Plant tissues and transpiration

A leaf is built from several tissues:

• Epidermal tissue covers the leaf and is often coated in a waxy cuticle to reduce water loss.
• Palisade mesophyll, near the top of the leaf, has tightly packed cells full of chloroplasts, adapted for photosynthesis.
• Spongy mesophyll has air spaces between its cells, letting gases diffuse to and from the palisade layer.
• Xylem and phloem, found in the leaf's veins, transport substances into and out of the leaf.

Stomata are tiny pores, mostly on the underside of a leaf, that let gases — carbon dioxide, oxygen and water vapour — move in and out. Each stoma is surrounded by a pair of guard cells, which change shape to open or close the pore, controlling both gas exchange and water loss from the leaf.

Transpiration is the loss of water vapour from a plant's leaves, mostly through the stomata. It is driven by evaporation from cells inside the leaf, which pulls a continuous stream of water up from the roots through the xylem — the transpiration stream. This stream also carries dissolved mineral ions, and helps keep plant cells turgid.

Four factors affect the rate of transpiration:

• Light intensity — more light makes stomata open wider for photosynthesis, increasing transpiration.
• Temperature — higher temperature speeds up evaporation, increasing transpiration.
• Humidity — higher humidity reduces the concentration gradient for water vapour, decreasing transpiration.
• Air movement (wind) — more wind removes water vapour from around the leaf, steepening the concentration gradient and increasing transpiration.