Flower

FLOWER, a term popularly used for the blossom of a plant, and so by analogy for the finest part or aspect of anything. Here it is dealt with only botanically. The flower is characteristic of the highest group of plants—the flowering plants (Phanerogams) —and is the association of more or less leaf-like organs which are concerned with the production of the seed. In modern botany the group is often known as the seed-plants or Spermophyta (see ANGIOSPERMS and GYMNOSPERMS). As the seed develops from the ovule, fertilised by the pollen, the essential structures for seed production are the pollen-bearer or stamen and the ovule-bearer or carpel. In comparative morphology, these are known as sporophylls because they bear the microspores (pollen-grains), developed in the microsporangia (pollen-sacs) or the megaspore, contained in the megasporangium (ovule). In the more primitive gymnosperms (q.v.) the micro- or mega- sporophylls are gen erally associated in separate cones, to which the term "flower" has been applied. But it is to more definite and elaborate struc tures in the higher angiosperms (q.v.) that "flower" generally refers.

Bracts.

Flowers are produced from flower-buds in the axils of leaves called bracts; "bract" is properly restricted to the leaf from which the primary floral axis arises, while leaves which arise between the bract and the flower are bracteoles. Their arrange ment is similar to that of foliage leaves. In many cases, bracts protect the young flower, but they are sometimes undeveloped, and usually fall off early. Sometimes, especially with bracteoles, no flower-buds arise in their axils. At the base of the general umbel in umbelliferous plants is often a whorl of bracts, the general involucre, while partial involucres or involucels are found at the base of the umbellules. In Compositae the involucre sur rounds the heads of flowers (fig. 12). When bracts become united, the outer ones often do not produce flowers. A sheathing bract or spathe, enclosing one or several flowers is common among monocotyledons and may, in some palms, reach 20 ft. in length and enclose 200,000 flowers. The spathe may be coloured (An thurium) or white (arum lily, Zantedeschia ethiopica). In grasses, the outer scales (glumes) of the spikelet are sterile bracts. Bracts may be changed into leaves (phyllody).

Inflorescence.

The arrangement of flowers on the axis is called the inflorescence. The primary axis of the inflorescence is the rachis; its branches, when present, are peduncles, which in turn may give rise to pedicels. A flower having a stalk is peduncu late or pedicellate; one with no stalk is sessile. To obviate any confusion, it is common to speak of the rachis as the primary floral axis, the peduncles as the secondary floral axes, and so on. The peduncle may be simple, bearing a single flower, or branched. When it proceeds from radical leaves, it is a scape. The floral axis may be shortened, as in daisy and fig, or flattened, convex or concave in form and bearing numerous flowers, as in marigold or in fig. Adhesions occasionally take place between the peduncle and the bracts, as in the lime-tree (fig. I). The termination of the part upon which the whorls of the flower are arranged is the receptacle.

There are two types of inflorescence—one in which the flowers arise as lateral shoots from a primary axis, which goes on elongat ing, and the lateral shoots never exceed in their development the length of the primary axis beyond their point of origin ; the flowers are thus axillary and the inflorescence is indeterminate. In the other type, the primary axis ends in a single flower, but lateral axes arise from the axils of bracts and repeat the process, the development of each lateral axis being stronger than that of the primary axis beyond its point of origin. The flowers are thus terminal and the inflorescence is definite. In indeterminate in florescences, the lower or outer flowers expand first (centripetal) ; in determinate inflorescences, it is the upper or inner flowers that open first (centrifugal). In some inflorescences (mixed), the primary axis has an arrangement belonging to one type, the branches one of the opposite type.

Amongst indefinite forms, the simplest is when a lateral shoot in the axil of a single foliage leaf ends in a single flower. In this case the flower is solitary. A more complicated inflorescence is commoner. Thus if the primary axis, as in fig. 2, is elongated and gives off pedicels ending in a single flower, a raceme is produced, which becomes a panicle if the secondary axes branch. If the lower flower-stalks are developed more strongly than the upper, and thus all the flowers are on a level, a corymb is formed, which may be simple (fig. 4) or branched. If the pedicels are wanting, so that the flowers are sessile, the result is a spike. If this bears unisexual flowers it is a catkin; if it becomes succulent and sur rounded by a spathe it is a spadix, which may be simple (fig. 7) or branched. A spike with female flowers only and covered with scales, as in the hop, is a strobilus. In grasses there are usually numerous sessile flowers in small spikes (spikelets) ; if these are borne not on the primary but on secondary axes, they form a panicle.

If the primary axis is contracted, other forms result. When it is so short that the secondary axes arise from a common point and spread out as radii of nearly equal length each ending in a single flower or dividing again similarly, an umbel (fig. 8) is produced. If there are numerous flowers on a flattened, convex or concave receptacle, having short pedicels or none, a capitulum is formed, as in marigold and scabious (fig. 3) . If the margins of the recep tacle are developed upwards, they may ultimately give rise to the hypanthodium of the fig.

In compound indefinite inflorescences, the lateral shoots, de veloped centripetally, bear numerous bracteoles from which floral shoots arise, which may have a centripetal arrangement similar to that on the mother shoot, or a different one. Thus we may have a group of racemes arranged in a racemose manner; compound umbels as in most Umbelliferae (fig. 8) ; a raceme of capitula ; a raceme of umbels, as in ivy, and so on.

The simplest type of definite inflorescence is where, as in Anemone nemorosa, the axis terminates in a single flower, no other flowers being produced on the plant. This is a solitary terminal inflorescence. When the primary floral axis, before ending in a flower, gives off lateral unifloral axes which repeat the process, the result is a cyme. A cyme with two axes is a biparous cyme or dichasium (fig. II); with one axis, it is uniparous. In the dead nettle (Lamium) the flowers arise in the axis of each foliage leaf and appear as if arranged in a whorl, but there is a central flower expanding first, and from its axis spring secondary axes bearing single flowers (fig. 1 o) . The inflorescence is thus a dichasium and the clusters are called verticillasters. Where numerous lateral axes arise from the primary axis, a cymose umbel is produced. When these lateral axes grow strongly and develop irregularly, as in species of muncus, the inflorescence is an anthela. In a uniparous cyme, the basal portion of the consecutive axes may become thickened and arranged more or less in a straight line, forming a false axis or sympodium, the inflorescence thus simulating a raceme. The uniparous cyme presents two forms ; in a scorpoid cyme the flowers are arranged alternately in a double row along one side of the sympodium, the bracts forming another double row on the opposite side (e.g., Boraginaceae). In a helicoid cyme, the flowers are in a single row and form a spiral round the false axis.

Compound definite inflorescences are by no means common, but in Streptocarpus polyanthus and in several calceolarias we probably have examples. Here we find scorpoid cymes of pairs of flowers, each pair consisting of an older and a younger flower.

Both the definite and indefinite types are represented in mixed inflorescences. Thus in the ragwort (fig. 12) the heads of flowers are developed centrifugally, while the florets open centripetally. Various combinations occur in different families.

Tabular View of Inflorescences \. indefinite Centripetal Inflorescences.

I. Flowers solitary, axillary, Veronica hederifolia.

II. Flowers in groups, pedicellate.

i. Elongated form (Raceme), Hyacinth; (Corymb), Prunus.

ii. Contracted form (Umbel), Cowslip.

III. Flowers in groups, sessile.Iii. Flowers in groups, sessile.

i. Elongated form (Spike), Plantago.

(Spikelet), grasses.

(Catkin), Hazel.

(Spadix), Arum.

(Strobilus), Hop.

ii. Contracted form (Capitulum), Daisy.

IV. Compound Indefinite Inflorescences.

a. Compound Spike, Rye-grass.

b. Compound Spadix, Palms.

c. Compound Raceme, Astilbe.

d. Compound Umbel, most Umbellifese.

C. Raceme of Capitula, Pet asites.

f. Raceme of Umbels, Ivy.

B. Definite Centrifugal Inflorescences.

I. Flowers solitary, terminal, Tulip.

II. Flowers in cymes.

I. Uniparous Cyme.

• a. Helicoid Cyme.

(i) Elongated, Alstroemeria.

(ii) Contracted, Wittsteinia corymbosa.

b. Scorpoid Cyme.

(i) Elongated, Forget-me-not.

(ii) Contracted, Erodium.

2.

Biparous Cyme (including Dichasium, Cymose Umbel).

a. Elongated, Cerastiuna.

b. Contracted (Verticillaster), Dead-nettle.

3. Compound definite Inflorescence. Many Calceolarias.

C. Mixed Inflorescences.

Raceme of Scorpoid Cymes, Horse-chestnut.

Scorpoid Cyme of Capitula, Vernonia scorpioides.

Compound Umbel of dichotomous Cymes, Viburnum. Capitulum of contracted Scorpoid Cymes (Glomerulus), Sea pink.

Cyme of Capitula, Ragwort.

Parts of the Flower.

The flower consists of the floral axis bearing the sporophylls (stamens and carpels) usually with pro tective envelopes. The axis is normally contracted, no internodes being developed and the portion bearing the floral leaves, the re ceptacle, is frequently a conical, flattened or hollowed expansion; rarely the axis is elongated and internodes are developed. Upon the receptacle the parts of the flower are crowded, usually form ing a series of whorls, the parts of which alternate, but sometimes arranged spirally, especially if the axis is elongated. In a typical flower (fig. 22) there are four distinct whorls, an outer calyx of sepals; within it, the parts alternating with those of the calyx, is the corolla of petals; next alternating with the parts of the corolla, the androeciunr of stamens; and in the centre, the gynoecium of carpels. Fig. 14 is a diagrammatic representation (floral diagram) of such a flower, supposed to be cut transversely, the parts of each whorl being distinguished by a different symbol. The sepals are usually greenish, their function is mainly protec tive, shielding the delicate internal organs before the flower opens. The petals are usually showy. Sometimes (usually in monoco tyledons) the calyx and corolla are similar; in such cases the term perianth is applied, and the parts of the calyx are petaloid. In some cases the petals resemble sepals (sepaloid). In plants, as Nympliaea alba (fig. 21), where a spiral arrangement of floral leaves occurs, these whorls pass insensibly into each other. When both calyx and corolla are present, the plant is diclilamydeous; when one is absent, monochlamydeous (fig. 15). Sometimes both are absent (achlamydeous), as in willow. The stamens in their most differentiated form, consist each of a stalk, the filament (fig. i 6) consisting of pollen-sacs containing the powdery pollen (microspores) which is ultimately discharged. The gynoecium or pistil terminates the floral axis. It consists of one or more carpels (megaspores), separate or combined (fig. 17). The pistil is com posed of the ovary (fig. 20), the lower portion enclosing the ovules destined to become seeds; and the stigma loose cellular tissue, the receptive surface on which the pollen is deposited and either sessile on the apex of the ovary (poppy) or separated by a prolonged style. The androccium and gynoecium are not present in all flowers. When both are present the flower is hermaphrodite and is represented by the symbol . When only one is present the flower is unisexual or diclinous, and is either male (staminate), or female (pistillate), ?. When all four whorls of leaves are present the flower is complete. Usually the successive whorls of the flower, disposed from below upwards or from without inwards upon the floral axis, are of the same number of parts, or are a multiple of the same number of parts, those of one whorl alter nating with those of the whorl next to it.

In the more primitive types of flower the receptacle is more or less convex, and the series of organs follow in regular suc cession, culminating in the carpels (fig. i8). This arrangement is hypogynous, the other series being beneath (hypo) the gynoe cium. In other cases, the apex of the growing point ceases to develop and the parts below form a cup around it, from the rim of which the outer members of the flower are developed (perig ynous, fig. 19). In many cases this is carried further and a cavity is formed roofed over by the carpels, so that the outer members of the flower spring from the edge of the receptacle which is immediately above the ovary (epigynous, fig. 2o).

Symmetry of the Flower.

When a flower consists of parts arranged in whorls it is cyclic, and if all the whorls have an -equal number of parts and are alternate it is eucyclic (figs. 14, In contrast to cyclic flowers are those where the parts are in spirals (acyclic). Flowers which are cyclic at one portion and spiral at another (as many Ranunculaceae) are hemicyclic. In spiral flowers there is usually a gradual passage from sepaloid through petaloid to staminal parts (water lily, fig. 21). In some cases the parts of one whorl are opposite or superposed to those of the next. The superposition of the stamens on the sepals in the Caryophyllaceae is due to the suppression of the petals.

A flower is symmetrical when each whorl consists of an equal number of parts, or when the parts of any one whorl are mul tiples of that preceding it. Thus, a symmetrical flower may have five sepals, five petals, five stamens and five carpels (fig. 14) or the number of any of these parts may be a multiple of five. In the staminal whorl especially it is common to find additional rows. In fig. 24 the parts are in fours; in figs. 25 and 26, in threes. The floral envelopes are rarely multiplied. Flowers in which the number of parts in each whorl is the same are isomerous; when the number in some whorls is different, the flower is anisomerous. It often happens that when fully formed, the number of parts in the pistillate whorl is not in conformity with that in the other whorls. In such circumstances a flower is called symmetrical, provided that the other whorls are normal (fig. 27). A flower in which the parts are arranged in twos is dimerous; in threes, fours or fives, trimerous, tetramerous or pentamerous respectively. Trimerous symmetry is the rule in the monocotyledons, pentam erous the commonest in the dicotyledons, though dimerous and tetramerous flowers also occur in the latter group.

The various parts of the flower have a definite relation to the central axis. Thus in a tetramerous flower, one sepal may be next the axis (superior or posterior), another next the bract (inferior or anterior) and the other two lateral (fig. 24). A plane passing through the anterior and posterior sepals and through the floral axis is the median plane of the flower; a plane cutting it at right angles and passing through the lateral sepals, is the lateral plane; whilst the planes which bisect the angles formed by the lateral and median planes are diagonal planes. In a pentamerous flower one sepal may be superior (Rosaceae and Labiatae) or inferior (pea family, figs. 28, 29) ; in the latter case the odd petal (vex illum) is then superior. In the Scrophulariaceae one of the two carpels is posterior, the other anterior, whilst in the Convol vulaceae the carpels are lateral.

When the different members of each whorl are alike, the flower is regular; differences in size and shape of the parts of a whorl make the flower irregular (fig. 28). When a flower can be divided by a single plane into two similar parts it is zygomorphic (as in Papilionaceae). Polysymmetrical flowers have a radial sym metry and can be divided by several planes into similar por tions; such are all regular, symmetrical flowers. When the parts of any whorl are not equal to, or some multiple of, the others, the flower is asymmetrical. This alteration in the symmetrical arrangement has been traced to suppression or non-development of parts, degeneration or imperfect formation, cohesion or union of parts of the same whorl, adhesion or union of parts of different whorls, multiplication of parts and deduplication (chlorosis) or splitting of parts. Cultivation has a great effect iri causing changes in the various parts of a plant. The changes in colour and form of flowers thus produced are endless.

As a convenient method of expressing the arrangement of the parts of a flower, floral formulae have been derived. The follow ing is a simple mode : the whorls are represented by the letters S (sepals), P (petals), A (stamens), C (carpels) and a figure marked after each indicates the number of parts in that whorl. Thus S5P5A5C5 means that the flower is perfect, isomerous and pentamerous. The flower of Sedum (fig. 22) would be represented by S5P5A5+5C5, where A5+5 indicates that the staminal whorl consists of two rows of five parts each.

Aestivation.

The manner in which the parts are arranged in the flower-bud with respect to each other before opening is the aestivation or praefloration, and distinctive names have been given to the different arrangements exhibited, both by the leaves individually and in their relation to each other. As regards each leaf of the flower, it is either spread out, as the sepals in the bud of the lime-tree, or folded upon itself (conduplicate), or slightly folded inwards or outwards at the edges, as in the calyx of some species of Clematis, or rolled up at the edges (involute or revolute), or folded transversely, becoming crumpled or cor rugated as in the poppy.

When the parts of a whorl are in an exact circle, and are applied to each other by the edges only, without overlapping or folding, aestivation is valuate. The edges of each of the parts may be turned inwards (induplicate) or outwards (reduplicate). When the parts of a single whorl are placed in a circle, each of them exhibiting torsion so that by one of its sides it overlaps its neigh bour, whilst its side is overlapped in like manner by that standing next to it, aestivation is twisted or contorted. When the flower expands, traces of twisting sometimes remain. These forms of aestivation occur in cyclic flowers and are included under circular aestivation. In spiral flowers the leaves may cover each other partially, like tiles on a house, or the parts may envelop each other completely (convolute). When the parts are five, as in many dicotyledons, there may be two parts external, two internal and a fifth which partially covers one of the inner parts by its margin and is in turn partially covered by one of the external parts (quincuncial), as in the corolla of Rosaceae. In the Leguminoseae, the vexillum is often large and folded over the others (vexillary) or the carina may perform a similar office (carinal). Circular aestivation is generally associated with a regular calyx and corolla while spiral aestivation is connected with irregular as well as regular forms.