Why do leaves have different shapes?

Every leaf does the same job, yet they come in endless shapes. Climate, light, and water explain a lot of it, and science is still working out the rest.

A maple leaf, a pine needle, and an ash leaf all do the same job: capture sunlight and turn it into food. So why do they look so different? Scientists have been asking that for more than a century. Some answers are well supported, and some are still open questions.

Size: why some leaves are huge and others tiny

Leaf size varies more than 100,000-fold between species. A global study of more than 7,600 species found big leaves are most common in hot, wet, sunny places, while small leaves dominate in hot, dry places and at high latitudes and elevations (Wright et al. 2017; Macquarie University).

The surprise was why. The old textbook answer was that big leaves overheat in the day. The study found that over much of the world, the risk of frost at night sets the limit instead. Big leaves are wrapped in a thicker layer of still air, so they trade heat with their surroundings more slowly and have a harder time recovering the heat they radiate away on clear nights (Macquarie University). Water matters too: on hot days, big leaves can stay cooler than the air if they have enough water to evaporate (Nicotra et al. 2011).

Teeth: a clue to climate

More than a century ago, botanists noticed that in colder climates, a larger share of trees and shrubs have toothed leaf edges, while smooth edges dominate in warm places. The pattern is strong enough that scientists use it in reverse: by counting the share of smooth-edged species in a fossil flora, they can estimate how warm that ancient landscape was, though only roughly (Royer 2012).

Why teeth help in the cold is still being worked out. One idea is that toothed edges give young leaves a head start: in one study, they photosynthesized and moved water faster in their first month, which would help a plant gain ground in a short, cool spring. The authors called the link preliminary (Royer and Wilf 2006). Many teeth also carry tiny water pores that may boost sap flow to young leaves (Nicotra et al. 2011).

Lobes

Lobes are deep cuts that shorten the distance across the leaf. That should help a leaf shed heat, and it trims away the tissue farthest from the main veins, the part most likely to dry out. Lobes and teeth may serve quite different purposes (Nicotra et al. 2011).

One tree, two kinds of leaves

Even leaves on the same tree differ. Shade leaves on the lower, inner branches are larger than sun leaves, often several times larger, and thinner, which helps them catch dim light. Sun leaves are smaller and thicker, with extra layers of food-making cells, and often more hairs and wax to limit water loss (Northern Woodlands). On northern red oak, the sun leaves at the outside of the crown are also more deeply lobed; one explanation is water supply, though researchers present that as a proposal rather than a settled answer (Nicotra et al. 2011).

Compound leaves

Ash, walnut, and hickory leaves are split into many leaflets on one stalk. A large study of Amazon forests found that compound-leaved trees grew faster and had lighter wood, supporting the idea that compound leaves go with fast growth. It found no good support for the older idea that they’re an adaptation to seasonal drought (Malhado et al. 2010). Genes help explain how they form: in many plants, compound leaves are linked to a family of master growth genes, called KNOX genes, staying active longer in the developing leaf (Nicotra et al. 2011).

Needles and scales

Pine needles are leaves, just very narrow ones. Most pines carry them in bundles of two to five, and a few species have more (University of Wisconsin). The singleleaf pinyon is an unusual exception, usually carrying just one needle per bundle (Virginia Tech Dendrology). Their waxy coating cuts water loss, and their breathing pores sit in sheltered pits, out of the wind. Keeping needles year-round lets conifers photosynthesize whenever conditions allow during a short growing season, and saves nutrients in cold soils where decay is slow (College of St. Benedict and St. John’s University). See are pine needles leaves? for more.

Drip tips

Many rainforest leaves end in a long, narrow point. About a third of Amazon tree species have one, and they’re most common where rain is heaviest. The usual explanation is that drip tips help leaves shed water, but experiments have given mixed results, so the evidence is weaker than textbooks suggest (Malhado et al. 2012).

Leaves that change shape

Some plants change their leaves as they grow up. Young, climbing English ivy has three- to five-lobed leaves, while adult, shrubby growth has unlobed, diamond-shaped to rounded leaves, and only the adult form flowers (UConn Plant Database). Sassafras mixes three shapes on one tree: an oval with no lobes, a two-lobed “mitten,” and a three-lobed leaf (Arnold Arboretum). Many water plants grow very different leaves underwater than above it (Nicotra et al. 2011).

The short answer

Leaf shape is a trade-off between catching light, managing water, and staying at a workable temperature, shaped by climate, by where on the plant the leaf grows, and by each plant’s family history. That’s also why shape is so useful for identifying trees.