📚 Two Systems: Xylem and Phloem | 两大运输系统：木质部与韧皮部
In vascular plants, long-distance transport of water, minerals, and organic nutrients is carried out by two specialised tissues: xylem and phloem. These tissues form a continuous vascular system connecting roots, stems, and leaves, and are essential for growth, metabolism, and survival.
维管植物体内，水分、矿物质和有机养料的长距离运输由两种特化的组织完成：木质部和韧皮部。这些组织形成贯穿根、茎、叶的连续维管系统，对植物的生长、代谢和存活至关重要。
1. Introduction to Plant Vascular Systems | 植物维管系统简介
Vascular plants possess complex conducting tissues that enable efficient transport over large distances. Xylem and phloem are the two principal components of the vascular system, often organised together in vascular bundles.
维管植物拥有复杂的输导组织，能够实现长距离高效运输。木质部和韧皮部是维管系统的两大主要组成部分，通常共同组成维管束。
The evolution of xylem and phloem allowed plants to colonise terrestrial habitats, as water and nutrients could be moved from soil to aerial parts and photosynthetic products could be distributed to non-photosynthetic tissues.
木质部和韧皮部的演化使植物能够征服陆地生境，因为水分和养分可以从土壤运输到地上部分，而光合产物则可分配到非光合组织。
In roots, stems, and leaves, the arrangement of xylem and phloem varies but always forms an interconnected network. This continuity ensures that no living cell is far from a supply of water and solutes.
在根、茎和叶中，木质部和韧皮部的排列方式各不相同，但总是形成一个相互连通的网络。这种连续性确保了每个活细胞离水分和溶质的供应源都不远。
2. Xylem Structure: Tracheids and Vessels | 木质部结构：管胞与导管
Xylem is a complex tissue composed of several cell types, chiefly tracheids and vessel elements, which are dead at maturity and have lignified walls. These cells form hollow tubes for water conduction.
木质部是一种复合组织，由多种细胞类型组成，主要是管胞和导管分子。这两种细胞在成熟时是死亡的，并且细胞壁木质化，形成中空的管道用于输导水分。
Tracheids are elongated cells with tapered ends and fully lignified secondary walls. Water moves between adjacent tracheids via pits, which are thin areas in the cell walls where only primary wall remains. Tracheids provide both conduction and structural support.
管胞是长而两端渐尖的细胞，具有完全木质化的次生壁。水分通过纹孔在相邻管胞间移动，纹孔是细胞壁上只保留初生壁的较薄区域。管胞同时提供输导和机械支持。
Vessel elements are shorter and wider than tracheids, with perforation plates at their end walls that allow open connection and efficient water flow. They are arranged end-to-end to form continuous vessels. Vessels are primarily found in angiosperms and represent an evolutionary advancement in water transport efficiency.
导管分子比管胞短而宽，端壁具有穿孔板，允许敞开式连接和高效水流。它们首尾相连形成连续的导管。导管主要出现在被子植物中，代表着水分运输效率的进化提升。
Both tracheids and vessels deposit lignin in patterns such as annular, spiral, scalariform, or pitted thickenings, preventing collapse under the negative pressures generated during transpiration.
管胞和导管均以环纹、螺纹、梯纹或孔纹等形式沉积木质素，以防止在蒸腾作用产生的负压下发生塌陷。
3. Xylem Fibres and Parenchyma | 木质纤维与薄壁组织
In addition to tracheary elements, xylem contains fibres and parenchyma cells. Xylem fibres are highly elongated, thick-walled, lignified cells that contribute mechanical strength without a primary role in conduction.
除管状分子外，木质部还含有纤维和薄壁细胞。木质纤维是高度伸长的、厚壁且木质化的细胞，主要提供机械强度而并非主要承担输导功能。
Xylem parenchyma is made of living cells with thin, unlignified primary walls. These cells are metabolically active and function in storage of starch, oils, and other substances, as well as in lateral transport of water and solutes through their plasmodesmata connections.
木质部薄壁组织由活细胞组成，具有薄的、未木质化的初生壁。这些细胞具有代谢活性，能够储存淀粉、油脂和其他物质，并通过胞间连丝进行水分和溶质的横向运输。
In some species, xylem parenchyma cells differentiate into contact cells that facilitate exchange between the vessel lumen and surrounding tissues, playing an important role in wound healing and refilling of embolised vessels.
在某些物种中，木质部薄壁细胞分化为接触细胞，促进导管腔与周围组织之间的物质交换，在创伤愈合和栓塞导管再填充中发挥重要作用。
4. Mechanism of Water Transport in Xylem | 木质部水分运输机制
Water moves through xylem vessels and tracheids predominantly under tension generated by transpiration. This process is passive at the level of the xylem conduits, requiring no direct metabolic energy for the ascent of sap.
水分主要通过蒸腾作用产生的拉力在木质部导管和管胞中移动。这一过程在木质部管道水平上是被动的，汁液上升不需要直接消耗代谢能。
The driving force is the evaporation of water from mesophyll cell walls into the leaf air spaces and then out through stomata. This creates a low water potential at the leaf surface, which is transmitted through the continuous water columns in the xylem all the way down to the roots.
其驱动力是水分从叶肉细胞壁蒸发进入叶内气隙，然后经由气孔散失。这会在叶片表面产生低水势，该低水势通过木质部中的连续水柱一直传递到根部。
Water uptake by roots occurs by osmosis along a water potential gradient from the soil into the root hair cells and then into the xylem apoplast or symplast pathway, eventually being pulled upwards.
根对水分的吸收沿水势梯度通过渗透作用进行，从土壤进入根毛细胞，随后进入木质部质外体或共质体途径，最终被向上拉拽。
5. Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory explains how water can rise to great heights in trees. It relies on the high tensile strength of water and the cohesion between water molecules due to hydrogen bonding.
内聚力-张力理论解释了水如何能够上升到树木的很高处。该理论依赖于水的高抗张强度以及水分子间因氢键而产生的高内聚力。
As water evaporates from leaf cell walls, surface tension draws water from the xylem into the cell wall, creating a negative pressure (tension) within the xylem. This tension pulls the water column upward because the cohesive forces prevent the column from breaking.
当水分从叶细胞壁蒸发时，表面张力将水从木质部吸入细胞壁，从而在木质部内产生负压（张力）。由于内聚力阻止水柱断裂，这一张力便向上牵拉水柱。
Adhesion of water molecules to the hydrophilic lignin and cellulose walls of xylem conduits also helps maintain the continuous water column and counteracts gravitational forces.
水分子与木质部管道壁上亲水的木质素和纤维素之间的附着力也有助于维持连续水柱，并抵消重力的作用。
Experimental evidence, such as diurnal changes in stem diameter and measurement of xylem tensions with pressure bombs, strongly supports the cohesion-tension mechanism.
实验证据，例如茎直径的昼夜变化以及使用压力室测量木质部张力，都强有力地支持内聚力-张力机制。
6. Phloem Structure: Sieve Tubes and Companion Cells | 韧皮部结构：筛管与伴胞
Phloem is the tissue responsible for translocation of organic solutes, mainly sucrose and amino acids, from sources to sinks. The main conducting elements are sieve tube elements, which are living but lack a nucleus, vacuole, and many ribosomes at maturity.
韧皮部是负责将有机溶质（主要是蔗糖和氨基酸）从源运输到库的组织。主要的输导分子是筛管分子，它们成熟时是活细胞，但缺乏细胞核、液泡和许多核糖体。
Sieve tube elements are arranged end-to-end, and their end walls are perforated by large sieve pores, forming sieve plates that allow rapid flow of phloem sap between elements.
筛管分子首尾相连，其端壁由大型筛孔穿孔，形成筛板，允许韧皮部汁液在各分子之间快速流动。
Each sieve tube element is closely associated with one or more companion cells, which are metabolically active parenchyma cells that retain a nucleus and abundant mitochondria. Companion cells provide ATP, proteins, and other essential metabolites to the sieve elements via plasmodesmata.
每个筛管分子与一个或多个伴胞紧密联系。伴胞是具有代谢活性的薄壁细胞，保留细胞核和丰富的线粒体。伴胞通过胞间连丝向筛管分子提供ATP、蛋白质和其他必需代谢物。
In some plants, particularly angiosperms, ordinary companion cells can differentiate into transfer cells with wall ingrowths that increase surface area for active loading of solutes into the sieve tube.
在某些植物，尤其是被子植物中，普通伴胞可以分化为传递细胞，其细胞壁内突生长增加了表面积，利于将溶质主动装载到筛管中。
7. Phloem Fibres and Parenchyma | 韧皮纤维与薄壁组织
Phloem fibres, also known as bast fibres, are elongated sclerenchyma cells with thick lignified walls. They provide mechanical support and protection to the vascular bundles, and in some species are of commercial importance, e.g., flax and hemp.
韧皮纤维，也称为韧皮部纤维，是细长且细胞壁木质化的厚壁组织细胞。它们为维管束提供机械支持和保护，在一些物种中具有商业价值，例如亚麻和大麻。
Phloem parenchyma consists of living, thin-walled cells that store starch, lipids, and other reserves. They are also involved in lateral transport and in the retrieval of leaked solutes from the apoplast back into the phloem stream.
韧皮部薄壁组织由活的薄壁细胞组成，储存淀粉、脂质和其他储备物质。它们还参与横向运输，并将从质外体渗漏的溶质重新回收至韧皮部流中。
Storage parenchyma in perennial plants can provide carbohydrates for early spring growth before photosynthetic leaves are fully developed, highlighting the role of phloem in seasonality.
多年生植物中的储藏薄壁组织可以在光合叶片完全发育之前为早春生长提供碳水化合物，这突显了韧皮部在季节性生长中的作用。
8. Translocation of Organic Solutes | 有机物溶质的转运
Translocation is the movement of assimilates, principally sucrose, through the phloem from sources (mature leaves, storage organs) to sinks (roots, fruits, meristems). Sucrose is the dominant transported sugar because it is non-reducing and less reactive during transport.
转运是指同化物（主要是蔗糖）通过韧皮部从源（成熟叶片、贮藏器官）向库（根、果实、分生组织）的移动。蔗糖是主要运输的糖类，因为它不具有还原性且在运输过程中反应性较低。
Amino acids, plant hormones, and some inorganic ions are also transported in the phloem sap, making it a versatile distribution system for organic and signaling molecules.
氨基酸、植物激素和一些无机离子也通过韧皮部汁液运输，使其成为有机分子和信号分子的多功能分配系统。
The direction of translocation is not fixed: phloem can transport solutes upwards and downwards simultaneously in different sieve tubes, depending on the relative positions of sources and sinks.
转运方向不是固定的：韧皮部可以在不同筛管中同时向上和向下运输溶质，具体方向取决于源与库的相对位置。
The transport is active at the loading and unloading stages, requiring metabolic energy, but flow through the sieve tube lumen is a passive, pressure-driven bulk flow.
运输在装载和卸载阶段是主动的，需要代谢能，但通过筛管腔的流动则是被动的、压力驱动的集流。
9. Pressure Flow Hypothesis (Source-Sink) | 压力流动假说（源-库）
The pressure flow hypothesis, also called the mass flow hypothesis, explains the mechanism of phloem translocation. At the source, sucrose is actively loaded into the sieve tube, decreasing the water potential inside the sieve element.
压力流动假说，也称为集流假说，解释了韧皮部运输的机制。在源端，蔗糖被主动装载到筛管中，降低了筛管分子内部的水势。
Water then enters the sieve tube from the adjacent xylem by osmosis, causing a rise in hydrostatic pressure at the source. At the sink, sucrose is actively unloaded, raising the water potential, so water moves out of the phloem, resulting in a lower hydrostatic pressure.
随后，水分通过渗透作用从邻近的木质部进入筛管，导致源端静水压升高。在库端，蔗糖被主动卸载，水势升高，因此水分流出韧皮部，造成较低的静水压。
The difference in hydrostatic pressure between the source and sink generates a pressure gradient that drives a bulk flow of phloem sap from high to low pressure, carrying solutes along the gradient.
源库之间的静水压差产生压力梯度，驱动韧皮部汁液从高压处向低压处集流运动，溶质随之沿梯度运输。
This hypothesis is supported by experimental data such as the presence of aphid stylets and measurement of phloem sap flow rates and pressure differentials. It emphasises the close functional coupling between xylem and phloem.
该假说得到了实验数据的支持，例如利用蚜虫口针进行观测以及测量韧皮部汁液流速和压力差。它强调了木质部与韧皮部在功能上的紧密偶联。
10. Radial Distribution of Xylem and Phloem | 木质部与韧皮部的径向分布
In roots, the vascular tissue is typically arranged in a central stele, with xylem forming a solid core or star-shaped pattern and phloem positioned between the xylem arms, providing protection against stretching forces.
在根中，维管组织通常位于中柱内，木质部形成实心核心或星状排列，而韧皮部位于木质部辐射臂之间，以此抵御拉伸力的影响。
In herbaceous dicot stems, vascular bundles are arranged in a ring with xylem located towards the inside and phloem towards the outside. This arrangement allows the xylem to be compressed against the strong cortical tissues, while phloem is closer to the periphery for delivery of assimilates.
在草本双子叶植物的茎中，维管束呈环状排列，木质部朝向内侧，韧皮部朝向外侧。这种安排使木质部可以紧靠坚韧的皮层组织，而韧皮部则靠近外围以输送同化物。
In woody dicots and gymnosperms, secondary growth displaces the phloem outward and crushes older primary phloem; the functional secondary phloem remains near the vascular cambium, while the secondary xylem accumulates as wood.
在木本双子叶植物和裸子植物中，次生生长将韧皮部推向外侧并压碎旧的初生韧皮部；有功能的次生韧皮部保持在维管形成层附近，而次生木质部则积累成为木材。
Monocot stems have scattered vascular bundles, each surrounded by a bundle sheath and typically lacking secondary growth. Here, xylem and phloem are enclosed within each individual bundle.
单子叶植物的茎则具有散生的维管束，每个束被维管束鞘包围，通常缺乏次生生长。木质部和韧皮部被包裹在每个独立的维管束内。
11. Comparison of Xylem and Phloem | 木质部与韧皮部比较
| Feature / 特征 | Xylem / 木质部 | Phloem / 韧皮部 |
|---|---|---|
| Main conducting cells / 主要输导细胞 | Tracheids, vessel elements / 管胞、导管分子 | Sieve tube elements / 筛管分子 |
| Cell state at maturity / 成熟时细胞状态 | Dead, empty lumen / 死细胞，中空腔 | Living, no nucleus / 活细胞，无细胞核 |
| Cell wall / 细胞壁 | Lignified secondary wall / 木质化次生壁 | Primary wall, no lignin / 初生壁，无木质素 |
| Transported substance / 运输物质 | Water and mineral ions / 水分与矿质离子 | Sucrose, amino acids, hormones / 蔗糖、氨基酸、激素 |
| Direction of flow / 运输方向 | Upwards (roots to leaves) / 向上（根至叶） | Bidirectional (source to sink) / 双向（源至库） |
| Driving force / 驱动力 | Transpiration pull, cohesion-tension / 蒸腾拉力，内聚力-张力 | Pressure gradient (mass flow) / 压力梯度（集流） |
| Perforations / 穿孔结构 | Perforation plates in vessels / 导管穿孔板 | Sieve plates with sieve pores / 筛板及筛孔 |
| Companion/associated cells / 伴随细胞 | Xylem parenchyma / 木质部薄壁细胞 | Companion cells / 伴胞 |
12. Adaptations and Importance | 适应性与重要性
Xylem vessels with wide diameters and perforation plates reduce resistance to water flow, enabling tall trees to supply their crowns with water. Lignin deposition strengthens the walls to withstand large negative pressures without collapse.
具有宽内径和穿孔板的导管降低了水流阻力，使得高大乔木能够向树冠供水。木质素的沉积加强了管壁，以承受较大的负压而不坍塌。
The close association between sieve tubes and companion cells ensures adequate ATP supply for active loading and unloading of solutes, which is critical for maintaining the pressure gradient in phloem transport.
筛管与伴胞的紧密关联确保了主动装卸溶质所需的充足ATP供应，这对维持韧皮部运输中的压力梯度至关重要。
Pits and plasmodesmata in both systems allow symplastic continuity between cells, facilitating lateral exchange and coordinated responses to environmental stresses such as drought or wounding.
两个系统中的纹孔和胞间连丝使细胞间保持共质体连续性，有助于横向交换和对干旱或损伤等环境胁迫的协调响应。
The functional interdependence of xylem and phloem is exemplified by the water-phloem cycle: water from xylem enters the phloem at sources and returns to the xylem at sinks, creating a hydraulic loop that sustains long-distance transport in plants.
木质部与韧皮部在功能上的相互依存关系体现在水-韧皮部循环中：水从木质部在源端进入韧皮部，在库端重新返回木质部，形成一个水力环路，维持植物的长距离运输。
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