How water reaches the leaf
A land plant takes its water from the soil, and the leaf where it is needed may be twenty metres above the soil. Chapter 1 built the pipe; this is the journey along it.
Osmosis
The movement of water molecules from a place where water is more concentrated to a place where it is less concentrated, through a membrane that lets water through. This is how the soil water crosses into the root hair.
The route, step by step
- Soil water surrounds the root.
- It is taken in through the root hairs, by osmosis.
- It passes through the cortex of the root.
- Then through the endodermis, the layer inside the cortex.
- It enters the xylem of the root.
- It travels up the xylem of the stem, on to the veins of the leaf, and is delivered to the mesophyll cells.
Distribution of the water throughout the leaf then happens along the veins spread through it. A leaf is veined the way it is so that no cell in it is far from a water supply.
Every stage of that was a page in chapter 1
The root hair, long and narrow for absorbing water. The cortex, a region rather than a tissue, filled with parenchyma. The xylem vessel, an open tube made of dead cells with their end walls dissolved. The mesophyll, which is the palisade and spongy parenchyma holding the chloroplasts. This page adds no new tissue at all — it just sends water through the ones you already have.
Beyond the syllabus — osmosis is bigger than this
Osmosis is defined here only as far as this chapter needs it, which is enough to follow water into a root hair. It governs a great deal more than that — how a cell keeps its shape, why parenchyma is turgid, what happens to a plant that is not watered — and it deserves a fuller treatment of its own than a photosynthesis chapter can give it.