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-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.]
[Illustration: Fig. 422. The same, cut through lengthwise: _a_, the
hilum; _c_, chalaza; _d_, outer seed coat; _e_, inner seed-coat; _f_,
the albumen; _g_, the minute embryo.]
[Illustration: Fig. 423. Seed of a St. John's-wort, divided lengthwise;
here the whole kernel is embryo.]
386. =The Kernel, or Nucleus=, is the whole body of the seed within the
coats. In many seeds the kernel is all _Embryo_; in others a large part
of it is the _Albumen_. For example, in Fig. 423, it is wholly embryo;
in Fig. 422, all but the small speck (_g_) is albumen.
387. =The Albumen or Endosperm= of the seed is sufficiently
characterized and its office explained in Sect. III., 31-35.
388. =The Embryo= or _Germ_, which is the rudimentary plantlet and the
final result of blossoming, and its development in germination have been
extensively illustrated in Sections II. and III. Its essential parts are
the _Radicle_ and the _Cotyledons_.
389. =Its Radicle or Caulicle= (the former is the term long and
generally used in botanical descriptions, but the latter is the more
correct one, for it is the initial stem, which merely gives origin to
the root), as to its position in the seed, always points to and lies
near the micropyle. In relation to the pericarp it is
_Superior_, when it points to the apex of the fruit or cell, and
_Inferior_, when it points to its base, or downward.
[Illustration: Fig. 424. Embryo of Calycanthus; upper part cut away, to
show the convolute cotyledons.]
390. =The Cotyledons= have already been illustrated as respects their
number,--giving the important distinction of _Dicotyledonous_,
_Polycotyledonous_ and _Monocotyledonous_ embryos (36-43),--also as
regards their thickness, whether _foliaceous_ or _fleshy_; and some of
the very various shapes and adaptations to the seed have been figured.
They may be straight, or folded, or rolled up. In the latter case the
cotyledons may be rolled up as it were from one margin, as in
Calycanthus (Fig. 424), or from apex to base in a flat spiral, or they
may be both folded (_plicate_) and rolled up (_convolute_), as in Sugar
Maple (Fig. 11.) In one very natural family, the Cruciferae, two
different modes prevail in the way the two cotyledons are brought round
against the radicle. In one series they are
_Accumbent_, that is, the edges of the flat cotyledons lie against the
radicle, as in Fig. 425, 426. In another they are
[Illustration: Fig. 425. Seed of Bitter Cress, Barbarea, cut across to
show the accumbent cotyledons. 426. Embryo of same, whole.]
_Incumbent_, or with the plane of the cotyledons brought up in the
opposite direction, so that the back of one of them lies against the
radicle, as shown in Fig. 427, 428.
[Illustration: Fig. 427. Seed of a Sisymbrium, cut across to show the
incumbent cotyledons. 428. Embryo of the same, detached whole.]
391. As to the situation of the embryo with respect to the albumen of
the seed, when this is present in any quantity, the embryo may be
_Axile_, that is occupying the axis or centre, either for most of its
length, as in Violet (Fig. 429), Barberry (Fig. 48), and Pine (Fig. 56);
and in these it is straight. But it may be variously curved or coiled in
the albumen, as in Helianthemum (Fig. 430), in a Potato-seed (Fig. 50),
or Onion-seed (Fig. 60), and Linden (Fig. 414); or it may be coiled
around the outside of the albumen, partly or into a circle, as in
Chickweed (Fig. 431, 432) and in Mirabilis (Fig. 52). The latter mode
prevails in Campylotropous seeds. In the cereal grains, such as Indian
Corn (Fig. 67) and Rice (Fig. 430a), and in all other Grasses, the
embryo is straight and applied to the outside of the abundant albumen.
[Illustration: Fig. 429. Section of seed of Violet; anatropous with
straight axile embryo in the albumen. 430. Section of seed of Rock Rose,
Helianthemum Canadense; orthotropous, with curved embryo in the albumen.
430a. Section of a grain of Rice, lengthwise, showing the embryo outside
the albumen, which forms the principal bulk.]
[Illustration: Fig. 431. Seed of a Chickweed, campylotropous. 432.
Section of same, showing slender embryo coiled around the outside of the
albumen of the kernel.]
392. The matured seed, with embryo ready to germinate and reproduce the
kind, completes the cycle of the vegetable life in a phanerogamous
plant, the account of which began with the seed and seedling.