Structure of Stems, Roots and Leaves | 根、茎、叶的结构

📚 Structure of Stems, Roots and Leaves | 根、茎、叶的结构

In the Cambridge A Level Biology syllabus, plant structure is not just a list of tissues to label; it is the key to understanding how water, mineral ions and assimilates move through a flowering plant. The stem, root and leaf are organised into three tissue systems – dermal, vascular and ground – and their microscopic arrangement reflects a balance between support, transport, photosynthesis and water conservation.

在剑桥 A Level 生物大纲中，植物结构不仅是要识别的组织名称，更是理解水分、矿质离子和同化产物如何在有花植物中运输的关键。茎、根和叶由皮组织、维管组织和基本组织三大组织系统构成，其显微排列体现了支持、运输、光合作用和保水之间的平衡。

1. Plant Tissue Systems | 植物组织系统

The dermal tissue system forms the outer protective covering. In young stems and leaves it is the epidermis, often coated with a waxy cuticle; in roots the epidermis lacks a thick cuticle and produces root hairs. The ground tissue system includes cortex, pith and mesophyll, and is mainly responsible for storage, support and photosynthesis. The vascular tissue system contains xylem and phloem, the conducting tissues arranged in characteristic patterns in each organ.

皮组织系统形成外保护层。在幼茎和叶中为表皮，通常覆盖蜡质角质层；在根中表皮缺乏厚角质层并产生根毛。基本组织系统包括皮层、髓和叶肉，主要负责储存、支持和光合作用。维管组织系统包含木质部和韧皮部，这些输导组织在每个器官中排列成特征性模式。

Cambridge questions often ask you to identify the tissue systems in unfamiliar diagrams. Use position and cell shape: epidermis is the outermost cell layer, xylem usually has large open vessels with thickened walls, and phloem lies adjacent to xylem with smaller sieve tubes and companion cells.

剑桥试题常要求在不熟悉的结构图中识别组织系统。可根据位置和细胞形状判断：表皮是最外层细胞，木质部通常有大而空的导管和加厚细胞壁，韧皮部紧邻木质部，具有较小的筛管和伴胞。

2. Root External Structure and Root Tip | 根的外部结构与根尖

Roots anchor the plant, absorb water and mineral ions, and often store starch. The root tip is protected by the root cap, which secretes mucilage to reduce friction as the root pushes through soil. Behind the cap are zones of cell division, elongation and differentiation.

根固定植物，吸收水分和矿质离子，并常储存淀粉。根尖由根冠保护，根冠分泌黏液以减少根在土壤中推进时的摩擦。根冠后方依次是细胞分裂区、伸长区和分化区。

The zone of cell division contains the apical meristem, where mitosis produces new cells. In the zone of elongation, cells take up water and expand, pushing the root tip forward. In the zone of differentiation, cells mature into specialised tissues such as xylem, phloem and root hairs. Root hairs are unicellular extensions of epidermal cells that massively increase the surface area for water uptake.

细胞分裂区包含顶端分生组织，通过有丝分裂产生新细胞。在伸长区，细胞吸水膨大，推动根尖向前。在分化区，细胞成熟为专门组织，如木质部、韧皮部和根毛。根毛是表皮细胞的单细胞突起，极大地增加了吸收水分的表面积。

• Root cap: protection and mucilage secretion.
• 根冠：保护并分泌黏液。
• Apical meristem: mitosis to produce new cells.
• 顶端分生组织：通过有丝分裂产生新细胞。
• Root hairs: increase absorption area.
• 根毛：增加吸收面积。

3. Root Transverse Section | 根的横切面

A typical young dicot root in transverse section shows a central vascular cylinder rather than a ring of bundles. The outermost epidermis surrounds the cortex, which consists of unspecialised parenchyma cells that may store starch. Inside the cortex is the endodermis, a single layer with specialised cell wall modifications.

典型的幼年双子叶植物根横切面显示中央维管柱，而不是维管束环。最外层表皮包围皮层，皮层由可储存淀粉的非特化薄壁细胞组成。皮层内部是内皮层，是具有特殊细胞壁修饰的单层细胞。

The pericycle lies just inside the endodermis and retains meristematic ability; it can produce lateral roots. In the centre, xylem often forms a star-shaped arrangement with phloem located between the xylem arms. This central position helps the root withstand pulling forces, while the arrangement ensures close contact between xylem, phloem and surrounding living cells.

中柱鞘位于内皮层内侧，保留分生能力，可产生侧根。在中央，木质部常形成星状排列，韧皮部位于木质部臂之间。这种中央位置有助于根抵抗拉伸力，同时该排列确保木质部、韧

Published by TutorHao | A-Level Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）