Leaf

• 1. Introduction

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  A leaf seems like the simplest thing in the world. Yet in a single square centimeter of one, there may be anywhere from 1,000 to 100,000 stomata, the microscopic pores that breathe in carbon dioxide and breathe out water vapor. Each pore is held open by the pressure inside a pair of guard cells. Open them and the plant feeds. Close them and it keeps from drying out. This is the leaf's daily negotiation. It is a principal appendage of the stem of a vascular plant, usually borne above ground and built for one job above all others: photosynthesis. The questions that follow are stranger than the green flatness suggests. Why does a maple turn red only as it dies? Why do the leaves of a sunflower spiral by a fraction tied to the Fibonacci numbers? And how did one plant on Earth keep the same two leaves alive for more than a thousand years?

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• 2. The Sugar Engine

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  Glucose and sucrose are the point of the whole apparatus. Green plants are autotrophic, meaning they make their own food rather than taking it from other living things. The leaf captures the energy in sunlight and uses it to build simple sugars from carbon dioxide and water. Those sugars can be stored as starch, processed into proteins or cellulose, or burned through cellular respiration to power the cell.

  Water arrives by a route called the transpiration stream. The leaf draws it up from the ground through a conducting tissue named xylem, while carbon dioxide diffuses in from the air through the stomata. Once the sugar is made, it has to travel to the shoots and roots where growth happens. For that, the plant uses a separate tissue called phloem. Xylem and phloem run parallel to each other, but their cargo usually moves in opposite directions.

  The flat, thin shape of a typical leaf is not an accident. Being dorsiventrally flattened maximizes the surface exposed to light and lets that light penetrate to the chloroplasts inside. The same laminar shape increases thermal contact with the surrounding air, which promotes cooling. Beyond making sugar, the leaf is the principal site of transpiration, the force that pulls the whole stream up from the roots.

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• 3. Built For The Climate

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  Many conifers carry thin needle-like or scale-like leaves, an advantage in cold climates with frequent snow and frost. These are interpreted as reduced forms of the larger megaphyllous leaves their Devonian ancestors once bore. For a desert plant, a xerophyte, the binding constraint is not light but drought, and the leaf is shaped accordingly.

  Fenestraria and some Haworthia species, including Haworthia tesselata and Haworthia truncata, take an unusual path. These window plants are partly buried, admitting light through a translucent leaf window so photosynthesis can happen on inner surfaces, away from harmful sun. Succulent leaves take the opposite approach to scarcity, storing water and organic acids in thick juicy tissue.

  Eucalyptus shows how much a single species can change with circumstance. Mature trees that dominate their neighbors tend to grow isobilateral, pendent leaves, with photosynthetic palisade tissue on both sides. As seedlings, limited by available light, the same trees carry erect or horizontal leaves with distinct upper and lower faces. Pendent leaves also suit windy conditions, a trait shared by many willows.

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• 4. Spirals And Angles

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  Phyllotaxis is the name for how leaves arrange themselves on a stem, and the patterns are startlingly mathematical. In the simplest models, each new node forms at the growing tip and is rotated from the one before it by a fixed amount called the divergence angle. When one leaf grows per node and the stem stays straight, the leaves trace a helix.

  That divergence angle is often written as a fraction of a full turn. A fraction of one half, or 180 degrees, gives an alternate arrangement, as in the fan-aloe Kumara plicatilis. Beech and hazel rotate by one third. Oak and apricot turn by two fifths, sunflowers and pear by three eighths, and willow and almond by five thirteenths.

  Those denominators are not random. Most divergence angles trace back to the Fibonacci sequence, which begins 1, 1, 2, 3, 5, 8, 13, each term the sum of the two before it. The rotation fractions are typically a Fibonacci number divided by the one two places later, which is exactly the case for one half, one third, two fifths, three eighths, and five thirteenths. The ratio between successive Fibonacci numbers approaches the golden ratio, and so many leaves separate by close to the golden angle.

  Not every plant obeys a fixed angle. In orixate phyllotaxis, named for Orixa japonica, the divergence angle is periodic rather than constant, cycling through 180, 90, 180, and 270 degrees. Where opposite leaves grow paired at each node, as in basil with its decussate quarter-turns, they form a double helix instead.

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• 5. Reading The Veins

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  Veins are among the most visible features of any leaf, and they carry the leaf's plumbing. They extend in from the petiole, transporting water and nutrients between leaf and stem, supporting the leaf mechanically, and sustaining its water status and photosynthetic capacity. The vein entering from the petiole is the primary or first-order vein, those branching from it are secondary, and each further branching is numbered higher with a narrower diameter.

  Two broad patterns dominate the angiosperms. In parallel venation, the primary veins run side by side and equidistant for most of the leaf's length, then converge toward the apex. This is typical of monocots and correlates with their elongated shape and wide base. In reticulate venation, a single central midrib, the costa, sends out secondary veins toward the margins, which branch into a dense net. The patches of tissue between the finest veins are called areoles.

  The finest veins do the most intimate work. Minor veins act as the sites of exchange, collecting the products of photosynthesis from the cells, while major veins move that cargo out of the leaf. There is a deep timeline behind all this. Veins appeared in the Permian, before angiosperms arrived in the Triassic, and branching veins were present in ancient seed plants as long as 250 million years ago. Among temperate woody plants, Ginkgo biloba is the only species that still shows the old forking, dichotomous venation.

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• 6. Anatomy Of A Blade

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  The epidermis is the leaf's skin, a single outer layer of cells covering both faces. It is sealed by a waxy cuticle that is impermeable to liquid water and water vapor, and that cuticle is generally thicker on leaves from dry climates than from wet ones. Chloroplasts are absent from epidermal cells, with one exception: the guard cells of the stomata. Those stomatal pores are usually more numerous on the lower, abaxial surface than the upper one, and more numerous in plants from cooler climates.

  Mesophyll fills most of the interior, its name Greek for middle leaf, and it is where photosynthesis happens. In ferns and most flowering plants it splits into two layers. The upper palisade layer holds vertically elongated cells packed with chloroplasts, while the spongy layer beneath has loosely branched cells with large air spaces between them. Sun leaves build a multi-layered palisade, while shade leaves or older leaves near the soil keep a single layer.

  The leaf's green color comes from chlorophyll in those mesophyll chloroplasts. Some plants carry accessory pigments such as carotenoids, giving leaves of different colors. A leaf with lighter or white patches and edges has its own name: it is called variegated.

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• 7. Color, Defense, And Disguise

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  Red in an autumn leaf is a death-time event. As trees respond to cold and reduced sunlight, they curtail chlorophyll production, and the carotenoids and xanthophylls already present are revealed as yellow and orange. Red anthocyanin pigments are now thought to be produced in the leaf as it dies. One idea is that they mask the yellow, since yellow leaves appear to attract herbivores such as aphids. The masking also reduces the risk of photo-oxidative damage as the leaf senesces, protecting the recovery of its nutrients.

  Because leaves are richer in protein, minerals, and sugars than woody stem tissue, they sit prominently in many animal diets, and the plants fight back. Defenses include tannins, which hinder protein digestion and taste unpleasant, along with lignins, poisons, and the crystalline silica phytoliths of grasses. Animals that specialize in eating leaves are called folivores.

  Some creatures turn the leaf into a tool or a hiding place. The caterpillars of certain leaf-roller moths fold a leaf over themselves to build a small home. Among the skipper butterflies, the Hesperiidae, there is perhaps the greatest variety of shelter types, cut and bound with silk. Female leaf-rolling weevils of the Attelabidae lay their eggs inside leaves and then roll them up for protection. Other animals go further and impersonate the leaf itself. Some chameleons and some katydids even mimic the side-to-side oscillation of foliage in the wind while evading a threat.

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• 8. Shedding And Endurance

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  Abscission is the formal name for the act of letting go. Deciduous plants in cold temperate regions shed their leaves in autumn, and where the dry season is severe, some plants drop them until the rain returns. When a leaf falls it leaves behind a leaf scar on the twig, and the shed leaves often return their retained nutrients to the soil where they land.

  Other plants choose permanence. Palms and conifers retain their leaves for long periods rather than dropping them on a schedule. The most extreme case belongs to Welwitschia, which keeps its two main leaves for a lifetime that may exceed a thousand years. Two leaves, alive for a millennium, are a reminder of how far the rules of the leaf can stretch. The longest leaves of all belong to the Raffia palm, Raphia regalis, which may reach up to 25 meters long and 3 meters wide, a single green blade longer than most trees are tall.

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