Plant Transport Systems: Xylem and Phloem Functions and Structures

Roots and stems are the primary plant organs containing the transport system.
The system consists of thin tubes designed to carry liquids throughout the plant, moving substances both up and down.
There are two specific types of transport tubes: xylem and phloem. These are known as vascular tissues.
Xylem and phloem tubes are bundled together into structures called vascular bundles.

Function: Xylem tubes carry water and minerals in solution from the soil up to the parts of the plant above the ground.
Structure: These tubes possess thickened, strong walls that contribute to the structural support of the plant.
Tissue State: Xylem is characterized as dead tissue.
Wood and Growth: In woody plants such as trees, new xylem is produced each year, which forms the characteristic rings in the trunk. Xylem is the biological material known as wood.
Etymology: The word "xylem" originates from the Greek word "xylon," which translates to "wood."

Function: Phloem tubes transport sugar solution from the leaves (where it is synthesized during photosynthesis) to every other part of the plant.
Structure: Phloem tubes are composed of living cells joined end to end.
Adaptation for Flow: There are holes at the end of each phloem cell (the sieve plates) that allow the sugar solution to pass through easily.
Sugar Mix: Sugar mixes with water to create a strong sugar solution for transport.

Directionality: Unlike the upward flow in xylem, sugar solution in phloem moves in two directions at a time (bidirectional transport). It can move up to growing shoots or down to the roots.
Speed and Directional Factors: The movement depends on the specific needs of plant parts and the current season.
Seasonal Variations:
- Spring: When shoots are actively growing, the majority of the sugar solution is directed toward the growing shoots.
- Autumn: As the plant prepares for dormancy or reproduction, sugar is stored, directed up toward fruits and seeds, or down into the roots for storage.

Root Hairs: These are extensions of the epidermis consisting of thin-walled, elongated cells. They provide a large surface area to maximize the absorption of water and minerals from the soil.
Epidermis: The outermost layer of the root.
Cortex: Located between the epidermis and the vascular tissues, the cortex is composed of unspecialized cells.
Vascular Tissues: These are centrally located in the root. Water and minerals move up the xylem to the stem, while sugar from the leaves moves down the phloem.
Root Respiration: The sugar delivered by the phloem is used by root cells for the process of respiration.
Root Tip: The area where the root is actively growing.
Root Cap: A protective structure that covers the growing root tip as it pushes through the soil.

Epidermis: The outermost layer of cells making up the stem.
Bark: In woody plants, a layer of cork cells called bark exists on top of the epidermis.
Cortex: Present between the epidermis and the vascular tissue. It serves as a storage site for several substances, including:
- Carbohydrates
- Resins
- Latex
- Oils
- Tannins
Vascular Bundles in the Stem: These contain both xylem and phloem tubes.
- Xylem tubes: Located toward the interior, they have strong, thick walls to carry water and minerals upward.
- Phloem tubes: Located toward the exterior, they have thin walls and carry sugar up and down the plant.
Growth: Specialized cells within the stem produce new xylem and phloem as the plant increases in size.

Test Yourself

Question 1: What is a transport system?
- Contextual Answer: A system of tubes (vascular tissues) that moves liquids and nutrients throughout an organism.
Question 2: What is a vascular bundle?
- Contextual Answer: A grouping or bundle of xylem and phloem tubes.
Question 3: How are xylem and phloem tissue a) the same? b) different?
- a) Both are vascular tissues used for transport and are bundled together.
- b) Xylem is dead tissue with thick walls moving water/minerals up; Phloem is living tissue with thin walls and holes in cell ends moving sugar up and down.
Question 4: Explain how xylem tubes help support a plant.
- Contextual Answer: They have thickened, strong walls that provide structural reinforcement.
Question 5: What happens at the root tip?
- Contextual Answer: This is where cell growth occurs, allowing the root to extend further into the soil.
Question 6: What is the job of the root cap?
- Contextual Answer: To protect the delicate growing root tip.