WINTER LEAVES
Leaves are shaped like hearts, pinwheels, arrowheads, fans, bowls, spoons and spines. They can be as smooth as glass or as wrinkled as old men. They can be bald or hairy. There are leaves that behave like probing fingers and leaves that behave like stomachs - with a memory.
What accounts for such incredible variety, from pine needles to Alocasia macrorrhiza, more aptly known as elephant ear? Why does a sycamore leaf look different from a maple leaf?
"Almost certainly leaf shape has an evolutionary underpinning," said John Constable, an assistant professor of biology at Fresno State University. "It's the result of particular conditions in the distant past favoring certain individuals with a particular trait or ability."
A lot of those explanatory details, of course, are lost in the mists of distant evolution. But leaf morphologists - scientists who study the form and structure of leaves - have parsed out some answers.
GREEN WITH ENVY
Any story about leaves (or indeed about life in general) must begin with photosynthesis - the fundamental process by which plants combine minerals and water from soil, carbon dioxide from air, a green pigment called chlorophyll and sunlight to produce carbohydrates, such as sugar, and oxygen.
When light strikes a leaf, it passes through a waterproof covering called the cuticle and a protective layer of cells called the epidermis. Beneath the epidermis is the mesophyll, which is packed with elongated cells containing chloroplasts, tiny packets filled with chlorophyll. This is where photosynthesis occurs.
The mesophyll is bisected by veins that transport water and nutrients and provide structural support. Poking through both epidermis and mesophyll are pores called stomata, which pull carbon dioxide into the leaf and exude oxygen and water.
Leaves are designed to optimize the capture of light in order to maximize photosynthesis. There are many ways to do this, but a couple of broad generalizations can be made: In wet, tropical regions, leaves tend to be big and broad because there is plenty of water, lots of competition for sunlight and the weather is relatively mild. Conversely, in colder and drier climes, leaves tend to be smaller, narrower and thicker.
It's all about the environment, said Katherine Preston, a botanist and lecturer at Stanford University. In a moist environment, the large surface area of a few, big leaves results in more photosynthesis, albeit at the cost of moisture lost. But that's not a problem because water is abundant.
In a dry environment, plants have smaller leaves, but more of them. Smaller leaves means less moisture lost; more leaves means the photosynthetic workload can be broadly shared.
OUT OF BOUNDARIES
Leaf shape is also affected by the boundary layer, the zone where air rubs up against the leaf surface. For plants in full sun, a thick boundary layer is vital, insulating leaves from heat and moisture loss. Many plants boost the efficacy by growing hairs called trichomes that trap even more air and moisture, further cooling the plant.
But a really big leaf in a lot of sun can be counterproductive, said Preston. Moisture trapped in the leaf's boundary layer can become too hot, increasing evaporation and harming the plant.
Some plants simply reduce leaf size and corresponding boundary layer. Compound leaves - many smaller leaflets - produce the same effect, as does the lobed leaves on trees like maples and oaks.
In other species, such as California lilacs, the solution is curling. "Curling a leaf tends to reduce the boundary layer on the upper surface where the sun is hitting, while making the layer underneath thicker," Preston explained.
Conifers do something similar. A pine needle, with almost no protective boundary layer, compensates by recessing its stomata in tiny pits. Above each stomata a pocket of dead air forms, trapping moisture.
THE DIRT ON DIRT
Soil quality dictates leaf shape, too. In nitrogen-rich soil, plants can make big, nitrogen-packed leaves capable of a high rate of photosynthesis. Bugs also require nitrogen, which makes such leaves look a lot like dinner plates. That's why many tropical plants grow their leaves big, fast and turn them over quickly.
In nitrogen-poor soils, the rate of photosynthesis is necessarily slower and leaves must remain on the plant longer. Conifer needles endure a couple of years or even a decade, their thin profile minimizing the risk of wind and storm damage.
Other plants, like those in chaparral, deploy leaves that are tough and thick. These leaves are packed with more carbon than nitrogen, making them harder to digest and less attractive to herbivores.
THE POINT TO IT
Many leaves have pointy ends. The leaf tips of the tropical Bo tree, for example, resemble long, curling tendrils. Their purpose, said Jon Rebman, curator of botany at the San Diego Natural History Museum, is to draw water off leaves.
"Drip tips do exactly what they sound like. In very wet places, the constant presence of water on leaves can encourage fungal growth, which could interfere with photosynthesis. Drip tips cause water to drain off leaves."
In deserts and other places where moisture is scarce, leaves are often succulent - plump, waxy storage containers for water. A few plants go even further. Lithops, called "living stones," are succulents with rocklike leaves mostly buried to further reduce desiccation.
Only a few "window leaves" are exposed at the surface, their outer cells transparent so sunlight can reach chloroplasts deeper within the plant.
KILLER LEAVES
Perhaps the most extravagant leaf adaptations belong to carnivorous plants.
In pitcher plants, natural selection has favored plants with deeper-cupped leaves, capable of forming water-filled chambers. Insects are drawn to these plants by bug-appealing colors or oozing nectar. They fall into the slippery cups and drown. Bacteria from rainwater or enzymes secreted by the plant dissolve the bug into its constituent nutrients. In some cases, insect larvae reside in the cup, eating the trapped prey. The plant lives off the larval waste.
Butterworts and sundews employ leaves coated with sticky glue like flypaper. A bug lands, becomes stuck, dies and is digested. To reduce the chance of escape, some species quickly curl the leaf over the stuck prey or form a shallow depression under the bug - a mini digestive pit.
The leaves of another carnivorous plant family - the bladderworts - create pouches sealed by a hinged door lined with touch-sensitive trigger hairs. The interior of the pouch is kept in partial vacuum. When curious prey trigger the door, the vacuum sucks the prey in.
And finally, of course, there are Venus' flytraps and aquatic waterwheel, which both actively snap up their prey. Their leaves are divided into two lobes hinged along the midrib and armed with trigger hairs. When the hairs are bent by an investigating prey, a cascade of electrochemical signals prompts leaf cells to rapidly pump water out of the midrib. The lobes, held open under tension, snap shut in less than a second.
But what about false alarms, such as a raindrop or debris landing on a leaf? The plants have a kind of simple memory. For the lobes to snap shut, at least two hairs must be triggered within 0.5 to 30 seconds of each other.
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