Plants have two transport tissues. Xylem vessels are hollow tubes of dead cells strengthened with lignin; they carry water and mineral ions from the roots up to the leaves. Phloem is living tissue that carries dissolved sugars from the leaves to the rest of the plant, both up and down; this is called translocation.
Water evaporates from leaves through stomata, pulling more water up the xylem: this flow is the transpiration stream. Transpiration is faster when it is warm, dry, windy and bright (bright light opens more stomata). Guard cells open and close each stoma, controlling water loss and letting carbon dioxide in for photosynthesis. A potometer can be used to estimate the rate of water uptake.
Worked example
Why does a plant lose more water on a windy day?
- Wind blows away water vapour from around the leaf.
- This keeps the concentration gradient steep.
Answer:So water evaporates and diffuses out faster.
Key words
- xylem
- carries water and minerals upwards
- phloem
- carries sugars around the plant
- transpiration
- loss of water vapour from leaves
- guard cells
- open and close stomata
New questions every time.
NGSS-referenced: HS-LS1-2