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instances they are doubled inwards at the edges, or induplicate; while in many they are twisted or contorted.
The arrangement of the flowers upon the stem likewise requires notice. They are frequently situated singly at the apex of a stalk or peduncle rising from the root, or are solitary. In most plants, however, the stem is variously divided into small branches bearing the flowers. When a number of flowers are placed along a stalk without foot-stalks or pedicels, it is called a spike (Fig. 17). A drooping spike, containing stamens or pistils only, and dropping from the branch when withered, is termed an amentum or catkin. When the flowers of the spike are each supported by a foot-stalk, it becomes a raceme (Fig. 18). When the raceme is branched it is a panicle (Fig. 19). When the outer branches of the raceme or panicle are so elongated that the flowers are brought nearly to the same level with the inner ones, it is a corymb (Fig. 20). When the branches of the corymb terminate in a flower and then produce lateral stalks, as in the elder, it is styled a cyme (Fig. 21). When the flowers are arranged upon stalks branching from the apex of the stem, an umbel is formed, which, like the corymb, may be either simple or compound as the flowers stand singly upon the branches of the umbel or are supported upon secondary umbels rising from the extremities of the latter (Figs. 22 and 23). Sometimes the flowers are arranged in a close head or capitulum without pedicels, being situated upon a common disk or receptacle, as in the Dandelion and other Compositæ. The flowers are in this case surrounded by a whorl of leaves like a calyx, to which the name of involucrum is given, a term applied to any whorl of leaves or bracts situated upon the flower-stalk. In the compound umbel there is usually a general involucrum beneath the primary umbel, while each secondary umbel has one of its own, to which the term involucel is applied.
All the parts of the flower must be regarded as modifications of the leaf. The sepals and petals are nothing more than leaves some-
