Tissues in Action · Lesson 4 of 11
Conducting Tissues - Complex Permanent Tissues
“Xylem brings the water while phloem runs the plant’s food-delivery service.”
• Explain why xylem and phloem are complex permanent tissues. • Identify the cellular components of xylem and phloem. • Compare the materials and directions of transport. • Describe dermal, ground and vascular tissue systems. • Predict how tissue damage affects transport in a plant.
Introduction
A tall tree has a major transport problem to solve every day. Water and minerals are absorbed from the soil by the roots, but most of the food is prepared in the leaves through photosynthesis. This means that important materials are produced or collected in one part of the plant but are needed in many other parts, sometimes several metres away.
For example, water absorbed by the roots must travel upward through the stem to reach the leaves, where it is used in photosynthesis. Minerals dissolved in this water must also be carried to growing regions and other living cells. At the same time, sugars made in the leaves must be transported to roots, fruits, flowers, seeds, stems, and other parts of the plant that need energy or store food.
A plant cannot depend on materials simply moving from one cell to the next over such long distances. That method would be far too slow, especially in a large tree. Instead, plants have specialised transport tissues that form long conducting pathways through the roots, stems, branches, and leaves.
These conducting tissues are not made from one enormous cell. They are complex tissues made of several different kinds of cells that work together. Each cell type has a particular role, but together they form an efficient transport system.
The two main conducting tissues in plants are xylem and phloem. Xylem mainly carries water and dissolved minerals from the roots to the upper parts of the plant. Phloem mainly transports food, especially sugars made in the leaves, to other parts of the plant where it is needed or stored.
In this way, xylem and phloem act like the plant's internal transport network. They connect distant parts of the plant and help ensure that every region receives the materials it needs for growth, repair, storage, and normal functioning.
A complex permanent tissue contains more than one type of cell working together to perform a shared function.
Xylem
Xylem is the complex permanent tissue that transports water and minerals from roots to other plant parts and also adds strength.
Xylem transports water and dissolved minerals from roots to stems, leaves and other plant parts. It also contributes mechanical strength. Its conducting elements form long pathways through which water can move.
| Xylem component | Structure or condition | Contribution |
|---|---|---|
| Tracheids | Elongated, tubular and thick-walled; mainly dead at maturity | Conduct water and add support. |
| Vessels | Tubular elements joined into continuous passages; mainly dead at maturity | Provide an efficient water-conducting route. |
| Xylem parenchyma | Living cells | The living component of xylem; assists in storage and related functions. |
| Xylem fibres | Thick-walled, sclerenchymatous cells | Provide strength. |
The dead conducting elements are useful because their empty interiors and strong walls provide continuous channels that do not collapse easily. Water loss from leaves through stomata contributes to a pull in the xylem. Living leaf cells and mostly dead xylem elements therefore participate in one coordinated transport process.
Phloem
Phloem is the complex permanent tissue that transports prepared food from leaves to regions where it is used or stored.
Phloem transports food, especially sugars made in leaves, to growing regions, roots, fruits and storage organs. Movement follows the plant’s needs, so food may travel from a producing region to different places of use or storage.
| Phloem component | Structure or condition | Contribution |
|---|---|---|
| Sieve tubes | Long living cells joined end to end by perforated walls | Form the main pathway for food transport. |
| Companion cells | Specialised living parenchyma cells | Regulate the activities of sieve tubes and help with loading and unloading sugars. |
| Phloem parenchyma | Living cells | Store food and substances such as resins, tannins and latex. |
| Phloem fibres | Mainly sclerenchymatous | Provide mechanical strength. |
Problem
Suppose a complete ring of bark and the phloem beneath it is removed from a tree trunk. What is likely to be disturbed first?
- 1.Leaves may continue producing sugars for some time.
- 2.Removing phloem interrupts the route that carries those sugars to lower regions.
- 3.Roots receive less food even if xylem can still move water initially.
- 4.Further inward damage can reach xylem and then interfere with water and mineral transport.
- 5.The effect depends on how deeply and completely the trunk has been damaged.
Individual tissues are organised into larger tissue systems. The dermal tissue system forms the outer covering and reduces water loss while protecting inner parts. The ground tissue system forms much of the plant body between the outer and conducting regions; it includes parenchyma, collenchyma and sclerenchyma. The vascular tissue system contains xylem and phloem.
| Tissue system | Main components | General role |
|---|---|---|
| Dermal | Outer protective tissues | Protection and reduction of water loss |
| Ground | Parenchyma, collenchyma and sclerenchyma | Storage, photosynthesis and support |
| Vascular | Xylem and phloem | Transport and additional support |
Quiz
Which description best matches Complex Permanent Tissue?
Which description best matches Xylem?
Which term matches this description: A complex permanent tissue contains more than one type of cell working together to perform a shared function.
Which term matches this description: Xylem is the complex permanent tissue that transports water and minerals from roots to other plant parts and also adds strength.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why are xylem and phloem called complex tissues?
- Name the only living component listed for xylem.
- Explain how sieve tubes and companion cells cooperate.
- A plant receives water but its roots are deprived of food from the leaves. Which conducting tissue is likely damaged?
- Distinguish the dermal, ground and vascular tissue systems.
- Explain how dead xylem cells can participate in water transport together with living leaf cells.
Key Takeaways
• Xylem conducts water and minerals and adds strength. • Phloem transports prepared food. • Each conducting tissue contains several cell types with related jobs. • Plant tissues are organised into dermal, ground and vascular systems.