How Water Climbs to the Top of a Tall Tree

Water is absorbed by the roots and carried upwards through tubes called xylem, which run from the roots through the stem to every leaf. Food made in the leaves travels in a separate set of tubes called phloem. Two pipelines, running in opposite directions.

This page covers everything in the CBSE Class 7 Science chapter's second half: water absorption and the xylem, food transport and the phloem, transpiration, and respiration in plants.

Roots are covered with thousands of fine root hairs, which vastly increase the surface touching the soil. Water, together with the dissolved minerals in it, passes into these root hairs and then into the xylem.

The xylem is a system of long, narrow tubes running continuously from root to leaf. Water moves through it in one direction only — upwards.

In a tall tree that means lifting water many metres with no pump at all. The pull comes largely from the leaves: as water evaporates from them, more is drawn up behind it, like sucking through a straw from the top.

For example, a stick of celery or a white flower left in coloured water shows the xylem clearly — the colour climbs the stem and appears in the petals.

Root hairs also explain why transplanting a plant roughly can kill it: the delicate hairs tear off easily, and without them the plant cannot take up enough water even in wet soil.

The phloem transports the food made in the leaves to every other part of the plant.

Unlike the xylem, phloem carries material in both directions — wherever food is needed or stored. It goes down to the roots, out to growing stems and tips, and into fruits and seeds.

Much of that food is stored. In a potato it is stored in an underground stem, in a carrot in the root, in a mango in the fruit, and in wheat and rice in the seeds. Every one of those foods we eat is plant food that travelled through phloem.

So the two tissues divide the work cleanly: xylem carries water and minerals upwards; phloem carries food in all directions.

A useful memory link: phloem carries food, and both words have an 'o'. The distinction matters because questions frequently ask which tissue does what, and reversing them loses the mark entirely.

Transpiration is the loss of water vapour from a plant, mostly from the leaves through the stomata.

It sounds wasteful, but it does two useful jobs.

First, it pulls water up the plant. As water vapour leaves the leaf, more water is drawn up the xylem to replace it, creating the continuous upward stream that reaches even the topmost leaves.

Second, it cools the plant. Evaporating water carries heat away, which is why standing under a large tree feels noticeably cooler than standing in the open beside it.

You can see transpiration directly: tie a clear polythene bag around a leafy branch on a sunny day and droplets of water collect inside it within a few hours.

The trade-off links back to the stomata. The plant must open its pores to take in carbon dioxide for photosynthesis, and water inevitably escapes through the same openings — so transpiration is partly a cost the plant pays for making food.

Yes. Plants respire exactly as animals do, breaking down glucose using oxygen to release energy:

glucose + oxygen → carbon dioxide + water + energy

Respiration happens in every cell of the plant, day and night, because the plant needs energy continuously.

Photosynthesis, by contrast, happens only in green parts and only in sunlight. The two processes are near opposites:

- photosynthesis makes glucose and releases oxygen; respiration uses glucose and releases carbon dioxide
- photosynthesis stores energy; respiration releases it
- photosynthesis needs light; respiration does not

During the day both run together, and photosynthesis is usually much faster, so the plant gives out oxygen overall. At night only respiration continues, so the plant gives out carbon dioxide.

That resolves the common belief that plants "breathe out oxygen". They do so on balance during daylight only — at night the net exchange runs the other way.