Botany Terms Starting With W
Botany Glossary: W
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Water Potential
/ WAW-ter poh-TEN-shul / · Old English waeter; Latin potentia, power
Water potential is the pressure needed to stop water from moving into a plant cell by osmosis, measured in units called pascals, with pure water set at zero and salt solutions or dried seeds at negative values below zero.
Pure water has the highest possible water potential. When salts or sugars are dissolved in water, they lower its water potential because the solute molecules get in the way of water movement. In plants, water enters roots from the soil, moves up the stem, and evaporates from leaves into the air.
Water potential combines pressure and dissolved-solute effects. It predicts the direction water tends to move.
Water always moves toward the driest-looking tissue. Water moves according to water potential differences.
In wilted lettuce, water moves into cells when the leaves are placed in fresh water. Movement follows differences in water potential across cell membranes.
Plasma Membrane Functions →Wax Layer
/ waks LAY-er / · Wax from Old English weax, related to German Wachs, possibly from Indo-European root meaning to weave.
Wax Layer hydrophobic coating of lipid compounds deposited on the outer surface of the plant cuticle that reduces water loss and provides protection.
The wax layer consists primarily of very long-chain aliphatic compounds including alkanes, alcohols, fatty acids, and esters typically containing 20 to 36 carbon atoms. These waxes are synthesized in epidermal cells and transported to the cuticle surface, where they crystallize into diverse structures visible under electron microscopy. Epicuticular wax forms three-dimensional crystals that can appear as plates, tubes, or filaments depending on chemical composition and environmental conditions.
The wax layer thickness typically ranges from 0.1 to 10 micrometers, reducing nonstomatal water loss by up to 95 percent compared to unwaxed surfaces. Beyond water regulation, the wax layer reflects ultraviolet radiation, repels water droplets and dust particles, and creates a physical barrier against fungal penetration.
The blue-gray bloom on grape skins and plums is actually crystalline wax that can be rubbed off with handling, and some plants like cabbage produce so much wax that raindrops bounce off leaves without wetting them. Carnauba wax from Brazilian palm leaves is the hardest natural wax known, melting at 85 degrees Celsius and widely used in commercial products.
The shiny appearance of all leaves does not come from wax. Some glossy leaves like magnolia have smooth cuticles with minimal surface wax, and their gloss comes from ordered cell surface geometry.
Order Magnoliales →Eucalyptus leaves develop thick wax layers containing unique compounds that give them a distinctive gray-green color and help prevent water loss in Australian dry climates. The lotus leaf produces microscopic wax crystals that create superhydrophobic surfaces, causing water to bead up and roll off, carrying dirt particles in the self-cleaning lotus effect.
Explore Lotus Flower →Wilting
/ WILT-ing / · Origin unknown, possibly from Middle Dutch welken meaning to wither or fade.
Wilting is the loss of rigidity in plant tissues caused by insufficient water pressure within cells, resulting in drooping leaves and stems.
Wilting occurs when turgor pressure drops below the threshold needed to maintain cell rigidity, typically when water loss through transpiration exceeds water uptake by roots. Temporary wilting happens during hot, dry conditions when transpiration rates peak, with plants recovering overnight as temperatures cool and stomata close. Permanent wilting point is reached when soil water potential drops to approximately negative 1.5 megapascals, preventing root water absorption even when stomata are closed.
During wilting, guard cells lose turgor and stomata close automatically, reducing further water loss but also limiting carbon dioxide uptake for photosynthesis. Some plants exhibit wilting as an adaptive response to reduce leaf surface area exposed to direct sunlight, minimizing transpirational water loss.
Certain plants like Mimosa pudica deliberately wilt their leaves within seconds of touch through rapid potassium ion movement that triggers water loss from specialized motor cells. Prayer plants show daily wilting and recovery cycles called nyctinasty, raising leaves at night to reduce heat loss even when soil moisture is adequate.
Wilting always indicates insufficient soil water. Plants wilt on hot afternoons even when soil moisture is adequate, because transpiration temporarily outpaces root water uptake; these plants typically recover turgor by evening when stomata close and leaf-to-air vapor pressure gradients decrease.
Tomato plants commonly exhibit midday wilting in summer heat despite adequate irrigation, recovering by evening as temperatures decline. Desert wildflowers like California poppies wilt and close their petals during the hottest part of the day, reopening when conditions improve to conserve precious water reserves.
Wood
WUD · From Old English wudu meaning forest or trees, related to Old Norse viðr.
Wood is the hard, fibrous structural tissue formed by the secondary xylem in the stems and roots of woody plants.
Wood develops through the activity of the vascular cambium, a lateral meristem that produces new xylem cells toward the interior of stems and roots. In temperate regions, cambial activity varies seasonally, creating distinct annual growth rings consisting of large-celled earlywood formed in spring and dense latewood produced in summer. Wood composition includes approximately 40 to 50 percent cellulose, 20 to 30 percent hemicellulose, and 20 to 30 percent lignin, with the lignin providing rigidity and decay resistance.
Hardwoods from angiosperms contain vessel elements and fibers, while softwoods from gymnosperms consist primarily of tracheids. The oldest living wood is found in bristlecone pines, with some trees exceeding 5,000 years of continuous growth.
Some tropical trees produce tension wood on upper sides of branches that actively pulls limbs upward through cellulose gel formation, generating forces exceeding 10 megapascals. Fossilized wood can retain its cellular structure for over 200 million years, allowing paleobotanists to identify extinct tree species from their preserved anatomy.
Different Types of Trees →Hardwood and softwood refer to the actual hardness of the timber. These terms classify wood by the plant group it comes from: hardwoods derive from angiosperms and softwoods from gymnosperms, so balsa, one of the lightest and softest materials known, is classified as a hardwood because it comes from a flowering tree.
Oak trees produce ring-porous wood with large vessels concentrated in earlywood, creating distinctive grain patterns valued in furniture making. Teak wood from Southeast Asian forests contains natural oils that provide exceptional resistance to moisture and insect damage, making it prized for shipbuilding and outdoor applications.