x When placed before a specific epithet, indicates the taxon is of known hybrid origin. The basic number of chromosomes in a polyploid series.

xanthic (Gr. xanthos: yellow, yellowish) adj. Yello, yellowish. Also xanthous.

xanthotoxin n. A naturally occurring furocoumarin compound found in several species of plants. It is a photoactive substance that forms DNA adducts in the presence of ultraviolet A irradiation.

xanthophyll (Gr. xanthos: yellow; phyllon: leaf) n. Yellow, orange, or red fat-soluble pigments, found in marigold petals, etc. An oxygen-containing carotenoid found in plants, one of the pigments involved in photosynthesis. Xanthophylls are responsible for the yellow colour of leaves in the autumn. Also lutein.

xanthous See xanthic.

xenia (Gr. xenia: hospitality bonds) n. The immediate influence or effect of the pollen on structures, as the seed or fruit, other than the embryo. Cf. metaxenia.

xenogamous adj. See xenogamy.

xenogamy (Gr. xenos: strange, foreign, stranger; gamos: marriage) n. Pollination between flowers of separate plants. Cross-fertilization. Cf. geitonogamy.

xenogenesis n. Fancied or supposed reproduction of an organism unlike the parents.

xeric (Gr. xros: dry) adj. Characterized by a scanty supply of moisture, tolerating, or adapted to, arid conditions; xeric conditions exist in arid habitats, extremely cold habitats, and in salt marshes. Certain plants are adapted to live in such conditions. Cf. hydric, mesic.

xero- Prefix meaning dry.

xerochastic adj. Said of capsules that open only when they dry out.

xeromorph (Gr. xros: dry; morph, forme) n. A plant having structural features usually associated with plants of arid habitats (such as hard or succulent leaves) but not necessarily drought-tolerant. Cf. scleromorph, xerophyte.

xeromorphic (Gr. xros: dry; morph, forme) adj. Possessing obvious physical adaptations for a dry environment, such as the succulent, water storing stem of a cactus. Describing the structural modifications of certain plants (xerophytes) that enable them to reduce water loss, particularly from their leaves and stems.

xerophile (Gr. xros: dry; philos: friend) adj. Said of a plant able to grow in very dry conditions because it has adapted to restrict water loss by shedding leaves at the beginning of the dry season or by growing waxy leaves or dense hairs.

xerophilous (Gr. xros: dry; philos: friend) adj. Growing in arid places.

xerophyte (Gr. xros: dry; phyton, plante) n. A plant adapted to life in dry environments. A drought-tolerant plant. A plant that is adapted to live in conditions in which there is either a scarcity of water in the soil, or the atmosphere is dry enough to provoke excessive transpiration, or both. Xerophytes have special structural (xeromorphic) and functional modifications, including swollen water-storing stems or leaves (see succulent) and specialized leaves that may be hairy, rolled, or reduced to spines or have a thick cuticle to lower the rate of transpiration. Examples of xerophytes are desert cacti and many species growing on sand dunes and exposed moorlands. Some halophytes have xeromorphic features. Cf. mesophyte, hydrophyte, xeromorph.

xerophytic adj. Adapted to dry or arid conditions, places where fresh water is scarce or where water absorption is difficult due to an excess of dissolved salts.

xiphoid (Gr. xiphos: sword; eid: to look like) adj. Sword-shaped, ensiform.

xylan n. A polysaccharide component of hemicellulose and containing only xylose molecules.

xylem (Gr. xylon: wood) n. A tissue that transports water and dissolved mineral nutrients in vascular plants. In flowering plants it consists of hollow vessels that are formed from cells (vessel elements) joined end to end. The end walls of the vessel elements are perforated to allow the passage of water. In less advanced vascular plants, such as conifers and ferns, the constituent cells of the xylem are called tracheids. In young plants and at the shoot and root tips of older plants primary xylem is formed by the apical meristems (see protoxylem; metaxylem). In plants showing secondary growth this xylem is replaced in most of the plant by secondary xylem, formed by the vascular cambium. The walls of the xylem cells are thickened with lignin, the extent of this thickening being greatest in secondary xylem. Xylem contributes greatly to the mechanical strength of the plant: wood is mostly made up of secondary xylem. Part of the vascular system of plants responsible for the transport of water, dissolved minerals and some organic substances. Xylem consists of two distinct cell types, both of which lack living cell contents: vessels, which are open-ended tubes with lignified walls of various patterns; and tracheids, which are smaller closed cells with tapering ends. The composition of the xylem defines the main difference between hardwoods and softwoods. Vessels are formed only in hardwoods derived from trees in the Magnoliopsida, whereas tracheids are found in both, and exclusively form the xylem of softwoods derived from conifers. Cf. phloem. See also fibre.

xylocarp n. A hard, woody fruit such as the coconut.

xyloglucan n. A polysaccharide component of hemicelluloses and containing both xylose and glucose molecules.

xyloid (Gr. xylon: wood; eid: to look like) adj. Resembling wood, ligneous.

xylostroma (Gr. xylon: wood; strma, strmatos: what one spreads) n. (pl. xylostromata) The felted, blackish stroma of some species of wood-destroying fungi.

xylostromata See xylostroma.