📚 Xylem and Phloem: The Two Transport Systems in Flowering Plants | 木质部与韧皮部：开花植物的两大运输系统
Flowering plants need to move water, mineral ions and organic assimilates over long distances between roots, stems and leaves. The xylem and phloem are the two transport tissues that make this possible. Xylem mainly carries water and dissolved minerals from the roots upward, while phloem carries sugars and other organic products from sources to sinks.
开花植物需要将水、矿物质离子和有机同化产物在根、茎、叶之间进行长距离运输。木质部和韧皮部就是实现这一功能的两大运输组织。木质部主要负责把水和溶解的矿物质从根部向上运输，韧皮部则把糖类及其他有机产物从源运输到库。
1. Why Plants Need Two Transport Systems | 为什么植物需要两套运输系统
Diffusion is only effective over very short distances. A leaf may sit several metres above the root, and storage organs may be far from photosynthetic tissues. Plants therefore need mass transport systems that move water, minerals and assimilates rapidly through specialised tubes driven by pressure gradients rather than by random molecular movement.
扩散只在很短的距离内有效。叶片可能位于根部上方数米处，贮存器官也可能远离光合组织。因此植物需要集流运输系统，通过特化的管状结构、依靠压力梯度而非分子随机运动，快速运输水、矿物质和同化产物。
The two systems solve different problems. Xylem must supply the enormous water loss from leaves during transpiration, so it is built as a low-resistance pipeline made of dead, hollow cells. Phloem must deliver specific solutes to specific places at specific times, so it is built from living cells that can load and unload sugars actively.
两套系统解决的问题不同。木质部必须补足叶片蒸腾造成的巨大水分损失，因此被建成由死亡、中空细胞构成的低阻力管道。韧皮部必须在特定时间把特定溶质送到特定部位，因此由能够主动装载和卸载糖的活细胞构成。
The rate of any transport process depends on the surface area available, the concentration gradient and the distance over which movement occurs. Mass flow through a tube follows a simple relationship:
任何运输过程的速率都取决于可利用的表面积、浓度梯度和运输距离。通过管道的集流可用一个简单关系表示：
rate of mass flow ∝ (cross-sectional area × pressure gradient) ÷ path length
A wide lumen, a steep pressure gradient and a short path all increase flow. This is why xylem vessel elements have a wide lumen and no end walls, and why phloem sieve tubes are arranged end to end with perforated sieve plates.
宽大的管腔、陡峭的压力梯度和较短的路径都会提高流量。这就是木质部导管分子具有宽管腔且没有端壁，而韧皮部筛管以端壁带孔的筛板首尾相接的原因。
2. Xylem and Phloem at a Glance | 木质部与韧皮部速览
Before studying the detail, it helps to compare the two tissues side by side. The table below summarises the features most often examined.
在学习细节之前，先把两种组织并排对比会很有帮助。下表总结了最常考的特征。
| Feature 特征 | Xylem 木质部 | Phloem 韧皮部 |
| Cell state 细胞状态 | Dead at maturity, no cytoplasm 成熟时死亡，无细胞质 | Living at maturity 成熟时仍存活 |
| Cell types 细胞类型 | Vessel elements, tracheids, fibres, parenchyma 导管分子、管胞、纤维、薄壁细胞 | Sieve tube elements, companion cells, fibres, parenchyma 筛管分子、伴胞、纤维、薄壁细胞 |
| End walls 端壁 | Absent or perforated 缺失或穿孔 | Present as sieve plates 以筛板形式存在 |
| Walls 细胞壁 | Thick, lignified (lignin) 厚、木质化 | Thin, cellulose, not lignified 薄、纤维素、非木质化 |
| Main cargo 主要货物 | Water and mineral ions 水和矿物质离子 | Sucrose, amino acids, hormones 蔗糖、氨基酸、激素 |
| Direction 方向 | Unidirectional (roots to shoots) 单向（根到地上部） | Bidirectional (source to sink) 双向（源到库） |
| Driving force 驱动力 | Transpiration pull, tension 蒸腾拉力、张力 | Hydrostatic pressure gradient 静水压力梯度 |
Notice that the two tissues also differ in position. In a dicot stem, xylem lies towards the centre and phloem lies towards the outside, separated by the vascular cambium, which is why removing bark removes phloem but leaves the xylem intact.
注意，两者在位置上也不同。在双子叶植物茎中，木质部靠近中心，韧皮部靠近外侧，二者之间是维管形成层。因此剥去树皮会除去韧皮部，而木质部仍然完整。
3. Xylem: Structure and Adaptation | 木质部：结构与适应
Xylem is a complex tissue containing vessel elements, tracheids, fibres and xylem parenchyma. Vessel elements are the widest and most efficient conducting cells in angiosperms. Each one is elongated, has lignified secondary walls and loses its end walls completely, so a column of vessel elements forms a continuous open tube called a vessel.
木质部是一种复合组织，包含导管分子、管胞、纤维和木质部薄壁细胞。导管分子是被子植物中最宽、效率最高的输导细胞。每个细胞细长、具木质化次生壁，端壁完全消失，因此一列导管分子连成一条称为导管的开放管道。
Tracheids are narrower cells that retain tapered, overlapping end walls and conduct water through pits rather than through open perforations. They are the main conducting cells in gymnosperms such as pine, and because they are narrower, adhesion to the walls is proportionally more important in them.
管胞较窄，保留渐尖且相互重叠的端壁，水通过纹孔而不是开放穿孔流动。管胞是松树等裸子植物的主要输导细胞。由于管胞较窄，水与壁之间的附着力在其中相对更重要。
Lignin deposited in the secondary wall has three key effects. It makes the wall rigid and impermeable, it reinforces the tube against the tension generated by transpiration, and it prevents the tube collapsing inward. Lignin thickening may be spiral, ring-shaped, reticulate or pitted.
次生壁中沉积的木质素有三个关键作用：使细胞壁坚硬且不透水；增强管道以抵抗蒸腾产生的张力；防止管道向内塌陷。木质素加厚可呈螺旋状、环状、网状或具纹孔。
Because vessel elements die before they function, there is no cytoplasm, no nucleus and no cell membrane to resist flow. Their wide lumen and missing end walls give the lowest possible resistance to mass flow, which is exactly what a plant that may lose hundreds of litres of water a day requires.
由于导管分子在行使功能前已经死亡，其中没有细胞质、没有细胞核、也没有细胞膜阻碍流动。宽大的管腔和消失的端壁使集流的阻力尽可能低，这正是每天可能散失数百升水的植物所需要的。
Xylem also provides mechanical support. The lignified walls of vessels and tracheids, together with xylem fibres, are a major part of the strength of woody stems, which is why a plant wilted by water loss regains its upright position once the xylem refills.
木质部还提供机械支撑。导管和管胞的木质化壁以及木质部纤维构成了木本茎强度的主要来源。因此，因失水而萎蔫的植物在木质部重新充水后会恢复挺立。
4. Moving Water Up: The Cohesion-Tension Theory | 水分上行：内聚力-张力学说
Water is pulled up the xylem, not pushed, in the cohesion-tension mechanism. The process begins when mesophyll cells lose water by evaporation into the air spaces inside the leaf. Water vapour then diffuses out of the leaf through stomata, a loss called transpiration.
在内聚力-张力机制中，水是被向上"拉"而不是被"推"上去的。过程始于叶肉细胞的水蒸发进入叶内空气腔。随后水蒸气通过气孔扩散出叶片，这种损失称为蒸腾。
As water evaporates from the curved surfaces of the cell walls of mesophyll cells, it forms menisci whose curvature generates tension, or negative pressure. This tension is transmitted along the continuous columns of water in the xylem, all the way down to the roots, because water molecules are held together by hydrogen bonds.
当水从叶肉细胞壁的弯曲表面蒸发时，形成弯月面，其曲率产生张力（即负压）。由于水分子之间由氢键相连，这种张力沿着木质部中连续的水柱一直传递到根部。
Two properties of water make this possible. Cohesion is the attraction between water molecules that keeps the column unbroken, and adhesion is the attraction between water molecules and the hydrophilic walls of the xylem, which helps hold the column in place against gravity and assists capillary movement in narrow tubes.
水的两个性质使这成为可能。内聚力是水分子之间的吸引力，使水柱保持不断；附着力是水分子与木质部亲水壁之间的吸引力，有助于水柱在重力下保持位置，并促进细管中的毛细运动。
The mechanism can be summarised as a chain of causes and effects:
这一机制可归结为一条因果链：
in xylem water column → cohesion pulls water up → water enters root → transpiration stream
The water potential gradient that drives the whole system is generated by evaporation, so transpiration and xylem transport are two aspects of one process. Upward flow is fastest at midday, when stomata are open, temperature is high and the air is dry.
驱动整个系统的水势梯度由蒸发产生，因此蒸腾与木质部运输是同一过程的两个方面。正午气孔开放、温度高、空气干燥时，上行流速最快。
Xylem sap is under tension and is therefore in a metastable state. If the column breaks, air enters and forms an embolism, blocking the vessel. Plants limit this by having many narrow vessels, which contain the damage, and by closing stomata when water stress becomes severe.
木质部汁液处于张力之下，因而处于亚稳状态。如果水柱断裂，空气进入形成栓塞，堵塞导管。植物通过拥有许多狭窄导管来限制栓塞扩散（把损伤局限住），并在水分胁迫严重时关闭气孔。
5. Water and Ion Uptake by Roots | 根对水分和离子的吸收
Most water enters through root hair cells, which are long epidermal extensions that greatly increase the absorbing surface area. The cytoplasm of these cells contains a high concentration of solutes, so water moves in by osmosis down a water potential gradient.
大部分水通过根毛细胞进入。根毛是表皮细胞的细长突起，极大增加了吸收面积。这些细胞的细胞质溶质浓度高，因此水沿水势梯度通过渗透作用进入。
Water travels from the epidermis to the endodermis along three routes. In the apoplast pathway it moves through cell walls and the spaces between cells. In the symplast pathway it moves through the cytoplasm, passing from cell to cell through plasmodesmata. The vacuolar pathway passes through the vacuoles as well.
水从表皮到内皮层有三条途径。质外体途径中，水沿细胞壁和细胞间隙移动；共质体途径中，水通过细胞质，经胞间连丝在细胞之间穿行；液泡途径则还会经过液泡。
The endodermis forms a waterproof band called the Casparian strip, made of suberin, in its radial and transverse walls. The Casparian strip blocks the apoplast pathway, so all water and dissolved ions must pass through a selectively permeable membrane before entering the xylem.
内皮层细胞的径向壁和横向壁上有由木栓质构成的不透水带，称为凯氏带。凯氏带阻断质外体途径，因此所有水和溶解的离子在进入木质部之前都必须通过一层选择透过性膜。
This is the reason the endodermis is described as the control point of the root. Mineral ions such as nitrate, potassium and magnesium are taken in by active transport using carrier proteins and energy from ATP, so the plant can accumulate ions at concentrations higher than those in the soil solution.
这就是内皮层被称为根部控制点的原因。硝酸根、钾离子、镁离子等矿物质由载体蛋白利用 ATP 提供的能量主动运输吸收，因此植物能够在体内积累比土壤溶液浓度更高的离子。
Active ion accumulation lowers the water potential inside the root, drawing in more water and generating a small positive pressure called root pressure. Root pressure is too weak to lift water to the top of a tall tree, but it can cause guttation, the appearance of water droplets at the tips of leaves of small plants at night.
离子的主动积累降低了根内的水势，吸入更多水并产生一种称为根压的小幅正压。根压太弱，不足以把水提升到高大树木的顶端，但它可引起吐水现象，即小型植物夜间叶尖出现水滴。
6. Transpiration and the Factors That Affect It | 蒸腾作用及其影响因素
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It is a passive, inevitable consequence of gas exchange: when stomata open to admit carbon dioxide for photosynthesis, water vapour diffuses out along its own concentration gradient.
蒸腾作用是植物地上部分（主要经由气孔）散失水蒸气的过程。它是气体交换不可避免的被动结果：当气孔打开让二氧化碳进入以进行光合作用时，水蒸气沿自身的浓度梯度扩散出去。
The rate of transpiration depends on several environmental factors. Each one can be explained in terms of the water potential gradient between the leaf air spaces and the outside air, or in terms of stomatal opening.
蒸腾速率取决于多种环境因素。每一点都可以从叶内空气腔与外界空气之间的水势梯度，或从气孔开闭的角度解释。
Light: stomata open in light, so transpiration increases; in darkness most stomata close and the rate falls sharply.
光照：光下气孔开放，蒸腾增强；黑暗中大多数气孔关闭，速率骤降。
Temperature: higher temperature raises the kinetic energy of water molecules and increases the saturation water vapour concentration of the air, so the gradient becomes steeper.
温度：温度升高提高水分子的动能，并增加空气的饱和水蒸气浓度，使梯度更陡。
Humidity: in still, humid air the external water potential is high, the gradient is shallow and transpiration is slow.
湿度：在静止潮湿的空气中，外界水势高，梯度平缓，蒸腾慢。
Wind: moving air removes the boundary layer of humid air around the leaf, maintaining a steep gradient and increasing the rate.
风：流动空气带走叶片周围潮湿的界面层，维持陡峭梯度，从而提高速率。
Soil water availability: dry soil lowers the water potential of the root and eventually causes stomatal closure through the hormone abscisic acid.
土壤水分可利用性：土壤干燥降低根部水势，最终通过激素脱落酸引起气孔关闭。
Transpiration rates are measured with a potometer, which records the volume of water taken up by a cut shoot over time. Strictly, a potometer measures water uptake, which is only an approximation of transpiration, because some water is retained for growth and cell turgor.
蒸腾速率可用蒸腾计测量，它记录离体枝条在一段时间内吸水量的变化。严格来说，蒸腾计测量的是吸水量，只是对蒸腾的近似，因为一部分水被保留用于生长和细胞膨压。
Transpiration is not simply a wasteful process. It provides the driving force for the transport of water and mineral ions, it cools leaves by evaporation, and it maintains cell turgor, which keeps leaves spread out for maximum light absorption.
蒸腾并非单纯的浪费过程。它为水和矿物质离子的运输提供动力，通过蒸发为叶片降温，并维持细胞膨压，使叶片展开以获得最大的光吸收面积。
7. Phloem: Structure and Companion Cells | 韧皮部：结构与伴胞
Phloem is also a complex tissue. Its conducting cells are sieve tube elements, joined end to end with perforated end walls called sieve plates. Sieve tube elements are living cells but they lose their nucleus and most of their organelles as they mature, leaving a thin layer of cytoplasm and a wide lumen for flow.
韧皮部同样是复合组织。其输导细胞为筛管分子，以带孔端壁（筛板）首尾相连。筛管分子是活细胞，但成熟时失去细胞核和大部分细胞器，只留下一薄层细胞质和宽大的管腔供流动。
The loss of the nucleus creates a problem: sieve tube elements cannot make their own proteins or produce enough ATP to load sucrose actively. This problem is solved by companion cells, which lie alongside each sieve tube element and are joined to it by many plasmodesmata.
失去细胞核带来一个问题：筛管分子无法自行合成蛋白质，也无法产生足够 ATP 来主动装载蔗糖。这一问题由伴胞解决。伴胞紧邻每个筛管分子，并通过大量胞间连丝与之相连。
Companion cells have a dense cytoplasm, many mitochondria and numerous ribosomes, so they can supply the sieve tube with ATP, proteins and metabolic support. In many plants the companion cell has folded ingrowths of its wall that increase the surface area for transfer, and is then called a transfer cell.
伴胞细胞质浓稠、线粒体丰富、核糖体众多，因此能为筛管提供 ATP、蛋白质和代谢支持。在许多植物中，伴胞的细胞壁向内折叠形成突起，增加了转运表面积，此时称为转移细胞。
Also present are phloem fibres, which are dead sclerenchyma cells with thick lignified walls that give mechanical strength, and phloem parenchyma, which stores food. Phloem fibres are the reason the phloem-rich inner bark of flax and hemp can be spun into linen and rope.
韧皮部中还有韧皮纤维，即具厚木质化壁的死厚壁组织细胞，提供机械强度；以及储存养分的韧皮薄壁细胞。亚麻和大麻富含韧皮部的内层树皮能纺成亚麻布和绳索，原因正在于韧皮纤维。
The phloem does not carry pure sucrose solution. Phloem sap contains sucrose, amino acids, organic acids, potassium ions, plant hormones such as auxin and gibberellin, and in some plants messenger RNA and proteins that act as long-distance signals.
韧皮部运输的并非纯蔗糖溶液。韧皮部汁液含蔗糖、氨基酸、有机酸、钾离子、生长素和赤霉素等植物激素，在某些植物中还含有起长距离信号作用的信使 RNA 和蛋白质。
8. Translocation and the Pressure Flow Hypothesis | 转运与压力流假说
Translocation is the transport of assimilates in the phloem from source to sink. The widely accepted explanation is the pressure flow hypothesis, also called mass flow, proposed by Ernst Münch in 1930.
转运是指同化产物在韧皮部中由源向库的运输。目前被广泛接受的解释是压力流假说（也称集流假说），由恩斯特·明希于 1930 年提出。
At the source, sucrose is loaded into the sieve tube elements. In the apoplast route, sucrose is actively transported out of the companion cell into the cell wall, then taken back into the sieve tube by a sucrose-proton co-transporter, driven by a proton gradient set up by a proton-pumping ATPase.
在源端，蔗糖被装载进筛管分子。在质外体途径中，蔗糖被主动运出伴胞进入细胞壁，随后由蔗糖-质子共转运蛋白带回筛管，驱动力来自质子泵 ATP 酶建立的质子梯度。
Loading sucrose into the sieve tube lowers the water potential inside it, so water enters by osmosis from the adjacent xylem. The entry of water raises the hydrostatic pressure inside the sieve tube at the source end.
蔗糖被装载进筛管后，管内水势降低，水便从相邻木质部经渗透作用进入。水的进入提高了源端筛管内的静水压力。
At the sink, sucrose is unloaded, either by diffusion or by active transport, into cells that use it for respiration, convert it to starch, or store it. Unloading raises the water potential in the sieve tube, water leaves by osmosis and returns to the xylem, and the hydrostatic pressure falls.
在库端，蔗糖被卸载（通过扩散或主动运输）进入利用它进行呼吸、转化为淀粉或加以贮存的细胞。卸载使筛管内水势升高，水经渗透作用离开并返回木质部，静水压力随之下降。
The result is a pressure gradient between source and sink, and the whole mass of sap flows from the high-pressure source to the low-pressure sink:
结果便是源与库之间形成压力梯度，整股汁液由高压的源流向低压的库：
w down pressure gradient → sucrose unloading at sink → water leaves → low pressure at sink
Typical translocation rates are about 0.5 to 1 metre per hour, though rates up to several metres per hour have been recorded in some species. Transport direction is not fixed by the tissue itself but is determined by the position of sources and sinks at that moment.
转运速率通常约为每小时 0.5 至 1 米，有些物种记录到每小时数米。运输方向并非由组织本身固定，而是由当时源与库的位置决定。
9. Sources and Sinks
广告 / SponsoredAS & A Level Biology Exam Preparation and Practice剑桥国际AS & …生物 备考教辅 · 正版现货 · 与本文内容相关查看详情 / View on Taobao参考价 ¥253.0
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图