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Nervation

Definitions

  • Webster's Revised Unabridged Dictionary
    • n Nervation nẽr*vā"shŭn The arrangement of nerves and veins, especially those of leaves; neuration. "The outlines of the fronds of ferns, and their nervation, are frail characters if employed alone for the determination of existing genera."
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Century Dictionary and Cyclopedia
    • n nervation The arrangement or distribution of nerves. Specifically— In botany, the disposition of the fibrovascular bundles in the blades of leaves, the sepals or petals of flowers, the whig-like expansions of samaroid fruits, etc.: a character which has assumed special importance in the study of fossil plants, since it has been proved to have generic rank, while the form and outline of leaves have only specific rank. The nervation of leaves, as studied and classified by A. P. de Candolle (1827), Ginseppe Bianconi (1838), Baron von Ettingshausen (1854–61), Oswald Heer (1856), and later authors, is based primarily on the relative rank of the nerves, and secondarily on their course through the leaf. As regards the rank of the nerves, the leaves of dicotyledonous plants are usually either pinnately or palmately nerved. This refers to the primary nerves. In pinnately nerved leaves there is only one primary nerve, the midrib, which maybe regarded as a continuation of the petiole, and from which there are given off secondary nerves which proceed at various angles through the blade toward or to its margin. These secondaries may or may not give off other nerves called tertiaries, and even these may produce quaternary nerves. In palmately nerved leaves there arise, usually from the summit of the petiole, two or more (sometimes numerous) more or less divergent primary nerves, which may have nearly equal strength, but more commonly the central one is thickest and may still be denominated the midrib. In the latter case the others are called lateral primaries. Any or all of the primaries of a palmately nerved leaf may give off secondaries as in pinnately nerved leaves, but these more commonly proceed from the outer pair. Leaves of only three primaries are sometimes called triplinerved; those of five, quintuplinerved. Peltate leaves usually have a peltate nervation, which may be regarded as a modification of the palmate nervation. The pedate nervation is simply a case of palmate nervation in which there are several nearly equal primaries. The terms penninerved, palminerved, peltinerved, and pedalinerved were suggested by De Candolle for these several kinds of leaves. As regards the course of the nerves through the blade and their ultimate disposition, the following classes are distinguished: craspedodrome [⟨ Gr. κράσπεδον, edge, margin, + -δρομος, ⟨ δραμει%26ν, run], the nerves passing directly to the margin of the blade; camptodrome [⟨ Gr. καμπτός, verbal adj. of κάμπτειν, bend, curve], the nerves curving (usually forward) near the margin, and either losing themselves in the parenchyma, or joining, arching, or otherwise anastomosing within the margin; brochidodrome [⟨Gr, βροχίς (βροχιδ-), dim. of βρόχος, a noose, loop], the nerves forming loops within the blade of the leaf; acrodrome [⟨ Gr. α%148κρος, at the point], the nerves passing upward and forward and terminating in the apex or point of the leaf; dictyodrome [⟨ Gr. δίκτυον, a net], the nerves soon dividing up and losing themselves in the general network of the leaf (see explanation of nervilles, below); hyphodrome [⟨ Gr. υφή, a web], the nerves, of lower rank than primaries, solost in the thick, coriaceous tissues of the leaf as to be nearly or quite invisible at the surface; paryphodrome [⟨ Gr. παρυφή, a border woven along a robe], a strong nerve passing round the entire margin of the leaf, forming a sort of hem or border; marginal, a distinct nerve passing along the margin of the leaf, parallel to it, but separated from it by a narrow interval; parallelodrome [⟨ Gr. παράλληλος, parallel], the nerves running parallel to one another, either longitudinally, as in grasses, or horizontally from the midrib to the margin, as in the banana-tree; (10) campylodrome [⟨ Gr. καμπύλος, curved], the nerves passing in a gentle curve from base to apex of the leaf, the interval between them increasing gradually in width from either end to the middle. The last two classes are almost wholly restricted to monocotyledonous plants. Besides the above, there is the dichotomous or forking nervation of most ferns and some other plants. From the various nerves as thus described there usually proceed many much finer ones which join and anastomose in various ways, forming a network of meshes of different shapes, usually angular, and either rectangular, trapezoidal, or nearly square, the spaces inclosed by which are known as areolæ. To such nerves the term nervilles has been applied. Physiologically considered, all nerves consist of vascular bundles which pass from the branch through the petiole, if there is one, into the base of the leaf, the primary fascicle of which is subsequently divided up to furnish the various nerves of the leaf, the primary nerves further dividing to supply the secondaries, these to supply the tertiaries, etc., and no nerves or fibers originate within the leaf.
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Chambers's Twentieth Century Dictionary
    • ns Nervation the arrangement or distribution of nerves, esp. those of leaves
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Etymology

Chambers's Twentieth Century Dictionary
Fr.,—L. nervus; Gr. neuron, a sinew.