Two main types of plant tissue are used in transport - xylem and phloem. Xylem transports water and minerals. Phloem transports organic molecules such as the products of photosynthesis.
有两种主要的植物组织用于运输 - 木质部和韧皮部。木质部运输水和矿物质。韧皮部运输有机分子，如光合作用的产物。
Xylem 木质部
There are four types of xylem cells 有四种类型的木质部细胞:
- Xylem vessels 木质部血管: Consist of dead hollow cells because the walls are lignified and the cell contents disintegrate. The lignin makes the cell wall impermeable so they are in effect waterproof. It also makes the vessels extremely strong and prevents them from collapsing. They have a wide lumen and are linked end to end to create a long, hollow tube since the end cell walls have one or many perforations in them. This allows the transport of large volumes of water. The sidewalls have bordered pits (unlignified areas) to allow lateral movement of water. Xylem vessels are found in angiosperms. 由死亡的空心细胞组成，因为细胞壁被木质化，细胞内容物被瓦解。木质素使细胞壁不透气，因此它们实际上是防水的。它还使血管非常坚固，防止它们倒塌。它们有一个宽大的腔体，并端对端地连接起来，形成一个长长的空心管，因为末端的细胞壁上有一个或多个穿孔。这允许运输大量的水。侧壁有带边的凹坑（未木质化的区域），允许水的横向移动。木质部血管见于被子植物。
- Tracheids 管胞: Similar to vessels but with narrower lumens and connected by pits. They have tapered ends so that they dovetail together. Tracheids are found in conifers. 与血管相似，但管腔较窄，由凹坑连接。它们的两端是锥形的，因此它们可以对接在一起。管胞存在于针叶树中。
- Parenchyma 实质部: Living cells with thin cellulose walls. They can store water, which makes them turgid and so gives them a supporting role. 具有薄纤维素壁的活细胞。它们可以储存水分，这使它们具有膨胀性，因此使它们具有支持作用。
- Fibres 纤维: They provide strength because their walls are lignified (and therefore, dead). 它们提供强度，因为它们的壁是木质化的（因此是死的）。
Movement in the root 根中的运动
Water enters through the root hair cells and then moves across into the xylem tissue in the centre of the root. Water moves in this direction because the soil water has higher water potential, than the solution inside the root hair cells.
水通过根毛细胞进入，然后在根的中心移动到木质部组织。水向这个方向移动是因为土壤中的水比根毛细胞内的溶液具有更高的水势。
This is because the cell sap has organic and inorganic molecules dissolved in it. The root hairs provide a large surface area over which water can be absorbed.
这是因为细胞液中溶解了有机和无机分子。根毛提供了一个大的表面积，水可以在上面被吸收。
Minerals are also absorbed but, as you should be able to work out, their absorption requires energy in the form of ATP because they are absorbed by active transport. They have to be pumped against the concentration gradient.
矿物质也被吸收，但正如你应该能够想到的，它们的吸收需要ATP形式的能量，因为它们是通过主动运输吸收的。它们必须逆着浓度梯度进行泵送。
Water taken up by the root hairs moves across the cortex of the root either via the cytoplasm of the cells in between the root hair cell and the xylem (the symplast pathway) or through the cell walls of these cells (the apoplast pathway). The root hair cell will have higher water potential than the cell next to it. As always, water moves by osmosis to where the water potential is lower. In this way, as water is always being absorbed by the root hairs, water will always move towards the centre of the root.
根毛吸收的水通过根毛细胞和木质部之间的细胞质（症状体途径）或通过这些细胞的细胞壁（凋亡体途径）在根的皮层中移动。根毛细胞的水势会比它旁边的细胞高。一如既往，水通过渗透作用移动到水势较低的地方。这样一来，由于水总是被根毛吸收，水总是向根的中心移动。
When the water reaches a part of the root called the endodermis, it encounters a thick, waxy band of suberin in the cell walls. This is the Casparian strip and it is impenetrable. In order to cross the endodermis, the water that has been moving through the cell walls must now move into the cytoplasm.
当水到达根的一部分，称为内皮层时，它遇到了细胞壁中厚厚的、蜡质的亚皮素带。这就是卡斯帕里安带，它是不可穿越的。为了穿越内皮层，一直在细胞壁中移动的水现在必须进入细胞质。
Once it has moved across the endodermis, it continues down the water potential gradient until it reaches a pit in the xylem vessel. It enters the vessel and then moves up towards the leaves.
一旦它穿过内皮层，它就继续沿着水势梯度向下移动，直到到达木质部血管中的一个坑。它进入血管，然后向上移动到叶片。
Movement in the xylem 木质部中的运动
Water evaporates from the mesophyll cells into air spaces in the leaf. If the air surrounding the leaf has less water vapour than the air in the intercellular spaces, water vapour will leave the leaf through stomata.
水从中叶细胞蒸发到叶片的空气空间。如果叶片周围的空气中的水蒸气少于细胞间的空气，水蒸气就会通过气孔离开叶片。
This process is called transpiration and will continue as long as the stomata are open and the air outside is not too humid. On dry, windy days when water vapour is continually diffusing out and being removed, transpiration will increase in rate.
这个过程被称为蒸腾作用，只要气孔打开，外面的空气不太潮湿，这个过程就会持续下去。在干燥、多风的日子里，当水蒸气不断向外扩散并被清除时，蒸腾作用将增加。
Although this loss of water can cool the plant, it is essential that the plant does not lose too much water. Therefore water must be continuously supplied to the leaves. The xylem ensures that this happens. Xerophytes are plants which are well adapted to living where conditions are very dry. They may have rolled up leaves - for example, Marram grass which exposes the waterproof cuticle on the outside and means the stomata open into an inner humid space. Other Xerophytes store water in their stems and reduce the surface area of their leaves, which become spines - for example, Cactus.
虽然这种失水可以冷却植物，但植物不能失水太多，这是至关重要的。因此，必须不断向叶子提供水。木质部确保这一点。旱生植物是很适合生活在非常干燥的环境中的植物。它们的叶子可能是卷起来的，例如马兰草，它暴露了外面的防水角质层，意味着气孔打开，进入内部潮湿的空间。其他旱生植物将水储存在它们的茎中，并减少它们的叶子的表面积，使之成为刺，例如仙人掌。
Water is removed from the top of xylem vessels into the mesophyll cells down the water potential gradient. This removal of water from the xylem reduces the hydrostatic pressure exerted by the liquid so the pressure at the top is less than at the bottom. This pushes the water up the tube. The surface tension of the water molecules, the thin lumen of the xylem vessels and the attraction of the water molecules for the xylem vessel wall (adhesion), helps to keep the water flowing all the time and to keep the water column intact.
水从木质部血管的顶部沿着水势梯度进入中叶细胞。从木质部移走的水减少了液体施加的静水压力，因此顶部的压力小于底部的压力。这就把水推到了管子上。水分子的表面张力、木质部血管的薄腔以及水分子对木质部血管壁的吸引力（附着力），有助于保持水一直在流动，保持水柱的完整。
Pressure to push water up can also be increased from the bottom. By actively pumping minerals from cells surrounding the xylem into the xylem itself, more water is drawn into the xylem by osmosis.
推动水上升的压力也可以从底部增加。通过主动将木质部周围细胞中的矿物质泵入木质部本身，更多的水通过渗透作用被吸入木质部。
This increase in water pressure, called root pressure, certainly helps in the process but is less important than the simple movement of water down the water potential gradient, ultimately from the soil at the bottom, to the air at the top. This is because moving water this way does not require energy (it is passive).
这种水压的增加，称为根压，当然有助于这一过程，但不如水沿着水势梯度的简单运动重要，最终从底部的土壤到顶部的空气。这是因为以这种方式移动水不需要能量（它是被动的）。
Phloem 韧皮部
There are four types of phloem cells 韧皮部细胞有四种类型:
- Sieve tube elements 筛管元件: These are living, tubular cells that are connected end to end. The end cell walls have perforations in them to make sieve plates. The cytoplasm is present but in small amounts and in a layer next to the cell wall. It lacks a nucleus and most organelles so there is more space for solutes to move. The cell walls are made of cellulose so solutes can move laterally a well as vertically. Next to each sieve tube element is a companion cell. 这些是活的、管状的细胞，端与端相连。端部的细胞壁上有穿孔，形成筛板。细胞质是存在的，但数量很少，而且在细胞壁旁边的一层。它缺乏细胞核和大多数细胞器，因此有更多空间供溶质移动。细胞壁是由纤维素制成的，因此溶质可以横向和纵向移动。每个筛管元件的旁边是一个伴生细胞。
- Companion cell 配套细胞: Since the sieve tube element lacks organelles, the companion cell with its nucleus, mitochondria, ribosomes, enzymes etc., controls the movement of solutes and provides ATP for active transport in the sieve tube element. Strands of cytoplasm called plasmodesmata connect the sieve tube element and companion cell. 由于筛管元件缺乏细胞器，伴生细胞及其细胞核、线粒体、核糖体、酶等控制溶质的移动，并为筛管元件的主动运输提供ATP。称为质膜的几条细胞质线连接着筛管元件和伴生细胞。
- Parenchyma 实质部: Provides support through turgidity. 通过蠕动提供支持。
- Fibres 纤维: Provides support for the sieve tube elements. 为筛管元件提供支持。
Movement in the phloem 在韧皮部的运动
This process is called translocation and involves the movement of organic substances around the plant. It requires energy to create a pressure difference and so is considered an active process. 这一过程被称为转位，涉及到有机物质在植物周围的移动。它需要能量来产生压力差，因此被认为是一个主动过程。
Sucrose is loaded into the phloem at a source, usually a photosynthesizing leaf. For this to occur, hydrogen ions are pumped out of the companion cell using ATP. This creates a high concentration of hydrogen ions outside the companion cell. Sucrose is loaded (moved into companion cells) by active transport, against the concentration gradient.
蔗糖在一个来源处被装入韧皮部，通常是光合作用的叶片。要做到这一点，需要用ATP把氢离子从伴生细胞中抽出来。这在伴生细胞外产生了高浓度的氢离子。蔗糖通过主动运输被装入（移入伴生细胞），对抗浓度梯度。
However, the protein carrier involved in the loading, has two sites, one for sucrose and one for a hydrogen ion. When it is used to pump sucrose into the companion cell, hydrogen will move in the opposite direction, back down its concentration gradient. This is why a high concentration of ions is needed outside the cell.
然而，参与装载的蛋白质载体有两个位点，一个用于蔗糖，一个用于氢离子。当它被用来将蔗糖泵入伴生细胞时，氢气将向相反的方向移动，回到其浓度梯度上。这就是为什么需要在细胞外有高浓度的离子。
The sucrose can then diffuse down the concentration gradient into the sieve tube element via the plasmodesmata that connects the companion cell with the sieve tube element. This lowers the water potential of the sieve element so water enters by osmosis.
然后蔗糖可以通过连接伴生细胞和筛管元件的质膜沿着浓度梯度扩散到筛管元件中。这降低了筛管元件的水势，使水通过渗透作用进入。
At another point sucrose will be unloaded from the phloem into a sink (e.g. root). It is likely that the sucrose moves out by diffusion and is then converted into another substance to maintain a concentration gradient. Again, water will follow by osmosis.
在另一个点上，蔗糖将从韧皮部卸下，进入一个水槽（如根）。蔗糖很可能通过扩散移出，然后转化为另一种物质以保持浓度梯度。同样，水将通过渗透作用跟随。
This loading and unloading results in the mass flow of substances in the phloem. There is evidence to support this theory; the rate of flow in the phloem is about 10,000 times faster than it would be if it was due only to diffusion, the pH of the phloem sap is around 8 (it is alkaline due to loss of hydrogen ions), and there is an electrical potential difference across the cell surface (negative inside due presumably to the loss of positively charged ions).
这种装载和卸载的结果是在韧皮部的物质的质量流动。有证据支持这一理论；韧皮部的流动速度比仅由扩散引起的速度快约1万倍，韧皮部汁液的pH值约为8（由于氢离子的损失，它是碱性的），并且在细胞表面有一个电势差（据推测由于正电离子的损失，里面是负的）。