Two transport tissues
Plants move materials through two tissues. Xylem carries water and dissolved minerals upward from roots to leaves, in dead, hollow tubes. Phloem carries dissolved food (mainly sucrose) from leaves to where it is used or stored — translocation — through living cells, in both directions.
| Tissue | Carries | Direction |
|---|---|---|
| Xylem | water and minerals | roots → leaves (up) |
| Phloem | dissolved food (sucrose) | source → sink (both ways) |
Water uptake and the transpiration stream
Water enters roots through root hair cells by osmosis, drawn in by their large surface area. It rises through the xylem in a continuous transpiration stream, pulled up as water evaporates from the leaves — a flow that also carries dissolved minerals.
Transpiration
Transpiration is the loss of water vapour from a plant, mostly through pores called stomata. It is an unavoidable side-effect of opening the stomata to let CO₂ in for photosynthesis. It is faster in conditions that speed evaporation: high temperature, dry air, wind and bright light. Guard cells open and close the stomata, balancing CO₂ intake against water loss.
What speeds transpiration?
Anything that speeds evaporation: higher temperature, lower humidity, more wind, brighter light. Wilting on a hot, dry, windy day is transpiration outrunning uptake.
Mineral nutrition
Besides glucose, a plant needs mineral ions from the soil, absorbed by active transport. Nitrate is needed to make amino acids and proteins (deficiency stunts growth), and magnesium to make chlorophyll (deficiency yellows the leaves).
A worked example
A potted plant wilts on a hot, windy afternoon but recovers overnight. Explain why.
- On a hot, windy afternoon, transpiration is very fast.Heat and wind speed evaporation.
- Water is lost faster than the roots absorb it.Uptake cannot keep pace.
- Cells lose water and turgor, so the plant wilts.Turgid cells support the plant.
- Overnight it is cooler, calmer and darker, so transpiration slows and cells refill.Slower loss lets uptake catch up.
Fast transpiration outran uptake, so cells lost turgor and wilted; overnight the slower loss let it rehydrate.
Turgor and support
In a well-watered plant, water enters the cells by osmosis until each cell is firm and swollen — turgid. The pressure of the cell contents pushing against the cell wall is turgor pressure, and in non-woody plants it is the main source of support, holding stems upright and leaves spread out. If a plant loses too much water, its cells become flaccid and it wilts; in extreme cases the membrane pulls away from the wall (plasmolysis).
Turgid, flaccid, plasmolysed
Turgid = full and firm (water in, plant supported). Flaccid = limp (water lost, plant wilts). Plasmolysed = so much water lost that the membrane pulls away from the cell wall.
Measuring transpiration
The rate of water uptake — a good measure of transpiration — is measured with a potometer, which tracks how fast a leafy shoot draws water along a tube. By changing one condition at a time — temperature, humidity, air movement or light — while keeping the others constant, a student can show how each affects the rate, confirming that transpiration speeds up in warm, dry, windy, bright conditions.