venation [ vee-Ney-shuhn, vuh– ] noun: in botany, the arrangement of veins in a leaf
Veins transport nutrients and water to the leaf cells from the stem. Primary veins are the thickest, most noticeable veins. The thinner secondary veins branch off from the primary veins and the ultrathin tertiary veins branch off from the secondary veins. The arrangement of veins in a leaf is called venation.
Two common forms of venation that are the starting point for many plant identification systems are pinnate and palmate. In a pinnate leaf, the secondary veins are spaced along the central midvein in a feathered pattern as in the leaves of Betula nigra (river birch). Palmate veins form a branching pattern that radiates from the place where the petiole joins the leaf blade, like fingers from a hand, as displayed by the leaves of Cercis canadensis (eastern redbud). A third venation pattern is fan-shaped, as in gingko trees (though in some identification systems Gingko biloba is treated as a variation of two-lobed venation, as the Latin name hints). A fourth form, arcuate, has a strong midrib and curved secondary veins in a more heart-shaped arrangement exhibited by the leaves of Cornus alternifolia (alternate-leaf dogwood). The parallel venation of a fifth form appears in most monocotyledons (grass and grass-like flowering plants whose seeds usually contain only one embryonic leaf or cotyledon) like Cymophyllus fraserianus (Fraser’s sedge). (Refer to this Broadleaf Forms and Arrangements chart for illustrations of phyllotaxy, types, shapes, margins, and venation.)
Left to right: pinnate (B. nigra), palmate (C. canadensis cultivar), fan-shaped (G. biloba), arcuate (C. alternifolia), and parallel (C. fraserianus.)
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Reference
Hermsen EJ. 2021. Angiosperm leaf architecture. Digital Atlas of Ancient Life.