One plant keeps the same two leaves for over a thousand years
A leaf is the main side-branch of a plant stem, built above all for photosynthesis. Most are thin, flat and broad to catch light, but leaves can also become pea tendrils, cactus spines or pitcher traps, and Welwitschia holds on to just two leaves for a lifetime that may pass a thousand years.
Green plants make their own food. Leaves capture sunlight and use it to turn carbon dioxide and water into simple sugars like glucose and sucrose, which are stored as starch or built into proteins and cellulose. The main photosynthetic tissue, the palisade mesophyll, usually lies on the upper side of the blade, though mature Eucalyptus leaves have it on both faces. A waxy cuticle waterproofs the surface, and stomata only tens of micrometres across open and close to trade carbon dioxide, oxygen and water vapour with the air.
Broad leaves with branching veins are megaphylls, found in flowering plants, ferns and most gymnosperms, while lycopods carry single-veined microphylls of separate origin, such as those of the early Devonian Baragwanathia. Larger true leaves spread in several lineages independently during the Devonian, once atmospheric carbon dioxide had fallen sharply. The needles and scales of many conifers, useful where snow and frost are frequent, are thought to be shrunken versions of their Devonian ancestors’ broad leaves.
Leaf form tracks climate, light, grazing animals, nutrients and competition, and it can change as a plant ages: young eucalypts carry horizontal leaves, mature ones hanging leaves. Because leaves concentrate protein, minerals and sugars, animals eat them eagerly, so plants invest heavily in managing them. Deciduous trees in cold regions drop them in autumn and some dry-country plants during droughts, returning nutrients to the soil, while palms and conifers keep theirs for years.
Leaves also form the scales of gymnosperm cones and the parts of flowers. Unlike stems and roots, which keep growing while resources last, a leaf grows to a set shape and stops, which is why botanists can identify species by leaf margins, hairs and stalks.
Source: Leaf
