Part 24

Regular gamopetalous corollas are sometimes _campanulate_ or _bell-shaped_, as in (_Campanula_) (fig. 60); _infundibuliform_ or _funnel-shaped_, when the tube is like an inverted cone, and the limb becomes more expanded at the apex, as in tobacco; _hypocrateriform_ or _salver-shaped_, when there is a straight tube surmounted by a flat spreading limb, as in primula (fig. 61); _tubular_, having a long cylindrical tube, appearing continuous with the limb, as in _Spigelia_ and comfrey; _rotate_ or _wheel-shaped_, when the tube is very short, and the limb flat and spreading, as in forget-me-not, _Myosotis_ (when the divisions of the rotate corolla are very acute, as in _Galium_, it is sometimes called _stellate_ or _star-like_); _urceolate_ or _urn-shaped_, when there is scarcely any limb, and the tube is narrow at both ends, and expanded in the middle, as in bell-heath (_Erica cinerea_). Some of these forms may become irregular in consequence of certain parts being more developed than others. Thus, in _Veronica_, the rotate corolla has one division much smaller than the rest, and in foxglove (_Digitalis_) there is a slightly irregular companulate corolla. Of irregular gamopetalous corollas there may be mentioned the _labiate_ or _lipped_ (fig. 62), having two divisions of the limb in the form of lips (the upper one, u, composed usually of two united petals, and the lower, l, of three), separated by a gap. In such cases the tube varies in length, and the parts in their union follow the reverse order of what occurs in the calyx, where two sepals are united in the lower lip and three in the upper. When the upper lip of a labiate corolla is much arched, and the lips separated by a distinct gap, it is called _ringent_ (fig. 62). The labiate corolla characterizes the natural order Labiatae. When the lower lip is pressed against the upper, so as to leave only a chink between them, the corolla is said to be _personate_, as in snapdragon, and some other Scrophulariaceae. In some corollas the two lips become hollowed out in a remarkable manner, as in calceolaria, assuming a slipper-like appearance, similar to what occurs in the labellum of some orchids, as _Cypripedium_. When a tubular corolla is split in such a way as to form a strap-like process on one side with several tooth-like projections at its apex, it becomes _ligulate_ or _strap-shaped_ (fig. 63). This corolla occurs in many composite plants, as in the florets of dandelion, daisy and chicory. The number of divisions at the apex indicates the number of united petals, some of which, however, may be abortive. Occasionally some of the petals become more united than others, and then the corolla assumes a _bilabiate_ or _two-lipped_ form, as seen in the division of Compositae called Labiatiflorae.

Petals are sometimes suppressed, and sometimes the whole corolla is absent. In _Amorpha_ and _Afzelia_ the corolla is reduced to a single petal, and in some other Leguminous plants it is entirely wanting. In the natural order Ranunculaceae, some genera, such as _Ranunculus_, globe-flower and paeony, have both calyx and corolla, while others, such as clematis, anemone and _Caltha_, have only a coloured calyx. Flowers become double by the multiplication of the parts of the corolline whorl; this arises in general from a metamorphosis of the stamens.

Certain structures occur on the petals of some flowers, which received in former days the name of _nectaries_. The term nectary was very vaguely applied by Linnaeus to any part of the flower which presented an unusual aspect, as the crown (_corona_) of narcissus, the fringes of the Passion-flower, &c. If the name is retained it ought properly to include only those parts which secrete a honey-like substance, as the glandular depression at the base of the perianth of the fritillary, or on the petal of _Ranunculus_ (fig. 55), or on the stamens of Rutaceae. The honey secreted by flowers attracts insects, which, by conveying the pollen to the stigma, effect fertilization. The horn-like nectaries under the galeate sepal of aconite (fig. 58) are modified petals, so also are the tubular nectaries of hellebore (fig. 56). Other modifications of some part of the flower, especially of the corolla and stamens, are produced either by degeneration or outgrowth, or by _chorisis_, or _deduplication_. Of this nature are the scales on the petals in _Lychnis_, _Silene_ and _Cynoglossum_, which are formed in the same way as the ligules of grasses. In other cases, as in Samolus, the scales are alternate with the petals, and may represent altered stamens. In _Narcissus_ the appendages are united to form a crown, consisting of a membrane similar to that which unites the stamens in _Pancratium_. It is sometimes difficult to say whether these structures are to be referred to the corolline or to the staminal row.

Petals are attached to the axis usually by a narrow base. When this attachment takes place by an articulation, the petals fall off either immediately after expansion (_caducous_) or after fertilization (_deciduous_). A corolla which is continuous with the axis and not articulated to it, as in campanula and heaths, may be persistent, and remain in a withered or marcescent state while the fruit is ripening. A gamopetalous corolla falls off in one piece; but sometimes the base of the corolla remains persistent, as in _Rhinanthus_ and _Orobanche_.

The _stamens_ and the _pistil_ are sometimes spoken of as the essential organs of the flower, as the presence of both is required in order that perfect seed may be produced. As with few exceptions the stamen represents a leaf which has been specially developed to bear the pollen or microspores, it is spoken of in comparative morphology as a microsporophyll; similarly the carpels which make up the pistil are the megasporophylls (see ANGIOSPERMS). _Hermaphrodite_ or _bisexual_ flowers are those in which both these organs are found; _unisexual_ or _diclinous_ are those in which only one of these organs appears,--those bearing stamens only, being _staminiferous_ or "male"; those having the pistil only, _pistilliferous_ or "female." But even in plants with hermaphrodite flowers self-fertilization is often provided against by the structure of the parts or by the period of ripening of the organs. For instance, in _Primula_ and _Linum_ some flowers have long stamens and a pistil with a short style, the others having short stamens and a pistil with a long style. The former occur in the so-called thrum-eyed primroses (fig. 61), the latter in the "pin-eyed." Such plants are called _dimorphic_. Other plants are _trimorphic_, as species of _Lythrum_, and proper fertilization is only effected by combination of parts of equal length. In some plants the stamens are perfected before the pistil; these are called _proterandrous_, as in _Ranunculus repens_, _Silene maritima_, _Zea Mays_. In other plants, but more rarely, the pistil is perfected before the stamens, as in _Potentilla argentea_, _Plantago major_, _Coix Lachryma_, and they are termed _proterogynous_. Plants in which proterandry or proterogyny occurs are called _dichogamous_. When in the same plant there are unisexual flowers, both male and female, the plant is said to be _monoecious_, as in the hazel and castor-oil plant. When the male and female flowers of a species are found on separate plants, the term _dioecious_ is applied, as in _Mercurialis_ and hemp; and when a species has male, female and hermaphrodite flowers on the same or different plants, as in _Parietaria_, it is _polygamous_.

Stamens.

The stamens arise from the thalamus or torus within the petals, with which they generally alternate, forming one or more whorls, which collectively constitute the _androecium_. Their normal position is below the pistil, and when they are so placed (fig. 64, a) upon the thalamus they are hypogynous. Sometimes they become adherent to the petals, or are _epipetalous_, and the insertion of both is looked upon as similar, so that they are still hypogynous, provided they are independent of the calyx and the pistil. In other cases they are perigynous or epigynous (fig. 65). Numerous intermediate forms occur, especially amongst Saxifragaceae, where the parts are _half superior_ or _half inferior_. Where the stamens become adherent to the pistil so as to form a column, the flowers are said to be _gynandrous_, as in _Aristolochia_ (fig. 66). These arrangements of parts are of great importance in classification. The stamens vary in number from one to many hundreds. In acyclic flowers there is often a gradual transition from petals to stamens, as in the white water-lily (fig. 31). When flowers become double by cultivation, the stamens are converted into petals, as in the paeony, camellia, rose, &c. When there is only one whorl the stamens are usually equal in number to the sepals or petals, and are arranged opposite to the former, and alternate with the latter. The flower is then _isostemonous_. When the stamens are not equal in number to the sepals or petals, the flower is _anisostemonous_. When there is more than one whorl of stamens, then the parts of each successive whorl alternate with those of the whorl preceding it. The staminal row is more liable to multiplication of parts than the outer whorls. A flower with a single row of stamens is _haplostemonous_. If the stamens are double the sepals or petals as regards number, the flower is _diplostemonous_; if more than double, _polystemonous_. The additional rows of stamens may be developed in the usual centripetal (acropetal) order, as in Rhamnaceae; or they may be interposed between the pre-existing ones or be placed outside them, i.e. develop centrifugally (basipetally), as in geranium and oxalis, when the flower is said to be _obdiplostemonous_. When the stamens are fewer than twenty they are said to be _definite_; when above twenty they are _indefinite_, and are represented by the symbol [infinity]. The number of stamens is indicated by the Greek numerals prefixed to the term _androus_; thus a flower with one stamen is _monandrous_, with two, three, four, five, six or many stamens, di-, tri-, tetr-, pent-, hex- or polyandrous, respectively.

The function of the stamen is the development and distribution of the pollen. The stamen usually consists of two parts, a contracted portion, often thread-like, termed the _filament_ (fig. 25 f), and a broader portion, usually of two lobes, termed the _anther_ (a), containing the powdery _pollen_ (p), and supported upon the end of the filament. That portion of the filament in contact with the anther-lobes is termed the _connective_. If the anther is absent the stamen is abortive, and cannot perform its functions. The anther is developed before the filament, and when the latter is not produced, the anther is sessile, as in the mistletoe.

The filament is usually, as its name imports, filiform or thread-like, and cylindrical, or slightly tapering towards its summit. It is often, however, thickened, compressed and flattened in various ways, becoming _petaloid_ in _Canna_, _Marania_, water-lily (fig. 32); _subulate_ or slightly broadened at the base and drawn out into a point like an awl, as in _Butomus umbellatus_; or clavate, that is, narrow below and broad above, as in _Thalictrum_. In some instances, as in _Tamarix gallica_, _Peganum Harmala_, and _Campanula_, the base of the filament is much dilated, and ends suddenly in a narrow thread-like portion. In these cases the base may give off lateral stipulary processes, as in _Allium_ and _Alyssum calycinum_. The filament varies much in length and in firmness. The length sometimes bears a relation to that of the pistil, and to the position of the flower, whether erect or drooping. The filament is usually of sufficient solidity to support the anther in an erect position; but sometimes, as in grasses, and other wind-pollinated flowers, it is very delicate and hair-like, so that the anther is pendulous (fig. 105). The filament is generally continuous from one end to the other, but in some cases it is bent or jointed, becoming _geniculate_; at other times, as in the pellitory, it is spiral. It is colourless, or of different colours. Thus in fuchsia and _Poinciana_, it is red; in _Adamia_ and _Tradescantia virginica_, blue; in _Oenothera_ and _Ranunculus acris_, yellow.

Hairs, scales, teeth or processes of different kinds are sometimes times developed on the filament. In spiderwort (_Tradescantia virginica_) the hairs are beautifully coloured, moniliform or necklace-like, and afford good objects for studying rotation of the protoplasm. Filaments are usually articulated to the thalamus or torus, and the stamens fall off after fertilization; but in _Campanula_ and some other plants they are continuous with the torus, and the stamens remain persistent, although in a withered state. Changes are produced in the whorl of stamens by cohesion of the filaments to a greater or less extent, while the anthers remain free; thus, all the filaments of the androecium may unite, forming a tube round the pistil, or a central bundle when the pistil is abortive, the stamens becoming _monadelphous_, as occurs in plants of the Mallow tribe; or they may be arranged in two bundles, the stamens being _diadelphous_, as in _Polygala_, _Fumaria_ and Pea; in this case the bundles may be equal or unequal. It frequently happens, especially in Papilionaceous flowers, that out of ten stamens nine are united by their filaments, while one (the posterior one) is free (fig. 68). When there are three or more bundles the stamens are _triadelphous_, as in _Hypericum aegyptiacum_, or _polyadelphous_, as in _Ricinus communis_ (castor-oil). In some cases, as in papilionaceous flowers, the stamens cohere, having been originally separate, but in most cases each bundle is produced by the branching of a single stamen. When there are three stamens in a bundle we may conceive the lateral ones as of a stipulary nature. In Lauraceae there are perfect stamens, each having at the base of the filament two abortive stamens or staminodes, which may be analogous to stipules. Filaments sometimes are adherent to the pistil, forming a column (_gynostemium_), as in _Stylidium_, Asclepiadaceae, _Rafflesia_, and Aristolochiaceae (fig. 66); the flowers are then termed _gynandrous_.

The anther.

The _anther_ consists of lobes containing the minute powdery pollen grains, which, when mature, are discharged by a fissure or opening of some sort. There is a double covering of the anther--the outer, or _exothecium_, resembles the epidermis, and often presents stomata and projections of different kinds (fig. 69); the inner, or _endothecium_, is formed by a layer or layers of cellular tissue (fig. 69, cf), the cells of which have a spiral, annular, or reticulated thickening of the wall. The endothecium varies in thickness, 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 indication of two lobes soon appears. Upon these projections the rudiments of the pollen-sacs are then seen, usually four in number, two on each lobe. In each a differentiation takes place in the layers beneath the epidermis, by which an 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, two for each anther-lobe, and when these remain permanently complete it is a _quadrilocular_ or _tetrathecal_ anther (fig. 70). 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 _dithecal_. 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_ (hollyhock). 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, 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 is produced. That part of the anther to which the filament is attached is the _back_, the opposite being the _face_. 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_, indicating the line of dehiscence. The suture is often towards one side in consequence of the valves being unequal. The stamens may cohere by their anthers, and become _syngenesious_, as in composite flowers, and in lobelia, jasione, &c.

The connective.

The anther-lobes are united to the _connective_, which is either continuous with the filament or articulated with it. 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 attachment is very narrow, and an articulation exists, the anthers are movable (_versatile_) and are easily turned by the wind, as in _Tritonia_, grasses (fig. 105), &c., where the filament is attached only to the middle of the connective. The connective may unite the anther-lobes completely 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 oleander; or it is extended backwards and downwards, as in violet (fig. 71), forming a nectar-secreting spur. In _Salvia officinalis_ the connective is attached to the filament in a horizontal manner, so as to separate the two anther-lobes (fig. 72), one only of which contains pollen, the other being imperfectly developed and sterile. The connective is joined to the filament by a movable joint forming a lever which plays an important part in the pollination-mechanism. In _Stachys_ the connective is expanded laterally, so as to unite the bases of the anther-lobes and bring them into a horizontal line.

Antherdehiscence.

The opening or _dehiscence_ of the anthers to discharge their contents takes place either by clefts, by valves, or by pores. When the anther-lobes are erect, the cleft is lengthwise along the line of the suture--_longitudinal dehiscence_ (fig. 25). At other times the slit is horizontal, from the connective to the side, as in _Alchemilla arvensis_ (fig. 73) 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 longitudinal dehiscence may appear to be transverse. 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_, Rhododendron, _Vaccinium_ and _Solanum_ (fig. 74), where there are two, and _Poranthera_, where there are four; whilst in the mistletoe the anther has numerous pores for the discharge of the pollen. Another mode of dehiscence is the valvular, as in the barberry (fig. 75), where each lobe opens by a valve on the outer side of the suture, separately rolling up from base to apex; in some of the laurel tribe there are two such valves for each lobe, or four in all. In some Guttiferae, as _Hebradendron cambogioides_ (the Ceylon gamboge plant), the anther opens by a lid separating from the apex (_circumscissile_ dehiscence).