Xylem and Phloem: The Two Transport Systems of Plants | 木质部与韧皮部：植物两大运输系统

📚 Xylem and Phloem: The Two Transport Systems of Plants | 木质部与韧皮部：植物两大运输系统

Plants, as sessile organisms, have evolved two remarkable vascular tissues — xylem and phloem — to transport water, minerals, and organic nutrients over long distances. These two systems form the backbone of plant physiology and are a central topic in CIE A-Level Biology.

植物作为固着生物，演化出了两种非凡的维管组织——木质部与韧皮部——用于长距离运输水分、矿物质和有机营养物质。这两大系统构成了植物生理学的核心支柱，也是 CIE A-Level 生物学的重点内容。

1. Overview of Vascular Tissues | 维管组织概览

Vascular tissues are arranged in vascular bundles, which run continuously from roots through stems to leaves. In dicotyledonous stems, xylem typically lies on the inner side and phloem on the outer side, separated by a layer of meristematic tissue called the cambium. This arrangement allows for secondary growth.

维管组织以维管束的形式排列，从根经过茎持续延伸至叶。在双子叶植物茎中，木质部通常位于内侧，韧皮部位于外侧，二者之间由一层称为形成层的分生组织隔开。这种排列方式允许植物进行次生生长。

In roots, the vascular bundle is a central stele, with xylem forming a star-shaped core and phloem occupying the spaces between the xylem arms. In leaves, vascular bundles form veins, with a network that ensures every mesophyll cell is close to a transport pathway.

在根中，维管束形成中央中柱，木质部呈星状核心，韧皮部占据木质部臂之间的空隙。在叶中，维管束形成叶脉，其网络确保每个叶肉细胞都靠近运输通道。

2. Structure of Xylem | 木质部的结构

Mature xylem consists of four main cell types: vessel elements, tracheids, xylem fibres, and xylem parenchyma. The first two are the actual conducting cells, both of which are dead at functional maturity, lacking cytoplasm and end walls to create continuous pipelines.

成熟的木质部由四种主要细胞类型组成：导管分子、管胞、木纤维和木薄壁细胞。前两种是实际的输导细胞，它们在功能成熟时均已死亡，缺乏细胞质和端壁，形成连续的管道。

Vessel elements are wider and shorter than tracheids. Their end walls break down completely, forming perforation plates that allow unimpeded water flow. The secondary walls are thickened with lignin in various patterns — annular, spiral, scalariform, or pitted — which provides mechanical strength while preventing collapse under negative pressure.

导管分子比管胞更宽、更短。其端壁完全解体，形成穿孔板，允许水流无阻碍通过。次生壁以不同方式加厚木质素——环纹、螺旋、梯纹或孔纹——这提供了机械强度，同时防止在负压下塌陷。

Tracheids are elongated, tapering cells with bordered pits on their overlapping side walls. Water must pass through pit membranes between adjacent tracheids, creating higher resistance. All vascular plants have tracheids, while vessels are found mainly in angiosperms and some gymnosperms.

管胞是细长、两端尖锐的细胞，在重叠的侧壁上有具缘纹孔。水必须通过相邻管胞之间的纹孔膜，因此阻力较大。所有维管植物都有管胞，而导管主要存在于被子植物和一些裸子植物中。

3. Structure of Phloem | 韧皮部的结构

Phloem is composed of sieve tube elements, companion cells, phloem fibres, and phloem parenchyma. The conducting cells are sieve tube elements, which remain alive at maturity but have lost their nucleus, most organelles, and tonoplast. They are connected end-to-end by sieve plates — end walls perforated with numerous sieve pores.

韧皮部由筛管分子、伴胞、韧皮纤维和韧皮薄壁细胞组成。输导细胞是筛管分子，它们在成熟时仍然存活，但已失去细胞核、大部分细胞器和液泡膜。它们通过筛板——即布满众多筛孔的端壁——首尾相连。

Each sieve tube element is intimately associated with one or more companion cells, connected by numerous plasmodesmata. The companion cell retains a full complement of organelles — a large nucleus, abundant mitochondria, and dense ribosomes — and supports the metabolic needs of the enucleate sieve tube element.

每个筛管分子与一个或多个伴胞紧密相连，通过大量胞间连丝相通。伴胞保留完整的细胞器——大细胞核、丰富的线粒体和密集的核糖体——为无核的筛管分子提供代谢支持。

This structural interdependence is so critical that if a companion cell dies, the associated sieve tube element also dies. The ratio of companion cells to sieve tube elements varies; in some species, a single companion cell may serve multiple sieve elements.

这种结构上的相互依赖至关重要：如果伴胞死亡，相关的筛管分子也会死亡。伴胞与筛管分子的比例因物种而异；在某些物种中，一个伴胞可服务于多个筛管分子。

4. Water Movement in Xylem — Cohesion-Tension Theory | 木质部水分运动——内聚力-张力学说

The cohesion-tension theory explains how water rises to the tops of even the tallest trees. The driving force is transpiration: water evaporates from mesophyll cell walls into intercellular spaces and diffuses out through stomata. This evaporation creates a water potential gradient that draws water from xylem into surrounding cells.

内聚力-张力学说解释了水如何能上升到最高树木的顶端。驱动力是蒸腾作用：水从叶肉细胞壁蒸发进入细胞间隙，并通过气孔扩散出去。这种蒸发形成水势梯度，将水从木质部吸入周围细胞。

As water leaves the xylem, tension (negative pressure) develops within the columns of water. Water molecules are held together by hydrogen bonds — the property of cohesion. This cohesive strength is remarkable; water columns can withstand tensions exceeding 20 MPa before breaking.

当水离开木质部时，管道内的水柱产生张力（负压）。水分子通过氢键相互结合——这就是内聚力。这种内聚强度非常惊人；水柱在断裂前可承受超过 20 MPa 的张力。

Additionally, water adheres to the hydrophilic lignin and cellulose of xylem walls, a property known as adhesion. The combination of cohesion and adhesion maintains a continuous water column.

此外，水与木质部壁的亲水性木质素和纤维素相互附着，这一性质称为附着力。内聚力与附着力的结合维持了连续的水柱。

ψ = ψₛ + ψₚ (water potential = solute potential + pressure potential)

At the root, water enters root hairs by osmosis and moves via the apoplast, symplast, and transmembrane pathways to reach the xylem. Root pressure, caused by active mineral ion secretion into xylem, can contribute to water movement but is insufficient to explain tall trees.

在根部，水通过渗透作用进入根毛，并经由共质体、质外体和跨膜途径到达木质部。根压由木质部中矿物质离子的主动分泌引起，可辅助水分运动，但不足以解释高大树木的水分上升。

5. Transpiration and Limiting Factors | 蒸腾作用及其限制因素

Transpiration is defined as the loss of water vapour from aerial parts of plants, mainly through stomata. It is often described as a ‘necessary evil’ — necessary because it drives water transport and mineral distribution, and cools leaves; evil because excessive water loss can cause wilting and death.

蒸腾作用定义为植物地上部分水蒸气的散失，主要通过气孔进行。它常被描述为’必要的祸害’——必要是因为它驱动水分运输和矿物质分布，并冷却叶片；有害是因为过度失水会导致萎蔫甚至死亡。

Four environmental factors significantly affect the transpiration rate: light intensity, temperature, humidity, and wind speed. Higher light intensity causes stomata to open, increasing transpiration. Higher temperature increases the rate of evaporation and reduces relative humidity near the leaf. Higher humidity reduces the water potential gradient between leaf and air. Wind removes the boundary layer of humid air around the leaf, increasing the rate of diffusion.

四个环境因素显著影响蒸腾速率：光照强度、温度、湿度和风速。更高的光照强度使气孔张开，增加蒸腾。更高的温度提高蒸发速率并降低叶片附近相对湿度。更高的湿度减小叶片与空气之间的水势梯度。风移除了叶片周围的潮湿边界层，增加了扩散速率。

Factor 因素	Increase in factor 因素增强	Effect on transpiration 对蒸腾的影响
Light intensity 光照强度	Stomata open wider 气孔张开更大	Increase 增加
Temperature 温度	More evaporation 蒸发增强	Increase 增加
Humidity 湿度	Smaller gradient 梯度减小	Decrease 减少
Wind speed 风速	Boundary layer removed 边界层移除	Increase 增加

The potometer is a classic instrument used to measure water uptake by a cut shoot, which serves as an indirect measure of transpiration rate. In CIE exams, you may be asked to design experiments using a potometer and to identify sources of error, such as air bubbles or poor sealing.

蒸腾计是一种经典仪器，用于测量离体枝条的水分吸收，作为蒸腾速率的间接测量。在 CIE 考试中，你可能会被要求设计使用蒸腾计的实验，并识别误差来源，如气泡或密封不良。

6. Translocation in Phloem | 韧皮部的运输作用

Translocation is the movement of organic solutes, primarily sucrose, through the phloem from sources to sinks. A source is any region that produces or releases assimilates — mature leaves, germinating seeds, or storage organs in spring. A sink is any region that consumes or stores assimilates — growing roots, developing fruits, young leaves, or storage tissues.

运输作用是指有机溶质（主要是蔗糖）通过韧皮部从源到库的移动。源是任何产生或释放同化物的区域——成熟叶片、萌发的种子或春季的贮藏器官。库是任何消耗或储存同化物的区域——生长中的根、发育中的果实、幼叶或贮藏组织。

At different developmental stages, organs can switch between being sources and sinks. For example, a leaf is a sink during early development but becomes a source once it is fully expanded and photosynthetically active. Storage organs like tubers are sinks during growth but sources during sprouting.

在不同的发育阶段，器官可以在源和库之间转换。例如，叶在早期发育中是库，但一旦完全展开并具备光合作用能力后便成为源。块茎等贮藏器官在生长期间是库，但在萌发期间则成为源。

Sucrose, rather than glucose, is the primary transport sugar. The advantages are numerous: sucrose is non-reducing and therefore less reactive; it is more stable and less likely to be metabolised during transit; and it provides a more concentrated carbon transport form without affecting osmotic pressure as severely.

蔗糖而非葡萄糖是主要的运输糖。其优点众多：蔗糖是非还原糖，因此反应性较低；它更稳定，不易在运输途中被代谢；并且它提供了更浓缩的碳运输形式，同时不会如此严重地影响渗透压。

7. The Mass Flow Hypothesis | 集流假说

The mass flow (pressure flow) hypothesis, proposed by Münch in 1930, is the most widely accepted explanation for phloem transport. The mechanism operates as follows:

集流（压力流）假说由明希于 1930 年提出，是韧皮部运输最被广泛接受的解释。其机制如下：

At the source, sucrose is actively loaded into sieve tube elements, either directly from mesophyll cells via plasmodesmata (symplastic loading) or from the apoplast via active transport (apoplastic loading).

在源端，蔗糖被主动装载进入筛管分子，要么通过胞间连丝直接从叶肉细胞进入（共质体装载），要么通过主动运输从质外体进入（质外体装载）。
This accumulation lowers the water potential in the sieve tube, causing water to enter by osmosis from the adjacent xylem. This creates a high hydrostatic pressure at the source end.

这种积累降低了筛管内的水势，导致水通过渗透作用从邻近的木质部进入。这在源端产生高流体静压。
At the sink, sucrose is actively unloaded and metabolised or stored, raising the water potential. Water therefore leaves the sieve tube, reducing the hydrostatic pressure at the sink end.

在库端，蔗糖被主动卸出并代谢或储存，水势升高。因此水离开筛管，降低了库端的流体静压。
The pressure difference between source and sink drives mass flow of phloem sap through sieve tubes down the pressure gradient.

源端与库端之间的压力差驱动韧皮部汁液沿压力梯度通过筛管进行集流。

sucrose loading → water enters by osmosis → ↑ pressure at source → mass flow → ↓ pressure at sink → sucrose unloading

Supporting evidence includes: aphid stylet experiments that show sap under positive pressure; the presence of high sucrose concentrations in phloem sap; the rapid flow rates observed; and the fact that inhibitors of respiration at the source block translocation.

支持性证据包括：蚜虫口针实验显示筛管汁液处于正压下；韧皮部汁液中存在高浓度蔗糖；观察到的快速流速；以及源端呼吸抑制剂会阻断运输这一事实。

8. Loading and Unloading of Sucrose | 蔗糖的装载与卸出

Apoplastic loading is an energy-dependent process. Sucrose is actively transported into companion cells via H⁺/sucrose cotransporters. First, an H⁺-ATPase in the plasma membrane pumps H⁺ out of the companion cell, creating an electrochemical gradient. Then, sucrose from the apoplast is co-transported inward with H⁺ down this gradient.

质外体装载是一个依赖能量的过程。蔗糖通过 H⁺/蔗糖共转运蛋白被主动运输进入伴胞。首先，质膜上的 H⁺-ATPase 将 H⁺ 泵出伴胞，形成电化学梯度。然后，来自质外体的蔗糖与 H⁺ 一起沿该梯度共转运进入细胞内。

Symplastic loading relies on diffusion of sucrose through plasmodesmata from mesophyll cells to sieve elements. However, for this to work efficiently, sucrose must be converted to larger molecules such as raffinose or stachyose, which are too large to diffuse back through the narrow plasmodesmata, effectively trapping them in the phloem.

共质体装载依赖蔗糖通过胞间连丝从叶肉细胞扩散到筛管分子。然而，要使此过程高效运行，蔗糖必须转化为更大的分子如棉子糖或水苏糖，这些分子太大而无法通过狭窄的胞间连丝扩散回去，从而有效地将它们锁定在韧皮部中。

At the sink, unloading can occur symplastically through plasmodesmata or apoplastically against a concentration gradient, requiring energy. In many sinks, sucrose is hydrolysed by invertase to glucose and fructose, maintaining a low sucrose concentration that favours further unloading.

在库端，卸出可通过胞间连丝共质体进行，或逆浓度梯度质外体进行，后者需要能量。在许多库中，蔗糖被转化酶水解为葡萄糖和果糖，保持低蔗糖浓度以利于进一步卸出。

9. Structural Adaptations for Transport | 运输的结构适应性

Xylem vessels exhibit remarkable adaptations: lignified walls provide structural support against tension; the absence of end walls and protoplasts reduces resistance to flow; pits allow lateral water movement; and the various wall-thickening patterns combine flexibility with strength.

木质部导管展现出非凡的适应性：木质化壁提供对抗张力的结构支撑；缺乏端壁和原生质体降低了流动阻力；纹孔允许横向水分运动；不同类型的壁加厚模式兼具柔韧性与强度。

Phloem adaptations are equally elegant: sieve plates with wide pores permit efficient flow while allowing rapid blockage via callose deposition; the sieve tube element’s reduced cytoplasm minimises resistance; companion cells supply ATP and proteins; and P-proteins in some species seal damaged sieve tubes to prevent sap loss.

韧皮部的适应性同样精妙：具有宽孔的筛板允许高效流动，同时通过胼胝质沉积实现快速堵塞；筛管分子减少的细胞质将阻力降至最低；伴胞提供 ATP 和蛋白质；某些物种中的 P-蛋白可封闭受损的筛管以防止汁液流失。

Both tissues work together structurally: xylem provides the water that phloem requires for pressure-driven flow, and phloem supplies the organic carbon that xylem-forming cells need for growth and lignification.

两种组织在结构上协同工作：木质部提供韧皮部压力流所需的水，韧皮部则供应木质部形成细胞生长和木质化所需的有机碳。

10. Experimental Evidence and Investigative Techniques | 实验证据与调查技术

Several classic experiments support our understanding of phloem transport. Ringing experiments — removing a ring of bark (phloem) from a woody stem — result in swelling of the bark above the ring due to accumulated assimilates, demonstrating that phloem carries organic nutrients downward and upward.

几个经典实验支持了我们对韧皮部运输的理解。环割实验——从木本茎上移除一圈树皮（韧皮部）——导致环割上方树皮因同化物积累而肿胀，证明韧皮部向上和向下运输有机营养物质。

Aphid stylet experiments are particularly informative. When an aphid’s stylet is cut from its body while embedded in phloem, sap exudes from the severed stylet under positive pressure. This sap can be collected and analysed, confirming high sucrose concentrations and low hydrostatic pressures in sieve tubes.

蚜虫口针实验特别有信息量。当嵌入韧皮部的蚜虫口针从虫体切断后，汁液在正压下从断端渗出。这些汁液可以被收集和分析，证实筛管中蔗糖浓度高而流体静压较低。

Radioisotope tracing using ¹⁴C-labelled carbon dioxide allows researchers to track the movement of photosynthates from leaves to sinks. Autoradiography reveals the transport pathways, and scintillation counting quantifies translocation rates.

使用 ¹⁴C 标记的二氧化碳进行放射性同位素示踪，使研究者能够追踪光合产物从叶片到库的运动。放射自显影揭示运输路径，闪烁计数则可量化运输速率。

In CIE practical exams, you should be familiar with the use of microscopy to identify xylem and phloem in stem and root cross-sections. Key features to look for include the thick, lignified walls of xylem vessels (which appear red with phloroglucinol staining or blue with toluidine blue) and the thin-walled cells of phloem.

在 CIE 实验考试中，你应当熟悉使用显微镜识别茎和根横切面中的木质部与韧皮部。需要观察的关键特征包括木质部导管厚而木质化的壁（用间苯三酚染色呈红色，用甲苯胺蓝染色呈蓝色）以及韧皮部的薄壁细胞。

11. Comparison: Xylem vs Phloem | 木质部与韧皮部对比

Feature 特征	Xylem 木质部	Phloem 韧皮部
Direction of transport 运输方向	Roots → leaves (upwards only) 根 → 叶（仅向上）	Source → sink (multi-directional) 源 → 库（多方向）
Main transported substance 主要运输物质	Water and mineral ions 水和矿质离子	Sucrose, amino acids, hormones 蔗糖、氨基酸、激素
Conducting cells at maturity 成熟时的输导细胞	Dead (no cytoplasm) 死亡（无细胞质）	Alive but enucleate 存活但无核
Main driving force 主要驱动力	Transpiration (physical) 蒸腾（物理过程）	Pressure gradient (osmotic) 压力梯度（渗透过程）
Cell wall 细胞壁	Lignified, thick 木质化、厚壁	Cellulosic, thin 纤维素、薄壁
End walls 端壁	Perforation plates (absent/partial) 穿孔板（完全或部分消失）	Sieve plates 筛板
Energy requirement 能量需求	Passive (no ATP in transport) 被动（运输不需 ATP）	Active loading/unloading requires ATP 主动装载/卸出需 ATP
Associated supportive cells 相关支持细胞	None (dead cells self-supporting) 无（死细胞自我支撑）	Companion cells 伴胞

Understanding these differences is essential not only for exams but also for applied fields such as agriculture, where knowledge of vascular tissue function informs practices like grafting, ring-barking in fruit production, and the use of herbicides.

理解这些差异不仅在考试中至关重要，在应用领域同样如此，例如在农业中，维管组织功能的知识指导着嫁接、果树生产中的环剥以及除草剂的使用等实践。

12. Common Misconceptions and Exam Tips | 常见误解与考试提示

A frequent error is stating that xylem transports water ‘upwards only’. While the bulk flow is indeed upward, mineral ions can be redistributed laterally and even downward through xylem in some circumstances. More importantly, remember that transpiration stream direction is always from roots to leaves.

一个常见错误是说木质部’仅向上’运输水分。虽然集流确实向上，但在某些情况下矿质离子可以通过木质部横向甚至向下重新分布。更重要的是要记住，蒸腾流的方向始终是从根到叶。

Another misconception is that phloem sap flows in only one direction. In reality, individual sieve tubes may conduct in different directions depending on the relative positions of sources and sinks. However, within any given sieve tube at a particular time, flow is unidirectional from source to sink.

另一个误解是韧皮部汁液只朝一个方向流动。实际上，不同的筛管可能根据源和库的相对位置向不同方向输送。然而，在特定时间内任何给定的筛管内，流动是从源到库的单向流动。

For exam success, remember these key points:

为在考试中取得好成绩，请记住这些关键点：

Xylem transport is entirely passive, but the formation of xylem (secondary wall deposition, lignification) requires active processes and energy.

木质部运输是完全被动的，但木质部的形成（次生壁沉积、木质化）需要主动过程和能量。
Phloem loading and unloading always require energy; the mass flow itself is passive once the pressure gradient is established.

韧皮部装载和卸出始终需要能量；一旦压力梯度建立，集流本身是被动的。
Water potential equations and the direction of osmosis are recurring exam themes — practise applying ψ = ψₛ + ψₚ to sieve tube scenarios.

水势方程和渗透方向是反复出现的考试主题——练习将 ψ = ψₛ + ψₚ 应用于筛管情境。
Be precise with terminology: ‘transpiration’ is water loss, ‘translocation’ is solute transport, ‘mass flow’ is the mechanism.

术语要精确：’蒸腾’是水分流失，’运输’是溶质移动，’集流’是机制。

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Xylem and Phloem: The Two Transport Systems of Plants | 木质部与韧皮部：植物两大运输系统