A tall tree can lift hundreds of litres of water from its roots to its highest leaves every day — with no heart and no pump. Plants manage this using two separate transport tissues: xylem and phloem, running side by side through the roots, stem and leaves in bundles called vascular bundles.
Two pipelines: xylem and phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Carries | Water & dissolved minerals | Sucrose (sugars) & amino acids |
| Direction | Up only (roots → leaves) | Both ways (source → sink) |
| Cells | Dead, hollow tubes | Living cells |
| Process | Transpiration stream | Translocation |
- Xylem — dead, hollow, thick-walled tubes strengthened with lignin. They carry water and minerals one way, up from the roots, and also help support the plant.
- Phloem — living tubes that carry sucrose and amino acids made in the leaves to wherever they are needed (growing tips, roots, fruits). This movement is called translocation and can go up or down.
Root hairs and water uptake
Water enters through root hair cells. Each has a long, thin extension that gives a large surface area for absorbing water. Water moves in by osmosis — from the soil (high water potential) into the cell (lower water potential) through the partially permeable membrane. Minerals such as nitrate are taken up by active transport, using energy from respiration.
Transpiration
Transpiration is the loss of water vapour from the leaves, mainly through tiny pores called stomata. As water evaporates from the leaf, it pulls a continuous column of water up the xylem — the transpiration stream. Four factors change its rate:
- Temperature ↑ — faster evaporation, higher rate.
- Humidity ↑ — smaller concentration difference, lower rate.
- Wind / air movement ↑ — removes water vapour, higher rate.
- Light ↑ — stomata open for photosynthesis, higher rate.
Rate before = 5 ÷ 10 = 0.5 cm³/min. Rate after = 5 ÷ 4 = 1.25 cm³/min. The wind removes water vapour from around the leaf, keeping a steep concentration gradient, so evaporation and transpiration speed up.