8.2 Plant Transport

Root hairs are extensions of epidermal cells that greatly increase contact with soil water. Mineral ions may enter through channels or active transporters, lowering water potential in root cells so water follows by osmosis. Water can travel through cell walls and intercellular spaces by the apoplast route or cross membranes and cytoplasm by symplast and transmembrane routes. At the endodermis, the hydrophobic Casparian strip blocks the wall route and forces selective membrane crossing before xylem entry.

Xylem vessels are chains of dead cells whose end walls are absent or perforated. Their lignified walls resist collapse under tension, and pits allow lateral movement around blockages. Water molecules cohere through hydrogen bonding and adhere to hydrophilic vessel walls. When water evaporates from mesophyll cell walls and diffuses through stomata, curved air-water interfaces pull on the remaining water, reducing leaf water potential and drawing replacement water from xylem.

Transpiration is an unavoidable consequence of opening stomata for carbon dioxide. Warmth increases evaporation and diffusion, wind removes the humid boundary layer, and low atmospheric humidity steepens the water-vapor gradient. Light commonly opens stomata to support photosynthesis. Soil water shortage eventually causes guard cells to lose turgor or respond to abscisic acid, closing the pore. Closure conserves water but also restricts carbon dioxide and can lower photosynthesis.

A transverse section helps relate transport to support. In a young dicot stem, vascular bundles commonly form a ring, with xylem toward the center and phloem toward the outside. In a root, xylem is central and often star-shaped, with phloem between its arms. This arrangement supports the organ and shortens movement from absorbing tissues to xylem. A drawing should show relative tissue positions clearly rather than imitate every observed cell.