A leaf needs water for photosynthesis, but the water is in the soil, sometimes 100 meters below the top of a tall tree. And the roots need sugar, which is made in the leaves. Plants solve both problems with two kinds of tubes.
Key idea
Xylem carries water and minerals up from the roots. Phloem carries sugar from the leaves to wherever it is used or stored, up or down. Water leaves the plant as vapor through tiny pores in the leaves called stomata.
Why it works
Most of the lift comes from the top, not the bottom. As water evaporates from the leaves (transpiration), it pulls on the water behind it. Water molecules cling to each other, so the whole thin column in the xylem is drawn upward, like drinking through a very long straw. Root pressure gives only a small push.
Worked examples
Worked example: Celery and food coloring
A celery stalk stands in blue water. After a day, blue dots show in the cut stalk. Which tubes are they?
- The blue water moved up the stalk.
- Water moves up in the xylem.
Answer: Xylem.
Worked example: Sugar to a potato
A potato grows underground. How does sugar from the leaves get there?
- Sugar is made in the leaves.
- Phloem carries sugar down to the potato, where it is stored as starch.
Answer: Through the phloem.
Common mistake
Thinking water is pumped up from the roots like a heart pumps blood, or that xylem and phloem carry the same things. Most of the pull comes from water evaporating from the leaves. Check: when stomata close on a dry day, water flow slows.