1. Xylem:
* Function: Transports water and dissolved minerals upwards from the roots to the rest of the plant.
* Structure: Xylem is made up of dead, hollow cells called tracheids and vessel elements, connected end to end. These cells lack protoplasm and have thick, lignified walls for support.
* Mechanism:
* Root Pressure: The roots actively pump minerals into the xylem, creating a higher pressure than in the surrounding tissues. This pressure pushes the water upwards.
* Capillary Action: The narrow tubes of xylem create a strong capillary action, allowing water to move upwards against gravity.
* Transpiration Pull: As water evaporates from the leaves through transpiration, it creates a tension that pulls water upwards through the xylem.
2. Phloem:
* Function: Transports sugars (produced during photosynthesis) from the leaves to other parts of the plant, including roots, stems, and developing fruits.
* Structure: Phloem is composed of living cells called sieve tube elements and companion cells. Sieve tube elements have perforated end walls called sieve plates, allowing the flow of sugars. Companion cells provide metabolic support to the sieve tube elements.
* Mechanism:
* Pressure Flow Hypothesis: Sugars are loaded into the phloem at the source (leaves). This creates a high osmotic pressure, causing water to move into the phloem from the surrounding tissues. The pressure gradient pushes the sugars from the source to the sink (where sugars are used or stored).
Here's a breakdown of the process:
1. Absorption by Roots: Water and minerals are absorbed from the soil by root hairs.
2. Movement through Root Tissues: Water and minerals move through the root cortex and endodermis.
3. Xylem Transport: The water and minerals enter the xylem and are transported upwards to the stem and leaves.
4. Photosynthesis: In the leaves, the plant produces sugars through photosynthesis.
5. Phloem Transport: Sugars are loaded into the phloem and transported to other parts of the plant, such as roots, stems, fruits, and flowers.
6. Transpiration: Water evaporates from the leaves through stomata, creating a pulling force (transpiration pull) that helps move water through the xylem.
Factors affecting water and mineral movement:
* Transpiration rate: Higher transpiration rates increase the water movement through the xylem.
* Mineral concentration: Higher mineral concentrations in the soil lead to increased uptake by roots.
* Light intensity: Photosynthesis, which produces sugars, is influenced by light intensity, affecting phloem transport.
* Temperature: Temperature affects the rate of transpiration and the viscosity of water, influencing its movement.
This complex and interconnected system ensures the plant has access to water and minerals for growth and development, and efficiently distributes the products of photosynthesis to all its parts.
