Leaves take centre stage in the autumn. Who wouldn’t be captivated by the endless array of shades and colours on trees and shrubs? The canny gardener doesn’t mind too much when the fall starts.

Collecting the leaves does take time, but I find a leaf blower is a handy tool and a lawnmower does the job on the grass. Just keep sweeping them up as they fall and you’ll end up with a pile of crumbly leafmould after 18 months.

Leafmould makes the perfect, weed-free mulch and massively improves soil structure when it’s dug into a bed. Leafmould also comes into its own when I’m preparing my homemade potting mixes. It’s a time-consuming job, but the ingredients are free and the results knock spots off anything you could buy. Leafmould bulks out the compost much better than coir, municipal green waste, or heaven forbid, peat.

While I was enjoying the beauty of leaves and marvelling at their usefulness, I started wondering why they come in such different sizes, not only here but around the world. Why are banana leaves so much bigger than those tiny wee things you find on heather?

It’s all down to what makes leaves grow. Photosynthesis provides vital energy for plants. Sunlight reacts with carbon dioxide and water to produce glucose and oxygen. And plants have had to develop strategies to cope with the widely differing weather and light levels across the planet.

Plants use the temperature of their leaves to prevent scorching in the tropics and freezing near the poles. They aim to draw enough moisture from the soil to cool leaves through transpiration. This balances moisture with the intensity of the sun. The fiercer the sun, the more energy is produced through photosynthesis, and this leads to larger leaves.

The opposite is equally true: the further away from the equator you are, the smaller the leaf. The north of Scotland is blanketed in heather, not banana trees. Leaves here are usually larger than heather, but shrubs with large leaf areas are hard if not impossible to grow.

Unless we live in sheltered parts of the eastern lowlands, we need to think twice before embarking on the likes of sweet chestnut, however tempting the nuts. Unfortunately large-leafed walnuts are just as challenging.

Australian scientists have identified another reason for our difficulties. In last month’s edition of the journal Science, Dr Ian Wright and colleagues published the results of a worldwide study of the leaf size of nearly 8000 plant species. They found the difference between day and night temperatures was critical.

During the growing season, plants take longer to recover from low overnight temperatures and can’t reach and maintain the warmth needed to produce large leaves. This helps explain why the leaves on our trees and shrubs are much smaller than tropical ones.

The scientists also noted that there was smaller heat loss in leaves closer to the ground than in the canopy because they benefited from warmth rising from the soil. This resulted in slightly larger leaves.

Although leaf size differs hugely between global regions we can also see wide variation in our own gardens. And although Wright claims overnight temperature loss is critical, he agrees there are plenty other reasons for these differences. Trees are more stunted and have smaller leaves in poor soil, and shallow-rooted trees, such as birch, can’t draw up as much moisture as hazel, so have much smaller leaves.

But Wright hopes his research will encourage plant breeders to develop cultivars with the best balance of moisture and photosynthesis for different part of the world, especially during climate change. Breeders certainly have a long history of developing larger leaved domesticated species. Look how much bigger a Bramley leaf is than a crab apple, or a Victoria than a Blackthorn.