Capillarity and Surface Tension

Capillary Rise (Capillarity)

• Water rise in a glass capillary tube is due to adhesion and surface tension forces.
• Capillarity occurs due to adhesion between water and polar groups on the capillary wall, surface tension (cohesive forces between water molecules), and gravity acting on the water column.

Forces in Capillary Rise

• Strong cohesive forces between water molecules act to pull the bulk water up the tube.
• The lifting forces are balanced by the downward force of gravity acting on the water column.

Driving Force in Water Movement

• The driving force for water movement in xylem is provided by transpiration (evaporation of water from the leaf).

Water Uptake in Roots

• The region of most active water uptake lies near the root tip (0.5 cm to 10 cm from the tip).
• Water is taken up more rapidly in the zone of cell maturation, where vascular tissue (xylem) has begun to differentiate.

Root Hair Development

• Root hairs are thin-walled outgrowths of epidermal cells that increase the absorptive surface area and extend the absorptive capacity into larger volumes of soil.
• Root hairs can reach lengths of 0.1 mm to 10 mm and diameters of 10 μm.
• Root hairs increase the contact of the root with soil water by penetrating capillary spaces and extending into a cylinder of soil.

Importance of Root Hairs

• Root hairs permit the root to access water in capillary spaces and increase the absorptive surface area, enhancing water uptake.