Water moves from root hairs to the stem through a combination of processes including osmosis, capillary action, and root pressure, eventually entering the xylem for upward transport.
Here's a more detailed breakdown:
-
Absorption by Root Hairs: Root hairs, which are tiny extensions of epidermal cells on plant roots, significantly increase the surface area available for water absorption from the soil.
-
Movement Across the Root Cortex: Water absorbed by the root hairs travels across the root cortex (the tissue between the epidermis and the vascular cylinder) via two main pathways:
-
Apoplast Pathway: This pathway involves movement through the cell walls and intercellular spaces. It allows for easy and rapid movement of water and dissolved nutrients until it reaches the endodermis.
-
Symplast Pathway: This pathway involves movement through the cytoplasm of cells, connected by plasmodesmata (small channels that connect adjacent plant cells). Water enters the symplast by crossing the plasma membrane of a root hair cell.
-
-
The Endodermis and the Casparian Strip: The endodermis is a layer of cells surrounding the vascular cylinder (stele) in the root. Its key feature is the Casparian strip, a band of suberin (a waxy, waterproof substance) embedded in the cell walls. The Casparian strip is impermeable to water and ions. This forces water and minerals moving via the apoplast pathway to enter the symplast of the endodermal cells. This provides the plant with control over what enters the xylem.
-
Entry into the Xylem: Once inside the endodermal cells' symplast, water and dissolved minerals are released into the xylem vessels and tracheids, which are specialized water-conducting cells.
-
Upward Movement in the Xylem: The water then travels upward through the xylem to the stem. Several factors contribute to this upward movement:
-
Root Pressure: The accumulation of ions in the xylem can lower the water potential, causing water to move into the xylem and generate a positive pressure that pushes water upward. This is more significant in smaller plants.
-
Capillary Action: The narrow diameter of the xylem vessels and tracheids, combined with the adhesive forces between water molecules and the xylem walls (adhesion) and the cohesive forces between water molecules themselves (cohesion), contribute to capillary action.
-
Transpiration-Cohesion-Tension Mechanism: This is the primary driver of water movement in plants. Transpiration (the evaporation of water from leaves) creates a negative pressure (tension) in the leaves. This tension pulls water upward through the xylem, relying on the cohesive properties of water to maintain a continuous column of water from the roots to the leaves.
-
In summary, water moves from the soil through root hairs, across the root cortex (via apoplast and symplast pathways), through the endodermis (where the Casparian strip forces symplastic entry), into the xylem, and then upwards through the stem via root pressure, capillary action, and, most importantly, the transpiration-cohesion-tension mechanism.
