Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/147

ANTHERS.] B T A N Y 137 generally becoming thinner towards the part where the anther opens, and there disappears entirely. The walls of the cells are frequently absorbed, so that when the anther attains maturity the fibres are alone left, and these by their elasticity assist in discharging the pollen. The anther is developed before the filament, and is always sessile in the first instance, and sometimes continues so. It appears at first as a simple cellular papilla of meristem, upon which an in dication of two lobes soon appears. Upon these projections the rudiments of the pollen-sacs are then seen, usually four in number, two on each lobn. In each a differentiation takes place in the layers beneath the epidermis, by which riu outer layer of small-celled tissue surrounds an inner portion of large cells. Those central cells are the mother- cells of the pollen, whilst the small-celled layer of tissue external to them becomes the endothecium, the exothecium being formed from the epidermal layer. In the young state there are usually four pollen-sacs or cavities produced, two for each anther-lobe, and when these remain permanently complete, it is a quadrilocular or tetrathecal anther (fig. 220). The four cavities are some times placed in apposition, as in Poranthera, and Tetratheca juncea, and at other times two are placed above and two below, as in Persea gratissima. Sometimes, however, only two cavities remain in the anther, by union of the sacs in each lobe, in which case the anther is said to be bilocular or ditkecal. Sometimes the anther has a single cavity, and becomes unilocular, or monothecal, or dimidiate, either by the disappearance of the partition between the two lobes, or by the abortion of one of its lobes, as in Styphelia laeta and Althaea officinalis. Occasionally there are numerous cavities in the anther, as in Viscum and Rafflesia. The form of the anther-lobes varies. They are generally of a more or less oval or elliptical form, or they may be globular, as in Mercurialis annua ; at other times linear or clavate, curved, flexuose, or sinuose or anfractuose, as in Bryony and Gourd. According to the amount of union of the lobes and the unequal development of different parts of their surface an infinite variety of forms are pro duced. That part of the anther to which the filament is attached, and which is generally towards the petals, is the back, the opposite being the/ctce. The division between the lobes is marked on the face of the anther by a groove or furrow, and there is usually on the face a suture, indi cating the line where the membranous coverings open to discharge the pollen. The suture is often towards one side in consequence of the valves being unequal. The stamens may cohere by their anthers, and become syngene- sious or synantherous, as in Composite flowers, and in Lobelia, Jasione, Viola, &c. The anther-lobes are united to the connective, which may bo either continuous with the filament or articulated with it. It consists of parenchyma, in which the spiral vessels of the filament terminate. From the connective a parti tion or septum extends across each antherine loculus, dividing it either partially or completely. The septum sometimes reaches the suture When the filament is continuous with the connective, and is prolonged so that the anther-lobes appear to be united to it throughout their whole length, and lie in apposition to it and on both sides of it, the anther is said to be adnate or adherent ; when the filament ends at the base of the anther, then the latter is innate or erect. In these cases the anther is to a greater or less degree fixed. When, however, the attach ment is very narrow, and an articulation exists, the anthers are movable, and are easily turned by the wind. This is well seen in what are called versatile anthers, as in Tritonia, Grasses, &c. (fig. 212), where the filament is attached only to the middle of the connective ; and it may occur also in cases where it is attached to the apex, as in pendulous anthers. The connective may unite the untnor-lobes com pletely, or only partially. It is sometimes very short, and is reduced to a mere point, so that the lobes are separate or free. At other times it is prolonged upwards beyond the lobes, assuming various forms, as in Acalypha, and Nerium Oleander ; or it is extended backwards and downwards, as in Violet (fig. 221), and Ticorea febrifuga. In Salvia officinalis the connective is attached to the filament in a horizontal mariner, so as to separate the two anther-lobes, and then it is called distractile (fig. 222) ; one of the lobes only being perfect or fertile, containing pollen, the other imperfectly developed and sterile. In Stachys the connective is expanded laterally, so as to unite the bases of the antherine lobes, and bring them into a horizontal line. The opening of the anthers to discharge their contents is denominated dehiscence. This takes place either by clefts, by hinges, or by pores. When the anther-lobes are erect, the cleft takes place lengthwise along the line of the suture, constituting longitudinal dehiscence (fig. 175). At other times the slit takes place in a horizontal manner, from the connective to the side, as in Alchemilla arvensis (fig. 223) and in Lemna ; the dehiscence is then transverse. When the anther-lobes are rendered horizontal by the enlargement of the connective, then what is really longi tudinal dehiscence may appear to be transverse. In other cases, when the lobes are united at the base, the fissure in each of them may be continuous, and the two lobes may appear as one. The cleft does not always proceed the whole length of the anther-lobe at once, but often for a time it extends only partially. In other instances the opening is confined to the base or apex, each loculament opening by a single pore, as in Pyrola, Tetratheca juncea, Rhododen dron, Vaccinium, and Solanum (fig. 224), where there are Fig. 222. Fig. 225. FIG. 221. Two stamens of Pansy (7iVa tricolor), with their two anther lobes nn.1 the process p extending beyond them. One of the stamens has been depilvcd of its spur, the other shows its spur, c. FIG. 222. Anther of Salvia officinalis. If, fertile lobe full of pollen; Is, barrel) lobe without pollen; e, distractile connective; /, filament. FIG. 223. Stamen of Lady s Mantle (Alchenrilla), with the anther opening transversely. FIG. 224. Stiuncn of a species of Nightshade (Solanum), showing the dtvergenco of the anther-lobes at the base, and the dehiscence by pores at the apex a. FIG. 225. The stamen of the Barberry (Herberts wlffaru), showing one of the valves of the anther v, curved upwards, bearing the pollen on its inner surface. two, and Poranthera, where there are four ; whilst in the Mistleto the anther has numerous pores for the discharge of the pollen. Another mode of dehiscence is called

IV 18