The Elements of Botany Part 19

[Ill.u.s.tration: Fig. 395. Capsule of Iris, with loculicidal dehiscence; below, cut across.]

[Ill.u.s.tration: Fig. 396. Pod of a Marsh St. John's-wort, with septicidal dehiscence.]

371. Regular _Dehiscence_ in a capsule takes place in two ways, which are best ill.u.s.trated in pods of two or three cells. It is either

_Loculicidal_, or, splitting directly into the _loculi_ or cells, that is, down the back (or the dorsal suture) of each cell or carpel, as in Iris (Fig. 395); or

_Septicidal_, that is, splitting through the part.i.tions or _septa_, as in St. John's-wort (Fig. 396), Rhododendron, etc. This divides the capsule into its component carpels, which then open by their ventral suture.

[Ill.u.s.tration: Fig. 397, 398. Diagrams of the two modes.]

[Ill.u.s.tration: Fig. 399. Diagram of septifragal dehiscence of the loculicidal type. 400. Same of the septicidal or _marginicidal_ type.]

372. In loculicidal dehiscence the valves naturally bear the part.i.tions on their middle; in the septicidal, half the thickness of a part.i.tion is borne on the margin of each valve. See the annexed diagrams. A variation of either mode occurs when the valves break away from the part.i.tions, these remaining attached in the axis of the fruit. This is called _Septifragal_ dehiscence. One form is seen in the Morning-Glory (Fig.

400).

373. The capsules of Rue, Spurge, and some others, are both loculicidal and septicidal, and so split into half-carpellary valves or pieces.

374. =The Silique= (Fig. 401) is the technical name of the peculiar pod of the Mustard family; which is two-celled by a false part.i.tion stretched across between two parietal placentae. It generally opens by two valves from below upward, and the placentae with the part.i.tion are left behind when the valves fall off.

375. =A Silicle or Pouch= is only a short and broad silique, like that of the Shepherd's Purse, Fig. 402, 403.

[Ill.u.s.tration: Fig. 401. Silique of a Cadamine or Spring Cress.]

[Ill.u.s.tration: Fig. 402. Silicle of Shepherd's Purse.]

[Ill.u.s.tration: Fig. 403. Same, with one valve removed.]

[Ill.u.s.tration: Fig. 404. Pyxis of Purslane, the lid detaching.]

376. =The Pyxis= is a pod which opens by a circular horizontal line, the upper part forming a lid, as in Purslane (Fig. 404), the Plantain, Henbane, etc. In these the dehiscence extends all round, or is _circ.u.mscissile_. So it does in Amaranth (Fig. 387), forming a one-seeded utricular pyxis. In Jeffersonia, the line does not separate quite round, but leaves a portion for a hinge to the lid.

377. Of Multiple or Collective Fruits, which are properly ma.s.ses of fruits aggregated into one body (as is seen in the Mulberry (Fig. 408), Pine-apple, etc.), there are two kinds with special names and of peculiar structure.

[Ill.u.s.tration: Fig. 405. A fig-fruit when young. 406. Same in section.

407. Magnified portion, a slice, showing some of the flowers.]

[Ill.u.s.tration: Fig. 408. A mulberry. 409. One of the grains younger, enlarged; seen to be a pistillate flower with calyx becoming fleshy.

410. Same, with fleshy calyx cut across.]

378. =The Syconium or Fig-fruit= (Fig. 405, 406) is a fleshy axis or summit of stem, hollowed out, and lined within by a mult.i.tude of minute flowers, the whole becoming pulpy, and in the common fig, luscious.

379. =The Strobile or Cone= (Fig. 411), is the peculiar multiple fruit of Pines, Cypresses, and the like; hence named _Coniferae_, viz.

cone-bearing plants. As already shown (313), these cones are _open pistils_, mostly in the form of flat scales, regularly overlying each other, and pressed together in a spike or head. Each scale bears one or two naked seeds on its inner face. When ripe and dry, the scales turn back or diverge, and in the Pine the seed peels off and falls, generally carrying with it a wing, a part of the lining of the scale, which facilitates the dispersion of the seeds by the wind (Fig. 412, 413). In Arbor-Vitae, the scales of the small cone are few, and not very unlike the leaves. In Cypress they are very thick at the top and narrow at the base, so as to make a peculiar sort of closed cone. In Juniper and Red Cedar, the few scales of the very small cone become fleshy, and ripen into a fruit which closely resembles a berry.

[Ill.u.s.tration: Fig. 411. Cone of a common Pitch Pine. 412. Inside view of a separated scale or open carpel; one seed in place: 413, the other seed.]

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.

[Ill.u.s.tration: Fig. 414. Seed of a Linden or Ba.s.swood cut through lengthwise, and magnified, the parts lettered: _a_, the hilum or scar; _b_, the outer coat; _c_, the inner; _d_, the alb.u.men; _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 sh.e.l.l of the seed; the inner is almost always thin and delicate.

[Ill.u.s.tration: Fig. 415. A winged seed of the Trumpet-Creeper.]

[Ill.u.s.tration: Fig. 416. One of Catalpa, the kernel cut to show the embryo.]

[Ill.u.s.tration: 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 princ.i.p.al 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.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 420. Seed of a Violet (anatropous): _a_, hilum; _b_, rhaphe; _c_, chalaza.]

[Ill.u.s.tration: Fig. 421. Seed of a Larkspur (also anatropous); the parts lettered as in the last.]

[Ill.u.s.tration: Fig. 422. The same, cut through lengthwise: _a_, the hilum; _c_, chalaza; _d_, outer seed coat; _e_, inner seed-coat; _f_, the alb.u.men; _g_, the minute embryo.]

[Ill.u.s.tration: 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 _Alb.u.men_. For example, in Fig. 423, it is wholly embryo; in Fig. 422, all but the small speck (_g_) is alb.u.men.