The New Student's Reference Work/Venation

From Wikisource
Jump to navigation Jump to search

Vena'tion, the arrangement of veins in leaves. The details of this arrangement are endless, but they may be reduced to two principal types. The two main types are known as closed and as open venation. In the former case, while the veins branch, they always run into one another, never having tree ends among the leaf-cells or in the leaf-margins. As a consequence, such leaves have even outlines, that is, they are not toothed or lobed. A very common form of the closed venation is that in which the main veins run parallel from base to apex, as in the grass-blade, and leaves of other monocotyledons. Such leaves are called parallel-veined. In other cases of closed venation, as in palms, bananas etc., the main veins are not parallel. The closed venation is characteristic of the monocotyledons. In open venation, characteristic of dicotyledons, the ultimate branches end freely, either among the leaf-cells or in the leaf-margins. Such leaves are said to be net-veined or reticulate, and they are much inclined to become toothed, lobed or compound. There are two forms of open venation: pinnate or feather-veined leaves, in which there is a main central vein or rib (midrib), from which the whole vein system, arises; and palmate leaves, in which three or more main ribs rise together at the base of the leaf, and diverge upward. In the ferns the venation is open, but the veins fork repeatedly, such a venation being called dichotomous.