Mitchell Gunn, of Balclutha Primary School, asks :-

Why has the leaf I found at school got white fur on the back? Is it to keep the moisture in it?

Janice Lord, a botanist at the University of Otago, responded.

Many leaves are furry or hairy. Even when a leaf looks smooth, a magnifying glass, or microscope will often reveal tiny hairs, particularly on the back of the leaf. Botanists use a range of scientific terms to describe the different hairs on plants, including glandular (a stalk with a “pin head” containing, for example, a chemical that deter bugs), stellate (branched like a snowflake) and my personal favourite, arachnoid, meaning spidery.

The type of hairs on a plant can be important in identification, for example, yellow-flowered daisies you probably have around school, like Dandelion, Catsear, Hawkbit and Hawksbeard, differ in whether their leaf hairs are simple, segmented or stellate. Dense white fur on the underside of a leaf, scientifically called tomentum, is particularly common in the daisy family. The tomentum of Tikumu, large native mountain daisies, is so dense it can be stripped off (see picture) and was used for clothing and insulation by southern Maori. White furry leaves are also seen on plants in cold, high light areas like high mountains or in dry areas, like deserts.

So why are leaves furry? You are right in thinking it has something to do with water. Leaves produce food for the plant via photosynthesis. In order to turn sunlight into carbohydrates, chloroplasts inside the leaf need carbon-dioxide and water. Pores in the leaf allow carbon dioxide in, but they also allow water to escape. When a plant has plenty of water this isn’t a problem, in fact losing water through the pores can stop the leaf from overheating, just like your sweat cools you down.

However in cold or dry places where water can be limited, the plant needs to reduce water loss while avoiding overheating. Plants in these places often protect their pores in deep furrows under a layer of hairs, often on the underside of the leaf. This way the leaf can let in carbon dioxide and cool down without losing too much water to evaporation.

Plants that are silvery furry all over can even further limit water loss and overheating by reflecting back the high levels of light that are typical of high mountains and deserts.