382. The shape and the markings, so various in different seeds, depend mostly on the outer coat. Sometimes this fits the kernel closely; sometimes it is expanded into a _wing_, as in the Trumpet-Creeper (Fig. 415), and occasionally this wing is cut up into shreds or tufts, as in the Catalpa (Fig. 416); or instead of a wing it may bear a _Coma_, or tuft of long and soft hairs, as in the Milkweed or Silkweed (Fig. 417). The use of wings, or downy tufts is to render the seeds buoyant for dispersion by the winds. This is clear, not only from their evident adaptation to this purpose, but also from the fact that winged and tufted seeds are found only in fruits that split open at maturity, never in those that remain closed. The coat of some seeds is beset with long hairs or wool. _Cotton_, one of the most important vegetable products, since it forms the principal clothing of the larger part of the human race, consists of the long and woolly hairs which thickly cover the whole surface of the seed. There are also crests or other appendages of various sorts on certain seeds. A few seeds have an additional, but more or less incomplete covering, outside of the real seed-coats called an 383. =Aril, or Arillus.= The loose and transparent bag which encloses the seed of the White Water-Lily (Fig. 418) is of this kind. So is the _mace_ of the nutmeg; and also the scarlet pulp around the seeds of the Waxwork (Celastrus) and Strawberry-bush (Euonymus). The aril is a growth from the extremity of the seed-stalk, or from the placenta when there is no seed-stalk. [Illustration: Fig. 418. Seed of White Water Lily, enclosed in its aril.] 384. A short and thickish appendage at or close to the hilum in certain seeds is called a CARUNCLE or STROPHIOLE (Fig. 419). [Illustration: Fig. 419. Seed of Ricinus or Castor oil plant, with caruncle.] 385. The various terms which define the position or direction of the ovule (erect, ascending, etc.) apply equally to the seed: so also the terms anatropous, orthotropous, campylotropous, etc., as already defined (320, 321), and such terms as HILUM, or _Scar_ left where the seed-stalk or funiculus falls away, or where the seed was attached directly to the placenta when there is no seed-stalk. RHAPHE, the line or ridge which runs from the hilum to the chalaza in anatropous and amphitropous seeds. CHALAZA, the place where the seed-coats and the kernel or nucleus are organically connected,--at the hilum in orthotropous and campylotropous seeds, at the extremity of the rhaphe or tip of the seed in other kinds. MICROPYLE, answering to the _Foramen_ or orifice of the ovule. Compare the accompanying figures and those of the ovules, Fig. 341-355. [Illustration: Fig. 420. Seed of a Violet (anatropous): _a_, hilum; _b_, rhaphe; _c_, chalaza.] [Illustration: Fig. 421. Seed of a Larkspur (also anatropous); the parts lettered as in the last.]