Xylem, phloem and transpiration
Xylem vessels are dead, hollow tubes with no end walls and lignified walls (strength, waterproof) with pits. Water evaporates from mesophyll cells and diffuses out through stomata (transpiration), creating tension; cohesion between water molecules (hydrogen bonds) and adhesion to vessel walls pull a continuous column up: cohesion–tension. Water crosses the root via the apoplast (cell walls) and symplast (cytoplasm), but the Casparian strip forces it into the symplast at the endodermis. Transpiration increases with light, temperature and wind and decreases with humidity; measured with a potometer (water uptake). Xerophytes reduce water loss (sunken stomata, hairs, rolled leaves, thick cuticle). Phloem sieve tubes with companion cells move sucrose by mass flow from source to sink: sucrose loaded by co-transport lowers ψ, water enters, hydrostatic pressure pushes sap.
≈ 50 min