木质部和蒸腾作用驱动的物质运输

Plants acquire water and nutrients from the soil but require these substances even in faraway tissues. How do plants transport these substances against the force of gravity?

Water and dissolved minerals enter the plant through permeable root hair cells in the epidermis and travel to the xylem, where the fluid is referred to as xylem sap. The sap is distributed from the roots to the rest of the plant by bulk flow, a pressure-driven movement of liquid over long distances, through specialized transport cells in the xylem. 

Tracheids and vessel elements function as a flexible pipe system. Their cells lack cytoplasm and organelles and have impermeable, lignified cell walls, which prevents xylem sap from leaking out.

The primary mechanism that pulls xylem sap through the plant is transpiration from the leaves. Air space between mesophyll cells in the leaves becomes saturated with water vapor during the process of photosynthesis and cellular respiration. 

The air outside the plant is drier than the air within the leaves. When the stomata are open, water vapor diffuses out along its concentration gradient, traveling from the moist air pockets within the leaf to the atmosphere outside the plant.

A film of liquid water adheres to the cell wall of each mesophyll cell, facilitating gas exchange during photosynthesis and cellular respiration. When water vapor exits through the stomata, some of the liquid water evaporates, generating tension that pulls water from the leaf veins into the mesophyll cells to replenish the film.

Negative pressure in the leaf veins then draws the xylem sap away from the roots, up the stem, and into the leaves, transporting water and minerals throughout the plant.