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For leaves to grow in whorls is fairly rare except in plant species with very short internodes and some other genera (Gallium, Nerium, Elodea etc.). Leaf whorls occur in some trees such as Brabejum stellatifolium and other species in the family Proteaceae (e.g., in the genus Banksia). In plants such as these, crowded internodes within the leaf whorls alternate with long internodes between the whorls.
Also known as the Indian devil, the Blackboard Tree is a species of trees in the family Apocynaceae. The tree is self-supporting and can grow up to 40 m tall. The whorled leaves are simple, broad, and fleshy.Blackboard tree is usually found in southern Asia, Australia, and the Solomon Islands. The wood is soft, thus not very useful. However, the leaves being richin nutrients like alkaloids, are often used in traditional medicine.Leaf venation in two species of Ceanothus. C. leucodermous has glaucous leaves with three main veins from the base. This is a thorny (spiny) chaparral shrub with rigid, sharp-pointed branchlets. It is common on Palomar Mountain in San Diego County. C. palmeri has pale green leaves with one main vein from the base. This shrub is also common on Palomar Mountain, particularly in Doane Valley.
This perennial herbaceous plant is ï¿½–2' tall and more or less erect; it is either unbranched or sparingly branched along the upper half of its central stem. The central stem is yellowish green or medium green and terete; it has vertical lines of short pubescence. Whorls of 4-6 leaves occur along the nodes of the central stem; because the internodes of this stem are fairly short, these leaves are produced in abundance. Relative to the orientation of the central stem, the leaves are usually ascending, although sometimes they are widely spreading or drooping. The leaves are 2-3" long, 2-3 mm. across, and linear in shape; their margins are entire (toothless) and strongly revolute (rolled downward). Sometimes whorls of smaller secondary leaves are produced from short lateral stems that develop from the axils of leaves along the central stem. The upper leaf surfaces are yellowish green or medium green and glabrous to sparsely short-pubescent; they are narrowly grooved along the middle where the midribs occur. The lower leaf surfaces are whitish green and short-pubescent; they are partially obscured by the rolled leaf margins. The leaves are sessile or they have very short petioles (less than 2 mm. long). From the axils of middle to upper leaves, umbels of flowers are produced on short peduncles (flowering stalks); there can be 1-4 umbels of flowers at each node. Individual umbels span ï¿½–1ï¿½" across, consisting of 7-20 pedicellate flowers. (Source: www.illinoiswildflowers.info)