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xiii of the foramen or of the radicle, with regard to the hilum, is the most important feature of the embryo. When the radicle is directed to the extremity of the seed opposite the hilum, the embryo is said to be antitropal; when it is next the hilum, it is termed orthotropal; when the embryo lies across the seed with its radicle towards one side, it is heterotropal, when so curved that both extremities are pointed towards the hilum, it becomes amphitropal. The cotyledons are variously arranged in the seed, being often curiously folded and twisted; they are sometimes leaf-like, and in some instances thick and fleshy. In a very few exogenous plants the cotyledons are wanting or are not distinguishable from the plumule. These characters of the embryo, being generally discernible only by the aid of a lens and distinguished with difficulty by the unpractised eye, have been little employed in the present work.

The floral organs above described are all placed upon the more or less expanded apex of the flower-stalk, denominated the disk, receptacle, or thalamus. In many flowers the sepals, petals, stamens, and pistils are all situated immediately upon the receptacle, forming so many distinct concentric whorls; but in some a different arrangement prevails,—a circumstance of which botanists have availed themselves in classification. Where the stamens are placed directly upon the disk, beneath the pistil, they are called hypogynous; where they are attached to the calyx, around the pistil, perigynous; when placed apparently upon the ovary, they are said to be epigynous. It frequently happens that the filaments of the stamens in hypogynous flowers are attached to the petals, while in epigynous ones the calyx and ovary are often so united that the stamens appear to be in connexion with both organs.

The manner in which the flower-buds open, technically called their æstivation, is a feature of some importance in distinguishing tribes of plants. The petals are sometimes so folded that their margins just meet—the æstivation is then valvate; in some flowers they overlap each other like tiles—it is then imbricate; in other