The Elements of Botany Part 15

301. The PISTIL, when only one, occupies the centre of the flower; when there are two pistils, they stand facing each other in the centre of the flower; when several, they commonly form a ring or circle; and when very numerous, they are generally crowded in rows or spirals on the surface of a more or less enlarged or elongated receptacle. Their number gives rise to certain terms, the counterpart of those used for stamens (284), which are survivals of the names of orders in the Linnaean artificial system. The names were coined by prefixing Greek numerals to _-gynia_ used for gyncium, and changed into adjectives in the form of _-gynous_. That is, a flower is

_Monogynous_, when it has a single pistil, whether that be simple or compound;

_Digynous_, when it has only two pistils; _Trigynous_, when with three; _Tetragynous_, with four; _Pentagynous_, with five; _Hexagynous_, with six; and so on to _Polygynous_, with many pistils.

302. =The Parts of a Complete Pistil=, as already twice explained (16, 236), are the OVARY, the STYLE, and the STIGMA. The ovary is one essential part: it contains the rudiments of seeds, called OVULES. The stigma at the summit is also essential: it receives the pollen, which fertilizes the ovules in order that they may become seeds. But the style, commonly a tapering or slender column borne on the summit of the ovary, and bearing the stigma on its apex or its side, is no more necessary to a pistil than the filament is to the stamen. Accordingly, there is no style in many pistils: in these the stigma is _sessile_, that is, rests directly on the ovary (as in Fig. 326). The stigma is very various in shape and appearance, being sometimes a little k.n.o.b (as in the Cherry, Fig. 271), sometimes a point or small surface of bare tissue (as in Fig. 327-330), and sometimes a longitudinal crest or line (as in Fig. 324, 341-343), or it may occupy the whole length of the style, as in Fig. 331.

303. The word Pistil (Latin, _Pistillum_) means a pestle. It came into use in the first place for such flowers as those of Crown Imperial, or Lily, in which the pistil in the centre was likened to the pestle, and the perianth around it to the mortar, of the apothecary.

304. A pistil is either _simple_ or _compound_. It is simple when it answers to a single flower-leaf, compound when it answers to two or three, or a fuller circle of such leaves conjoined.

305. =Carpels.= It is convenient to have a name for each flower-leaf of the gyncium; so it is called a _Carpel_, in Latin _Carpellum_ or _Carpidium_. A simple pistil is a carpel. Each component flower-leaf of a compound pistil is likewise a carpel. When a flower has two or more pistils, these of course are simple pistils, that is, separate carpels or pistil-leaves. There may be only a single simple pistil to the flower, as in a Pea or Cherry blossom (Fig. 271); there may be two such, as in many Saxifrages; or many, as in the Strawberry. More commonly the single pistil in the centre of a blossom is a compound one. Then there is seldom much difficulty in ascertaining the number of carpels or pistil-leaves that compose it.

306. =The Simple Pistil=, viewed morphologically, answers to a leaf-blade with margins incurved and united where they meet, so forming a closed case or pod (the ovary), and bearing ovules at the suture or junction of these margins: a tapering upper portion with margins similarly inrolled, is supposed to form the style; and these same margins, exposed at the tip or for a portion of the length, become the stigma. Compare, under this view, the three accompanying figures.

[Ill.u.s.tration: Fig. 323. An inrolled small leaf, such as in double-flowered Cherry blossoms is often seen to occupy the place of a pistil.]

[Ill.u.s.tration: Fig. 324. A simple pistil (of Isopyrum), with ovary cut across; the inner (ventral) face turned toward the eye: the ovules seem to be borne on the ventral suture, answering to leaf-margins: the stigma above seen also to answer to leaf-margins.]

[Ill.u.s.tration: Fig. 325. Pod or simple pistil of Caltha or Marsh-Marigold, which has opened, and shed its seeds.]

307. So a simple pistil should have a one-celled ovary, only one line of attachment for the ovules, a single style, and a single stigma. Certain variations from this normal condition which sometimes occur do not invalidate this morphological conception. For instance, the stigma may become two-lobed or two-ridged, because it consists of two leaf-margins, as Fig. 324 shows; it may become 2-locellate by the turning or growing inward of one of the sutures, so as to divide the cavity.

308. There are two or three terms which primarily relate to the parts of a simple pistil or carpel, and are thence carried on to the compound pistil, viz.:--

VENTRAL SUTURE, the line which answers to the united margins of the carpel-leaf, therefore naturally called a suture or seam, and the ventral or inner one, because in the circle of carpel-leaves it looks inward or to the centre of the flower.

DORSAL SUTURE is the line down the back of the carpel, answering to the midrib of the leaf,--not a seam therefore; but at maturity many fruits, such as pea-pods, open by this dorsal as well as by the ventral line.

PLACENTA, a name given to the surface, whatever it be, which bears the ovules and seeds. The name may be needless when the ovules grow directly on the ventral suture, or from its top or bottom; but when there are many ovules there is usually some expansion of an ovule-bearing or seed-bearing surface; as is seen in our Mandrake or Podophyllum, Fig.

326.

[Ill.u.s.tration: Fig. 326. Simple pistil of Podophyllum, cut across, showing ovules borne on placenta.]

[Ill.u.s.tration: Fig. 327. Pistil of a Saxifrage, of two simple carpels or pistil-leaves, united at the base only, cut across both above and below.]

[Ill.u.s.tration: Fig. 328. Compound 3-carpellary pistil of common St.

John's-wort, cut across: the three styles separate.]

[Ill.u.s.tration: Fig. 329. The same of shrubby St. John's-wort; the three styles as well as ovaries here united into one.]

[Ill.u.s.tration: Fig. 330. Compound 3-carpellary pistil of Tradescantia or Spiderwort; the three stigmas as well as styles and ovary completely coalescent into one.]

309. =A Compound Pistil= is a combination of two, three, or a greater number of pistil-leaves or carpels in a circle, united into one body, at least by their ovaries. The annexed figures should make it clear. A series of Saxifrages might be selected the gyncium of which would show every gradation between two simple pistils, or separate carpels, and their complete coalescence into one compound and two-celled ovary.

Even when the const.i.tuent styles and stigmas are completely coalescent into one, the nature of the combination is usually revealed by some external lines or grooves, or (as in Fig. 328-330) by the internal part.i.tions, or the number of the placentae. The simplest case of compound pistil is that

310. =With two or more Cells and Axile Placentae=, namely, with as many cells as there are carpels, that have united to compose the organ. Such a pistil is just what would be formed if the simple pistils (two, three, or five in a circle, as the case may be), like those of a Paeony or Stonecrop (Fig. 224, 225), pressed together in the centre of the flower, were to cohere by their contiguous parts. In such a case the placentae are naturally _axile_, or all brought together in the axis or centre; and the ovary has as many DISSEPIMENTS, or internal _Part.i.tions_, as there are carpels in its composition. For these are the contiguous and coalescent walls or sides of the component carpels. When such pistils ripen into pods, they often separate along these lines into their elementary carpels.

[Ill.u.s.tration: Fig. 331, 332. Pistil of a Sandwort, with vertical and transverse section of the ovary: free central placenta.]

311. =One-celled, with free Central Placenta.= The commoner case is that of Purslane (Fig. 272) and of the Pink and Chickweed families (Fig. 331, 332). This is explained by supposing that the part.i.tions (such as those of Fig. 329) have early vanished or have been suppressed. Indeed, traces of them may often be detected in Pinks. On the other hand, it is equally supposable that in the Primula family the free central is derived from parietal placentation by the carpels bearing ovules only at base, and forming a consolidated common placenta in the axis. Mitella and Dionaea help out this conception.

[Ill.u.s.tration: Fig. 333. Plan of a one-celled ovary of three carpel-leaves, with parietal placentae, cut across below, where it is complete; the upper part showing the top of the three leaves it is composed of, approaching, but not united.]

[Ill.u.s.tration: Fig. 334. Cross section of the ovary of Frost weed (Helianthemum), with three parietal placentae, bearing ovules.]

[Ill.u.s.tration: Fig. 335. Cross section of an ovary of Hyperic.u.m graveolens, the three large placentae meeting in the centre, so as to form a three-celled ovary. 336. Same in fruit, the placentae now separate and rounded.]

312. =One-celled, with Parietal Placentae.= In this not uncommon case it is conceived that the two or three or more carpel-leaves of such a compound pistil coalesce by their adjacent edges, just as sepal-leaves do to form a gamosepalous calyx, or petals to form a gamopetalous corolla, and as is shown in the diagram, Fig. 333, and in an actual cross-section, Fig. 334. Here each carpel is an open leaf, or with some introflexion, bearing ovules along its margins; and each placenta consists of the contiguous margins of two pistil-leaves grown together.

There is every gradation between this and the three-celled ovary with the placentae in the axis, even in the same genus, sometimes even in different stages in the same pistil (Fig. 335, 336).

-- 2. GYMNOSPERMOUS GYNCIUM.

313. The ordinary pistil has a closed ovary, and accordingly the pollen can act upon the contained ovules only indirectly, through the stigma.

This is expressed in a term of Greek derivation, viz.:--

_Angiospermous_, meaning that the seeds are borne in a sac or closed vessel. The counterpart term is

_Gymnospermous_, meaning naked-seeded. This kind of pistil, or gyncium, the simplest of all, yet the most peculiar, characterizes the Pine family and its relatives.

[Ill.u.s.tration: Fig. 337. A pistil, that is, a scale of the cone, of a Larch, at the time of flowering; inside view, showing its pair of naked ovules.]

[Ill.u.s.tration: Fig. 338. Branchlet of the American Arbor-Vitae, considerably larger than in nature, terminated by its pistillate flowers, each consisting of a single scale (an open pistil), together forming a small cone.]

[Ill.u.s.tration: Fig. 339. One of the scales or carpels of the last, removed and more enlarged, the inside exposed to view, showing a pair of ovules on its base.]

314. While the ordinary simple pistil is conceived by the botanist to be a leaf rolled together into a closed pod (306), those of the Pine, Larch (Fig. 337), Cedar, and Arbor-Vitae (Fig. 338, 339) are open leaves, in the form of scales, each bearing two or more ovules on the inner face, next the base. At the time of blossoming, these pistil-leaves of the young cone diverge, and the pollen, so abundantly shed from the staminate blossoms, falls directly upon the exposed ovules. Afterward the scales close over each other until the seeds are ripe. Then they separate that the seeds may be shed. As the pollen acts directly on the ovules, such pistil (or organ acting as pistil) has no stigma.

315. In the Yew, and in Torreya and Gingko, the gyncium is reduced to extremest simplicity, that is, to a naked ovule, without any visible carpel.

316. In Cycas the large naked ovules are borne on the margins or lobes of an obvious open leaf. All GYMNOSPERMOUS plants have other peculiarities, also distinguis.h.i.+ng them, as a cla.s.s, from ANGIOSPERMOUS plants.

Section XI. OVULES.

317. =Ovule= (from the Latin, meaning a little egg) is the technical name of that which in the flower answers to and becomes the seed.

[Ill.u.s.tration: Fig. 340. A cl.u.s.ter of ovules, pendulous on their funicles.]

318. Ovules are _naked_ in gymnospermous plants (as just described), in all others they are enclosed in the ovary. They may be produced along the whole length of the cell or cells of the ovary, and then they are apt to be numerous, or only from some part of it, generally the top or the bottom. In this case they are usually few or single (_solitary_, as in Fig. 341-343). They may be _sessile_, i. e. without stalk, or they may be attached by a distinct stalk, the FUNICLE or FUNICULUS (Fig.

340).

[Ill.u.s.tration: Fig. 341. Section of the ovary of a b.u.t.tercup, lengthwise, showing its ascending ovule.]