Transpiration, root pathways and xerophytes

Last time

Before we start: last time, in Plant plans, xylem, phloem and translocation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A leaf is a leaky straw

A single large tree can lose hundreds of litres of water on a hot day. Nearly all of it escapes through tiny pores in the leaves. That loss is not an accident. It is what pulls water all the way up from the soil, and this lesson follows the water from the root hair to the air, then looks at plants that must save every drop.

Transpiration

Transpiration is the loss of water vapour from the leaves of a plant, mainly through open stomata. Water evaporates from the wet surfaces of the mesophyll cell walls into the air spaces. Water vapour then diffuses out through the stomata. It happens because the water potential inside the air spaces is higher than in the air outside. Water vapour moves down this gradient.

Factors

Higher light intensity opens the stomata, so transpiration rises. Higher temperature increases evaporation and lowers humidity of the air outside. Low humidity makes the gradient steeper, and wind removes the humid air around the leaf. Both raise the rate. High humidity lowers the rate, because the gradient is less steep.