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Leaves. Venation, or nervation, in leaves is the arrangement of veins in leaf blades. Four main types are distinguished. (1) One or more longitudinal veins do not branch and are not connected by crosspieces (in many conifers). (2) The branches of the veins are not connected by crosspieces and reach the leaf margins (in most ferns); venation is often bifurcated. (3) Veins proceed along the leaf almost in parallel (in many grasses with linear leaves), or they form arcs converging at the base and apex of the blade (in the lily of the valley) and connected by delicate transverse cross-pieces. Venation of this type (parallel-veined and arc-veined) is characteristic of most monocotyledons but is also present in some dicotyledons with linear leaves. (4) From one or more large first-order longitudinal veins, smaller second-order veins branch out that give rise to still smaller third-order branches, and so forth. The veins are connected by crosspieces in different directions. All the basilar tissue of the leaf is divided into portions surrounded by small veins. These portions are penetrated by very tiny branches of the conducting system that end blindly (pinnate or reticular-veined venation). This type of venation is characteristic of most dicotyledons but it is also found in some monocotyledons (in the canna, arum family, and so forth).
Venation is called palmate when several veins (the main one thicker than the others) separate at the base of the blade in radial fashion. The oldest and most primitive form of venation is dichotomic venation. Study and description of venation is very important in identifying both recent and fossil plants (in the form of impressions and so forth) from the leaves.
Insects. Venation in insects is the arrangement of veins in the wing. The number and arrangement of veins is an important taxonomic feature of orders, families, and other systematic categories of insects. Venation is particularly important in the classification of fossil forms.
L. V. KUDRIASHOV