮部之间的维管形成层是一种侧生分生组织，在次生生长中产生次级木质部和韧皮部。

Pith | 髓

The pith occupies the centre of the stem and is composed of parenchyma cells, often storing starch. In herbaceous stems, the pith can also serve in water storage.

髓占据茎的中心，由薄壁组织细胞构成，通常储存淀粉。在草本植物茎中，髓还可储存水分。

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3. Structure of the Dicotyledonous Root | 双子叶植物根的结构

In contrast to the stem, the dicot root has a radial arrangement of vascular tissues. The root is divided into the epidermis (with root hairs), cortex, endodermis, pericycle and the central vascular cylinder (stele).

与茎不同，双子叶植物根的维管组织呈辐射状排列。根分为表皮（具根毛）、皮层、内皮层、中柱鞘和中央维管柱（中柱）。

Epidermis and Root Hairs | 表皮与根毛

The root epidermis is thin-walled and permeable. Root hairs are extensions of single epidermal cells that massively increase the surface area for water and mineral absorption. They are short-lived and found in the region of maturation.

根表皮细胞壁薄且具有通透性。根毛是单个表皮细胞的延伸，极大增加了水分和矿质吸收的表面积。根毛寿命短暂，存在于成熟区。

Cortex | 皮层

The root cortex is much wider than that of the stem, consisting of large, loosely packed parenchyma cells. Water and mineral ions pass through the cortex via the apoplast and symplast pathways before reaching the endodermis.

根的皮层比茎的皮层宽得多，由大型、排列疏松的薄壁细胞组成。水分和矿质离子通过质外体途径和共质体途径穿过皮层，抵达内皮层。

Endodermis | 内皮层

The endodermis is a single layer of cells forming the innermost boundary of the cortex. Each cell has a Casparian strip — a band of suberin and lignin in the radial and transverse walls that blocks the apoplast pathway, forcing water and solutes to pass through the selectively permeable plasma membrane.

内皮层是形成皮层最内边界的单层细胞。每个细胞具有凯氏带——径向壁和横向壁上由木栓质和木质素组成的带状增厚，阻断质外体途径，迫使水分和溶质通过选择透过性原生质膜进入中柱。

Pericycle and Vascular Cylinder | 中柱鞘与维管柱

Inside the endodermis is the pericycle, a layer of meristematic cells that can give rise to lateral roots. The central vascular cylinder contains the xylem and phloem. In a dicot root, the xylem forms a star-shaped or ridged core (typically with 2–5 arms, described as diarch to pentarch), with phloem lying in the grooves between the xylem arms.

内皮层内侧是中柱鞘，是一层具有分生能力的细胞，可产生侧根。中央维管柱包含木质部和韧皮部。在双子叶植物根中，木质部形成星形或脊状核心（通常有 2–5 个臂，称为二原型至五原型），韧

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