Fig. 415. A winged seed of the Trumpet-Creeper.
Fig. 416. One of Catalpa, the kernel cut to show the embryo.
Fig. 417. Seed of Milkweed, with a Coma or tuft of long silky hairs at one end.
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.
