449. Although in most cases the line of dehiscence is parallel with the anther-lobes, it is occasionally transverse 172". In Laurus, the transverse and hinged (444) dehiscence being combined 172, the face of the anther breaks up into four hinged lobes.

450. It may be conjectured that the transverse dehiscence of an anther is analogous to the transverse articulation of petioles (261).

451. The anthers frequently grow together by their margin; Ex. Composites. Such anthers are called syngenesious.

452. The Pollen is formed by a peculiar modification of the cellules of the parenchyma of the anther.

It is demonstrated that pollen is produced

in the interior of the central cells of
an anther by merismatic (59) increase.
Each original cell is thus divided into
two or three, or four other cells, which
gradually change into pollen grains.

171

During this process the original or mother cell is either absorbed, or remains in the form of a viscid or filamentary elastic matter, lying in the midst of the ripe pollen grains.

453. It consists of hollow cases, of extreme smallness, containing a fluid, or fovilla, in which float grains of starch and drops of oil.

454. Its shell is single, double, triple, or quadruple, and its sides are solid, pierced, or slit.

Its skins have received the following names :

1. Extine, for the outermost.

2. Intextine, for that next the outermost.

3. Exintine, for that next the innermost.

4. Intine, for the innermost.

455. It is usually furnished with apertures 1727 173, through which its lining is protruded in the form of a delicate tube, when the pollen comes in contact with the stigma.

This protrusion may be imperfectly excited by placing pollen grains in weak sulphuric acid.

456. The shape of pollen grains is very variable; the spherical 172, the triangular, the polygonal, the oblong, are common forms.

457. Its surface is smooth, or studded with points 172, or divided into facettes, or furnished with crested areolæ.

173

17

458. The pollen grains are usually distinct from each other, but in some cases they cohere in definite numbers; Ex. Acacia 172 or in irregular masses; Ex. Orchids" or are inclosed within a bag, which seems to be the lining of the anther (Endo9 thecium); Ex. Asclepiads

mo

459. In cases where the pollen grains cohere in masses, or are inclosed within bags, they are often connected with a cartilaginous or elastic process, called the caudicle", which adheres to a gland belonging to the stigma.

Although the caudicle is mentioned here, in consequence of its connection with the pollen, yet, like the gland, it is in reality a process of the stigma, as is proved by its development. It must not be confounded with the elastic matter which holds together the pollen grains (452).

460. The function of the pollen is to enable the ovules to produce an embryo.

22

Fig. 173. Various forms of the Pollen, after Fritzsche: 1. Thunbergia alata, as seen when lying in oil; 2. Thunb. fragrans; 3. Passiflora lutea, in oil; 4. Pass. incarnata; 5. Pass. stipulata; 6. Caryocar brasiliense, dry; 7. Cuphea lanceolata, dry; 8. Anona tripetala; 9. Philydrum lanuginosum; 10. A Pollen mass of Inga anomala, discharging a tube; 11. Leschenaultia formosa, dry; 12. Plumbago capensis, side view in oil; 13. End view of the same, in water; 14. Polemonium coeruleum; 15. Geranium sylvaticum, acted on by iodine; 16. Chrysanthemum carinatum, treated with concentrated sulphuric acid; 17. Armeria vulgaris, ditto; 18. Sida Abutilon, dry; 19. Scolymus grandiflorus, treated with sulph. ac fort.; 20. Ipomoea purpurea, in oil; 21. A section of the exine of Lavatera triloba, and one of its spines. treated with sulph. ac. fort.; 22. A fragment of the exine of Nerium splendens, showing one of the openings in it.

THE DISK.

85

XV. OF THE DISK.

461. WHATEVER intervenes between the stamens and the pistil receives the general name of disk.

174

α

462. It usually consists of an annular elevation, encompassing the base of the ovary, when it is sometimes called the cup; Ex. Pæony.

463. It also appears in the form of separate glands or tubercles at the base of an ovary 174 a or of a waxy lining of the tube of the calyx; Ex. Rose: or of tooth-like, hypogynous (428) processes; Ex. Gesnera, Crucifers.

The disk must on no account be confounded with the torus or receptacle, which is a modification of the growing point (227).

464. It is certain that the disk is a non-development of an inner row or rows of stamens, as is proved by the Moutan Pæony. 465. The disk is one of the parts which Linnæan botanists call nectary, because it secretes honey.

466. It contains starch, or similar matter, convertible into sugar, and so applicable to the nutrition of the stamens or pistil.

The honey found in the flower is the surplus after these organs have been supplied. It is supposed to be formed in consequence of the accumulation of water near the flower, and the absorption of oxygen by it. The increase of temperature observed in flowers at the time when the honey is formed is referred to this or some other strong chemical action.

XVI.-OF THE PISTIL.

467. THE organ which occupies the centre of a flower, within the stamens and disk, if the latter be present, is called the pistil 175.

468. It is the female apparatus of flowering plants, or the gynoceum. 469. It consists of one or more carpels.

470. Each carpel is distinguishable into three parts; viz. the ovary the style', and the stigma.

b

A

175

B

175 a

471. If several carpels are united by their whole surface, as is often the case, then there are as many ovaries, styles, and stigmas, as there are carpels, whether they are distinguishable or not.

472. The OVARY is a hollow case, inclosing ovules It contains one or more cavities, called cells.

175 B.

473. The STYLE is the part that connects the ovary and stigma.

474. It is frequently absent, and is no more essential to a pistil than a petiole to a leaf, or a filament to an anther.

475. It is commonly thread-shaped, or at least terete; but sometimes is thin, flat, and membranous, and assumes the form of a petal, as in Iris.

476. It is either articulated with the ovary, or continuous with it. It usually proceeds directly from the apex of the ovary; but in some cases arises from the side, or even the base of that organ; Ex. Alchemill, Chrysobalans.

In such cases the carpel is analogous to a hooded leaf.

477. The STIGMA is that part of the pistil which is adapted to receiving the influence of the pollen. It is generally at the end of the style or at the apex of the ovary, but not always; in Iris, it is placed in a cleft at the back of the style, and in Sarracenia 179, below the points of the convex style. It sometimes produces glandular or other processes, to which the pollen attaches itself in masses (459).

478. Nothing is, properly speaking, Stigma, except that part of the style just described. Nevertheless, the name is often inaccurately applied to mere divisions of the style, as in Labiates; or to the hairy surface of undivided styles, as in Lathyrus, or to the umbrella-shaped expansion of Sarracenia 176.

In the last case the true stigma is placed below the apex of the triangular segments of the umbrella-shaped style, and consists of a tuft of long free cells 179 a 6. 479. Sometimes the stigmas grow to the face of the anthers, which form themselves into a solid mass; Ex. Asclepias. In this case the styles remain separate.

480. A CARPEL is formed by a folded leaf, the upper surface of which is turned inwards, the lower outwards; and within which are developed one or a greater number of ovules (526).

481. When the carpels are stalked, they are said to be seated upon a

thecaphore, or gynophore; Ex. Cleome, Passiflora. Their stalk is analogous to the petiole of a leaf.

176

482. When the carpels are all distinct, or are separable with facility, they are apocarpous; when they all grow into a solid body, which cannot be separated into its constituent parts, they are syncarpous.

483. The ovary is the lamina of the leaf.

484. The style is generally an extension of the midrib, but not always (485). 485. The stigma is the denuded, secreting, humid apex of the style.

Fig. 176.--The umbrella-shaped style and flower of Sarracenia.

Fig. 177.-Structure of the channel found in the centre of the style of a Campanula; cc cellular tissue forming the sides, traversed by brandles of spiral vessels, vv; pp are cells of another form, which line the sides, and, together with others of a long and filamentous character ff, obstruct the channel in part. Fig. 178.-A section of the ovary, &c. of Babingtonia.

Fig. 179.-a The underside of the point of one of its lobes, with the back of the true stigma, which resembles a round tooth; b the stigma seen in front.