Botany Terms Starting With X
Botany Glossary: X
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Xerophyte
/ ZEER-oh-fyte / · Greek xeros (dry) + phyton (plant)
Xerophyte xerophyte is a plant adapted to dry conditions; has features that help it save water or collect water efficiently.
Xerophytes employ multiple water conservation strategies including thickened cuticles that reduce transpiration by 90 percent, reduced leaf surface area from small leaves or needles, underground stems, and water storage in fleshy tissues. Cacti store water in parenchyma cells comprising up to 90 percent of their stem mass and can survive 2 to 3 years without precipitation. Some xerophytes like agave plants open stomata only at night through crassulacean acid metabolism, reducing water loss while still acquiring carbon dioxide.
Deep root systems extending 15 to 50 meters enable some desert plants like mesquite to access groundwater.
Some xerophytes exhibit leaf orientation that minimizes sun exposure; desert shrubs often have vertically oriented leaves rather than horizontal leaves. Others produce reflecting pubescence that bounces away radiation.
Desert Horse Purselane →Xerophytes do not require water. All xerophytes require water but have adaptations that reduce loss rates or extend the time between water uptake events.
Desert Rose →In creosote bushes, small leaves on widely spaced branches reduce water loss to less than 20 percent of potential loss. In aloe vera, thick succulent leaves store water that can sustain the plant for months without rainfall.
Desert Flowers →Xerophytic
/ zeer-oh-FIT-ik / · Greek xeros (dry) + phyton (plant) + -ic
Xerophytic describes features that help a plant survive in dry conditions, such as thick leaves or reduced water loss.
Xerophytic traits include reduced stomatal density with fewer pores decreasing water loss, sunken stomata or stomatal crypts that trap moist air and reduce transpiration by 40 to 60 percent, and thick waxy cuticles that block water vapor diffusion. Leaf rolling in some grasses reduces exposed surface area by 75 percent during drought. Xerophytic bark with multiple corky layers insulates against heat stress and water loss.
Reflective hairs on leaves reduce radiation absorption, lowering leaf temperature and transpiration rates.
Xerophytic traits appear in diverse climates including hot deserts, cold tundra, exposed windswept coastlines, and saline soils where water availability is limited. These traits also appear in epiphytic plants like bromeliads.
Xerophytic describes only desert plants. Xerophytic traits occur in any dry habitat including rocky mountaintops, limestone cliffs, and salt marshes.
In oleander leaves, sunken stomata arranged in pits reduce water loss to 5 to 10 percent of exposed stomatal rates. In wax myrtle, a thick waxy coating covering the entire leaf surface reflects light and reduces transpiration.
Order Myrtales →Xylem
/ ZY-lem / · Greek xylon (wood)
Xylem is plant tissue that carries water and dissolved minerals from the roots to the rest of the plant.
Xylem consists primarily of elongated, dead cells called tracheids and vessel elements with thick walls reinforced by lignin, creating strong tubes that transport water under negative pressure tension. In a redwood tree, xylem cells can pull water upward over 100 meters against gravity through capillary action and the continuous water column cohesion. Xylem also transports dissolved mineral ions from roots and provides structural support; wood is essentially dead xylem.
The thickened cell walls of xylem provide strength for tall plant growth.
Xylem water moves upward when atmospheric pressure and root pressure push water or when tension from transpiration pulls water upward. The water column inside xylem vessels can develop tensions exceeding 5 megapascals.
Xylem actively transports water using energy. Water moves through xylem passively through pressure gradients and tension without requiring ATP.
In a celery stalk, food coloring can be drawn upward through xylem strands demonstrating water transport. In a tree trunk, xylem forms the heartwood and sapwood that conduct water and provide structure.