Section XV. THE SEED.

380. Seeds are the final product of the flower, to which all its parts
and offices are subservient. Like the ovule from which it originates, a
seed consists of coats and kernel.

[Illustration: Fig. 414. Seed of a Linden or Basswood cut through
lengthwise, and magnified, the parts lettered: _a_, the hilum or scar;
_b_, the outer coat; _c_, the inner; _d_, the albumen; _e_, the embryo.]

381. =The Seed-coats= are commonly two (320), the outer and the inner.
Fig. 414 shows the two, in a seed cut through lengthwise. The outer coat
is often hard or crustaceous, whence it is called the _Testa_, or shell
of the seed; the inner is almost always thin and delicate.

[Illustration: Fig. 415. A winged seed of the Trumpet-Creeper.]

[Illustration: Fig. 416. One of Catalpa, the kernel cut to show the
embryo.]

[Illustration: 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-sta