0610 · 8.1Xylem and phloem

Transport tissue functions

0:00A bean plant has roots underground and leaves held above the soil. Its roots have access to water and mineral ions, while its leaves make sugars by photosynthesis. But these substances are needed in different parts of the plant. How do they get there? The answer is a transport network running through the roots, stems and leaves. It contains two different tissues, called xylem and phloem. By the end of this video, you will understand what each tissue transports, how the structure of xylem makes it suitable for its jobs, and how to recognise both tissues in sections of a root, stem and leaf.

0:33First, a little background. A tissue is a group of cells working together to carry out particular functions. So xylem and phloem are not individual cells. They are tissues containing specialised cells. Plants need water for processes including photosynthesis. They also need mineral ions, which are dissolved mineral nutrients. Sugars provide material and energy for growth, while amino acids are used to make proteins. Transport connects the places where these substances are available with the places where they are needed.

1:03Let us give each transport tissue its correct job. Xylem transports water and mineral ions. Xylem also provides support. Phloem transports sucrose and amino acids. Those are the precise functions you need to state. Notice that the sugar named for phloem transport is sucrose, not glucose. You already know that photosynthesis produces glucose. Plants can convert this into sucrose for transport. So there is no contradiction between saying that photosynthesis makes glucose and that phloem transports sucrose.

1:34They describe different parts of the plant’s use of sugars. Think of xylem and phloem as two delivery networks carrying different loads. This analogy helps us separate their functions, but remember that these are biological tissues, not pipes installed inside a plant. We will focus on their functions, positions and the structure of xylem vessels. How water enters the plant, what drives its movement, and the details of transport in phloem belong in the other videos on this chapter.

Xylem vessel adaptations

2:01Now imagine designing a passage through which water can flow. You would want an open interior, a continuous route, and walls strong enough to keep the passage from collapsing. Xylem vessels have exactly these features. A vessel forms from cells joined end to end. In the mature vessel, there are no cross walls separating the cells. Instead, they form a long continuous tube. To understand why this matters, imagine a drinking straw divided into many little compartments by solid partitions.

2:28Those partitions would interrupt the route through the straw. Removing them creates one continuous passage. In a xylem vessel, the absence of cross walls means water and dissolved mineral ions can move along an uninterrupted tube. When you explain this adaptation, connect the structure directly to its function. The mature cells forming xylem vessels are dead and have no cell contents. There is no cytoplasm or nucleus occupying the passage.

2:53That leaves a hollow interior through which water and mineral ions can flow with little obstruction. Here, being dead does not mean being useless. The remaining cell walls form a useful transport structure. This is quite different from imagining water squeezing its way through a chain of ordinary living cells filled with cell contents. A mature xylem vessel is an open tube. The third feature is the wall. Xylem vessels have thick walls containing lignin.

3:19Lignin is a substance that strengthens the cell walls. The thick, strengthened walls help stop the vessels collapsing, keeping the transport passage open. They also provide mechanical support for the plant. Think of the walls as doing two related jobs. They support the tube itself, and they contribute to supporting the plant around it. So the structure is not just a collection of facts to memorise. No cell contents leave space for flow.

3:43No cross walls create a continuous route. Thick walls with lignin strengthen the vessels and provide support.

Identifying tissues in organs

3:50Now let us locate these tissues inside the plant. The plants we are considering are non-woody dicotyledonous plants, such as a young bean plant. Dicotyledonous means that the seed has two seed leaves. You do not need a detailed lesson on classification here, but you do need to recognise that the arrangements we are about to show belong to this type of plant. A cross section is made by cutting across an organ.

4:13Cutting across a stem, for example, reveals the arrangement of tissues inside it, rather than showing their length. Start with the root. In a typical young dicot root, the xylem occupies the centre and often appears as a star or cross. The phloem lies between the arms of the xylem. Look carefully at that relationship. The phloem is not another star surrounding the xylem. It occurs in separate regions between the xylem arms.

4:38This central arrangement is the useful recognition pattern. In an image, the tissues might not be coloured, and the star might not look perfectly symmetrical. Identify the arrangement rather than expecting an exact copy of a textbook drawing. Next, the stem. Here, the transport tissues occur in groups called vascular bundles. A vascular bundle is a group of transport tissues containing xylem and phloem. In a non-woody dicot stem, these bundles are arranged in a ring.

5:05Within each bundle, the xylem is towards the centre of the stem. The phloem is towards the outside. Imagine standing at the centre of the stem and looking outwards through one bundle. You would reach the xylem first, then the phloem. Be careful with the word outside. It means the outer side of the vascular bundle, not outside the stem itself. Both tissues are inside the stem. Also, the ring is a ring of bundles.

5:28Do not draw all the xylem as one solid central star in the stem. That is the root pattern. Finally, the leaf. The veins contain vascular bundles. In a typical leaf cross section, the xylem is towards the upper surface of the leaf, and the phloem is towards the lower surface. Use the leaf surfaces to orient yourself. Upper does not simply mean whichever side happens to be at the top of your screen.

5:51If a diagram is rotated, the tissues rotate with it. We now have three arrangements. In the root, central xylem with phloem between its arms. In the stem, a ring of bundles with xylem inward and phloem outward. In the leaf, xylem towards the upper surface and phloem towards the lower surface.

Exam-style identification and explanations

6:08Let us work through an identification example rather than just repeat those patterns. You are shown an unfamiliar cross section. Several separate bundles form a ring. In one bundle, region A faces the centre of the organ and region B faces its outer edge. Identify the organ and both tissues. First, examine the overall arrangement. There is a ring of separate vascular bundles. That matches a non-woody dicot stem, rather than the central star-shaped arrangement of a root.

6:36Second, find the centre of the stem. This gives you a reference point for deciding which side of each bundle is inward. Third, apply the stem arrangement. Xylem is on the inward side, so region A is xylem. Phloem is on the outward side, so region B is phloem. Our answer is based on position, not colour. That is important because real microscope images and examination diagrams may use completely different colours, or no colour at all.

7:02Now suppose the question asks you to state the functions of these tissues. Keep the answer direct. Xylem transports water and mineral ions. Xylem provides support. Phloem transports sucrose and amino acids. Each sentence gives a clear, creditworthy point. Here is another exam-style task. Relate the structure of xylem vessels to their function. The command word relate means that you should connect a structural feature with what it allows the vessel to do.

7:27Naming features alone does not show the relationship. A precise answer could be given in these sentences. The walls are thick and contain lignin, which strengthens the vessels and provides support. The mature vessels have no cell contents, leaving a hollow passage for water and mineral ions to flow through. The cells are joined end to end with no cross walls, forming a long continuous tube for transport. Notice how every answer includes both the feature and its consequence.

7:54That is the same reasoning we used when imagining a well-designed water passage.

Common mistakes and recap

7:58Before we finish, let us clear up some common mistakes. First, saying that phloem transports food is too vague when you are asked to state its function. Name sucrose and amino acids. Saying glucose instead of sucrose does not give the required substance. Second, do not say that xylem vessels have no walls. They have thick walls. What they lack are cell contents and cross walls. Without their remaining walls, there would be no supported tube.

8:25Third, do not assume that every organ has the same arrangement. Xylem is central in the typical young dicot root, inward within the stem bundles, and towards the upper surface within a leaf vein. Finally, do not confuse transport with support. Xylem does both. Its hollow continuous interior helps transport, while its thick lignin-containing walls provide strength. Let us bring everything together. Xylem transports water and mineral ions and provides support.

8:51Phloem transports sucrose and amino acids. Mature xylem vessels are dead, have no cell contents, and are formed from cells joined end to end with no cross walls. This makes a long, hollow, continuous transport tube. Their thick walls contain lignin, which strengthens them and helps support the plant. To identify the tissues, look at the organ and its arrangement. Root means central xylem with phloem between the arms. Stem means a ring of bundles, with xylem inward and phloem outward.

9:22Leaf means xylem towards the upper surface and phloem towards the lower surface. Once you connect what each tissue carries, how xylem is built, and where both tissues are found, the diagrams become much more than shapes to memorise. They become different views of the plant’s transport network.