📚 Structure of Stems, Roots and Leaves | 茎、根和叶的结构
In flowering plants, the vegetative body is organised into three principal organs: the stem, the root, and the leaf. Each organ exhibits a distinctive internal arrangement of tissues—dermal, ground, and vascular—that enables critical physiological processes such as transport, photosynthesis, anchorage, and storage. For A-Level Biology candidates following the Cambridge syllabus, a detailed knowledge of this anatomical organisation is essential for explaining how structure underpins function and how plants adapt to their environments.
在开花植物中，营养体分为三个主要器官：茎、根和叶。每个器官都表现出独特的内部组织排列——皮组织、基本组织和维管组织——这些排列使得运输、光合作用、固着和储存等关键生理过程得以进行。对于学习剑桥课程大纲的A-Level生物考生而言，详细了解这一解剖结构对于解释结构如何支撑功能以及植物如何适应环境至关重要。
1. Introduction to Plant Organs and Tissues | 植物器官和组织概述
Plants possess three organ systems that are composed of three fundamental tissue types. The dermal tissue system, represented by the epidermis, forms the outer protective covering. The ground tissue system fills the interior and includes parenchyma (photosynthesis and storage), collenchyma (flexible support), and sclerenchyma (rigid support). The vascular tissue system comprises xylem, which transports water and dissolved minerals upwards, and phloem, which translocates organic solutes such as sucrose and amino acids. These tissues are distributed differently in stems, roots, and leaves, reflecting the primary functions of each organ.
植物具有三个器官系统，它们由三种基本组织类型构成。皮组织系统以表皮为代表，形成外部保护层。基本组织系统填充内部，包括薄壁组织（光合作用和储存）、厚角组织（柔韧支持）和厚壁组织（刚性支持）。维管组织系统包含木质部（向上运输水分和溶解的矿物质）和韧皮部（运输蔗糖和氨基酸等有机溶质）。这些组织在茎、根和叶中的分布各不相同，反映了每个器官的主要功能。
2. The Stem: Epidermis and Cortex | 茎：表皮与皮层
The outermost layer of the stem is the epidermis, a single sheet of cells normally coated with a waterproof cuticle. This layer protects against desiccation and mechanical injury. In herbaceous stems, stomata may be present to permit gas exchange. Below the epidermis lies the cortex, a region composed mainly of thin-walled parenchyma cells. These cells often contain chloroplasts, thus contributing to photosynthesis in young stems. Additionally, strands of collenchyma are found beneath the epidermis, particularly in ribs, where their unevenly thickened primary cell walls provide flexible structural support.
茎的最外层是表皮，为一层细胞，通常覆有防水的角质层。这层结构可防止干燥和机械损伤。在草本茎中，可能存在气孔以允许气体交换。表皮之下是皮层，主要由薄壁细胞组成。这些细胞常含有叶绿体，因此有助于幼茎的光合作用。此外，厚角组织束位于表皮下方，尤其在茎棱处，其不均匀加厚的初生细胞壁提供了柔韧的结构支持。
3. Stem Vascular Bundles: Xylem and Phloem | 茎的维管束：木质部与韧皮部
In a typical dicotyledonous stem, vascular bundles are arranged in a distinct ring. Each bundle contains xylem oriented towards the centre of the stem and phloem situated towards the exterior. Separating the xylem and phloem is a thin layer of meristematic cells known as the vascular cambium. The xylem consists of dead, lignified cells, including vessels and tracheids, which facilitate water conduction and mechanical support. The phloem is composed of living sieve tube elements, which lack a nucleus at maturity, and their associated companion cells that perform metabolic functions. In monocot stems, vascular bundles are scattered throughout the ground tissue and lack a cambium; consequently, these stems do not undergo secondary thickening.
在典型的双子叶植物茎中，维管束排列成明显的环状。每个维管束包含朝向茎中心的木质部和朝向外侧的韧皮部。分割木质部和韧皮部的是一层薄的分生组织细胞，称为维管形成层。木质部由死亡的、木质化的细胞组成，包括导管和管胞，有助于水分输导和机械支持。韧皮部由活筛管分子（成熟时无细胞核）及执行代谢功能的伴胞组成。在单子叶植物茎中，维管束散生于基本组织中且无形成层，因此这些茎不进行次生加厚。
4. Stem Internal Structure: Cambium and Pith | 茎的内部结构：形成层与髓
The vascular cambium is responsible for secondary growth in perennial dicots. It divides periclinally to produce secondary xylem inwards and secondary phloem outwards, resulting in an increase in stem diameter. The central region of a young dicot stem is occupied by the pith, a mass of parenchyma cells that can store starch and other substances. Radiating outward from the pith are medullary rays, thin layers of parenchyma that pass between vascular bundles. These rays allow lateral transport of water and nutrients and connect the pith to the cortex and phloem.
维管形成层负责多年生双子叶植物的次生生长。它通过平周分裂向内产生次生木质部，向外产生次生韧皮部，从而导致茎直径增加。幼嫩双子叶茎的中心区域由髓占据，这是一团可储存淀粉和其他物质的薄壁细胞。从髓向外辐射的是髓射线，即穿过维管束之间的薄层薄壁组织。这些射线允许水分和养分的横向运输，并将髓与皮层和韧皮部连接起来。
5. Stem Adaptations and Secondary Growth | 茎的适应性与次生生长
Secondary growth not only provides structural support for large plants but also generates wood, which enhances conductive capacity. The activity of the cork cambium produces the periderm, a protective tissue that replaces the epidermis in older stems. Some stems are modified for specialised functions: rhizomes (e.g., iris) are horizontal underground stems for asexual reproduction and storage; tuberous stems (e.g., potato) are swollen for starch storage; and tendrils (e.g., grape) are slender, coiling structures that aid climbing. Despite these modifications, the basic tissue arrangement—epidermis, cortex, vascular bundles, and often pith—remains identifiable.
次生生长不仅为高大植物提供结构支持，还产生木材，从而增强输导能力。木栓形成层的活动产生周皮，这种保护组织取代了较老茎的表皮。有些茎发生变态以执行特殊功能：根状茎（如鸢尾）是水平的地下茎，用于无性繁殖和储存；块茎（如马铃薯）因储存淀粉而膨大；卷须（如葡萄）是细长卷曲的结构，有助于攀援。尽管存在这些变态，基本的组织排列——表皮、皮层、维管束以及通常存在的髓——依然可辨。
6. The Root: Epidermis and Root Hairs | 根：表皮与根毛
Roots are typically covered by a single-layered epidermis that, in most regions, lacks a cuticle to facilitate water uptake. Many epidermal cells extend outward to form root hairs, which dramatically increase the surface area available for absorption of water and mineral ions. Root hairs are short-lived and are constantly replaced in the zone of maturation. They work in close contact with soil particles and mycorrhizal fungi to absorb ions, often against concentration gradients, via active transport using ATP.
根的典型覆盖物是单层表皮，在大多数区域没有角质层，以利于水分吸收。许多表皮细胞向外延伸形成根毛，极大地增加了可用于吸收水分和矿物离子的表面积。根毛寿命短，在成熟区不断更新。它们与土壤颗粒和菌根真菌紧密接触，通过消耗ATP的主动运输来逆浓度梯度吸收离子。
7. Root Cortex and Endodermis | 根的皮层与内皮层
The cortex occupies the bulk of a young root. It consists of several layers of unspecialised parenchyma cells with numerous intercellular spaces, facilitating movement of water and gases. Starch grains are often stored within these cells. The innermost layer of the cortex is the endodermis, a continuous ring of cells whose radial and transverse walls are impregnated with suberin, forming the Casparian strip. This hydrophobic band blocks apoplastic movement, forcing water and solutes to travel through the cytosol of endodermal cells. In doing so, the endodermis selectively regulates which substances enter the vascular cylinder.
皮层占据了幼根的主要部分。它由数层未特化的薄壁细胞组成，细胞间有许多间隙，便于水分和气体的移动。这些细胞内常储存有淀粉粒。皮层的最内层是内皮层，为一连续细胞环，其径向壁和横向壁被木栓质浸渍，形成凯氏带。这一疏水条带阻断质外体运输途径，迫使水分和溶质通过内皮层细胞的胞质。通过这种方式，内皮层选择性地调控进入维管柱的物质。
8. Root Vascular Cylinder (Stele) | 根的维管柱（中柱）
Immediately inside the endodermis is the pericycle, a layer of parenchyma cells that retains meristematic capacity. It is responsible for initiating lateral root formation. The vascular tissues are centrally located. In dicot roots, the xylem typically forms a solid, star-shaped core with phloem bundles located in the angles between the xylem arms. This exarch arrangement (xylem maturing from the outside inward) is characteristic of roots. Monocot roots often contain a central pith surrounded by a ring of alternating xylem and phloem. The pericycle is always the outermost layer of the stele.
紧贴内皮层内侧的是中柱鞘，为一层保留分生能力的薄壁细胞。它负责起始侧根的形成。维管组织位置居中。在双子叶植物根中，木质部通常形成一个实心的星状核心，韧皮部束位于木质部臂之间的角内。这种外始式排列（木质部由外向内成熟）是根的特征。单子叶植物的根常常包含一个中心髓，周围有木质部和韧皮部交替排列的环。中柱鞘始终是中柱的最外层。
9. The Leaf: Epidermis and Cuticle | 叶：表皮与角质层
The leaf is enclosed by upper and lower epidermis, both of which consist of transparent cells to allow light penetration for photosynthesis. The epidermis is covered by a waxy cuticle, which minimises water loss through transpiration. The lower epidermis typically possesses a higher density of stomata, each consisting of a pore bounded by two specialised guard cells. These guard cells are the only epidermal cells that contain chloroplasts. Their regulated opening and closing control the diffusion of CO₂ into the leaf for carbon fixation and O₂ out as a by-product.
叶由上下表皮包裹，两者均由透明细胞组成，以使光透入进行光合作用。表皮上覆有蜡质角质层，最大限度地减少蒸腾失水。下表皮通常具有较高密度的气孔，每个气孔由一对特化的保卫细胞围成孔隙。保卫细胞是唯一含有叶绿体的表皮细胞。它们受调控的开闭控制CO₂扩散进入叶片用于碳固定，以及副产物O₂的排出。
10. Leaf Mesophyll and Photosynthesis | 叶肉组织与光合作用
The ground tissue of the leaf, the mesophyll, is differentiated into palisade and spongy layers. Palisade mesophyll sits directly beneath the upper epidermis and consists of elongated, cylindrical cells packed closely together. These cells contain a high concentration of chloroplasts, optimising light capture for the light-dependent reactions. The underlying spongy mesophyll comprises irregularly shaped cells separated by large intercellular air spaces. This structure creates a large internal surface area to promote the diffusion of CO₂ to chloroplasts and the removal of O₂, thereby supporting efficient gas exchange.
叶的基本组织——叶肉——分化为栅栏层和海绵层。栅栏组织直接位于上表皮下方，由紧密排列的长柱形细胞组成。这些细胞含有高浓度的叶绿体，从而优化了对光能的捕获以进行光依赖反应。下方的海绵组织由形状不规则的细胞组成，细胞间有较大的胞间空气间隙。这种结构创造了巨大的内部表面积，促进CO₂向叶绿体扩散以及O₂的排出，从而支持高效的气体交换。
11. Leaf Vascular Bundles and Stomatal Regulation | 叶维管束与气孔调节
The vascular bundles of the leaf, or veins, extend throughout the mesophyll. In each bundle, the xylem is positioned towards the upper epidermis and the phloem towards the lower epidermis, surrounded by a bundle sheath composed of compact parenchyma cells. These bundle sheath cells provide mechanical support and, in C4 plants, are the site of carbon fixation. Stomatal opening is an active process: H⁺ ions are pumped out of guard cells, driving the inward transport of K⁺. The resulting decrease in water potential causes water to enter by osmosis, increasing turgor pressure. Because the inner walls of guard cells are thicker than the outer walls, swelling causes the
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