Support and Transport Systems in Dicotyledonous Plants
Understanding the anatomy, transport tissues, water movement, and secondary growth in dicot plants
Learners need to know how dicot roots and stems are built, how xylem and phloem transport substances, and how water moves through the plant. This work also explains how plants stay upright and how secondary growth increases stem thickness.
1. Anatomy of Dicotyledonous Organs
Dicot Root Structure
- Epidermis: The outermost layer of the root, featuring unicellular root hairs that enhance the surface area for absorption of water and nutrients.
- Cortex: A broad region composed of parenchyma cells that serve primarily for storage and facilitate water movement.
- Endodermis: The innermost layer of the cortex, characterized by the Casparian strip, which is made of waxy suberin. This structure regulates the flow of water into the vascular cylinder.
- Vascular Cylinder (Stele): Contains the pericycle (responsible for lateral root growth), xylem (typically X-shaped in the center), and phloem (located between the arms of the xylem).
Dicot Stem Structure
- Epidermis: Covered by a waxy cuticle that minimizes water loss.
- Cortex: Comprises collenchyma for flexible support and parenchyma for storage.
- Vascular Bundles: Arranged in a ring formation, each bundle contains xylem (inner side), phloem (outer side), and a layer of vascular cambium in between.
- Pith: The central region of the stem, made of parenchyma, primarily for storage.
2. Transport Tissues: Xylem and Phloem
The two main vascular systems in dicotyledonous plants are xylem and phloem, each serving distinct functions:
| Feature | Xylem | Phloem |
|---|---|---|
| Main Function | Transports water and minerals upwards. | Transports manufactured sugars (translocation) bidirectionally. |
| Direction | Unidirectional (from roots to leaves). | Bidirectional (from leaves to all parts). |
| Structure | Composed of vessels and tracheids with lignified walls. | Composed of sieve tube elements and companion cells. |
| Cell State | Non-living (dead) at maturity. | Living cells (though sieve tubes lack a nucleus). |
3. Movement of Water and Transpiration
Water enters the plant through osmosis in root hairs and ascends via three primary forces:
Root Pressure
A pushing force generated by water entering the roots.
Capillarity
The upward movement of water due to cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to the xylem walls).
Transpiration Pull
A suction force created as water evaporates from the leaves.
Transpiration
Transpiration is the process of water vapor loss through the stomata. Factors influencing the rate of transpiration include:
Increasing Rate
High light intensity, elevated temperatures, and wind.
Decreasing Rate
High humidity.
Related Terms
Guttation: The loss of liquid water from leaf margins due to high root pressure, typically occurring at night.
Wilting: A condition that arises when transpiration exceeds water absorption, leading to a loss of turgor pressure in cells.
4. Secondary Growth
In dicot stems and roots, secondary growth results in an increase in the plant's girth. This process is facilitated by the vascular cambium, which produces new secondary xylem (wood) and secondary phloem.
Annual Rings
The formation of annual rings can be observed in trees, which can be used to determine a tree's age and provide insights into past climate conditions.
Exam Practice
Multiple-choice questions to revise the work learners must know
Xylem & Phloem
1. What is the main function of xylem tissue?
Water Movement
2. Which force creates suction that pulls water up the xylem?
Root Structure
3. What is the function of the Casparian strip in the endodermis?
Secondary Growth
4. What tissue is responsible for secondary growth in dicots?
Quick Check Questions
1. What is the difference between xylem and phloem?
Show Answer
Xylem transports water and minerals upwards and consists of dead cells. Phloem transports sugars bidirectionally and consists of living cells.
2. Name the three forces that move water up the xylem.
Show Answer
Root pressure, capillarity, and transpiration pull.
3. What is the function of the Casparian strip?
Show Answer
It regulates water flow into the vascular cylinder by forcing water to pass through the cytoplasm of endodermal cells.
4. How are vascular bundles arranged in a dicot stem?
Show Answer
Vascular bundles are arranged in a ring formation around the pith.
5. What can annual rings tell us about a tree?
Show Answer
Annual rings can determine the tree's age and provide insights into past climate conditions.
Did You Know?
The tallest tree in the world is a coast redwood named Hyperion, standing at 115.92 meters (380.3 feet). The water required to reach such heights is transported through xylem vessels using the transpiration pull!
Key Terms
What To Remember
- Dicot roots have epidermis with root hairs, cortex, endodermis with Casparian strip, and vascular cylinder with X-shaped xylem.
- Dicot stems have epidermis with cuticle, cortex, vascular bundles arranged in a ring, and central pith.
- Xylem transports water and minerals upward (dead cells); Phloem transports sugars bidirectionally (living cells).
- Water moves via root pressure, capillarity (cohesion/adhesion), and transpiration pull.
- Secondary growth occurs through vascular cambium, producing annual rings that indicate age and climate.
