The function of the flower is to produce seed. It is probable that all its varied forms and colours contribute to this supreme end. These forms and colours please the human fancy and add to the joy of living, but the flower exists for the good of the plant, not for the good of man. The parts of the flower are of two general kinds—those that are directly concerned in the production of seeds, and those that act as covering and protecting organs. The former parts are known as the essential organs; the latter as the floral envelopes.

Envelopes.—The floral envelopes usually bear a close resemblance to leaves. These envelopes are very commonly of two series or kinds—the outer and the inner. The outer series, known as the calyx, is usually smaller and green. It usually comprises the outer cover of the flower-bud. The calyx is the lowest whorl in Fig. 173.

The inner series, known as the corolla, is usually coloured and more special or irregular in shape than the calyx. It is the showy part of the flower, as a rule. The corolla is the second or large whorl in Fig. 173.

The calyx may be composed of several leaves. Each leaf is a sepal. If it is of one piece, it may be lobed or divided, in which case the divisions are called calyx-lobes. In like manner, the corolla may be composed of petals, or it may be of one piece and variously lobed. A calyx of one piece, no matter how deeply lobed, is gamosepalous. A corolla of one piece is gamopetalous. When these series are of separate pieces, as in Fig. 173, the flower is said to be polysepalous and polypetalous. Sometimes both series are of separate parts, and sometimes only one of them is so formed.

The floral envelopes are homologous with leaves. Sepals and petals, at least when more than three or five, are in more than one whorl, and one whorl stands below another so that the parts overlap. They are borne on the expanded or thickened end of the flower stalk; this end is the torus. In Fig. 173 all the parts are seen as attached to the torus. This part is sometimes called the receptacle, but this word is a common-language term of several meanings, whereas torus has no other meaning. Sometimes one part is attached to another part, as in the fuchsia (Fig. 174), in which the petals are borne on the calyx-tube.

Subtending Parts.—Sometimes there are leaf-like parts just below the calyx, looking like a second calyx. Such parts accompany the carnation flower. These parts are bracts (bracts are small specialized leaves); and they form an involucre. We must be careful that we do not mistake them for true flower-parts. Sometimes the bracts are large and petal-like, as in the great white blooms of the flowering dogwood: here the real flowers are several, small and greenish, forming a small cluster in the centre.

Essential Organs.—The essential organs are of two series. The outer series is composed of the stamens. The inner series is composed of the pistils.

Stamens bear the pollen, which is made up of grains or spores, each spore usually being a single plant cell. The stamen is of two parts, as is readily seen in Figs. 173, 174,—the enlarged terminal part or anther, and the stalk or filament. The filament is often so short as to seem to be absent, and the anther is then said to be sessile. The anther bears the pollen spores. It is made up of two or four parts (known as sporangia or spore-cases), which burst and discharge the pollen. When the pollen is shed, the stamen dies.

The pistil has three parts: the lowest, or seed-bearing part, which is the ovary; the stigma at the upper extremity, which is a flattened or expanded surface, and usually roughened or sticky; the stalk-like part or style, connecting the ovary and the stigma. Sometimes the style is apparently wanting, and the stigma is said to be sessile on the ovary. These parts are shown in the fuchsia (Fig. 174). The ovary or seed vessel is at a. A long style, bearing a large stigma, projects from the flower. See also Figs. 175 and 176.

Stamens and pistils probably are homologous with leaves. A pistil is sometimes conceived to represent anciently a leaf as if rolled into a tube; and an anther, a leaf of which the edges may have been turned in on the midrib.

The pistil may be of one part or compartment, or of many parts. The different units or parts of which it is composed are carpels. Each carpel is homologous with a leaf. Each carpel bears one or more seeds. A pistil of one carpel is simple; of two or more carpels, compound. Usually the structure of the pistil may be determined by cutting horizontally across the lower or seed-bearing part, as Figs. 177, 178 explain. A flower may contain a simple pistil (one carpel), as the pea (Fig. 177); several simple pistils (several separate carpels), as the buttercup (Fig. 176); or a compound pistil with carpels united, as the Saint John’s wort (Fig. 178) and apple. How many carpels in an apple? A peach? An okra pod? A bean pod? The seed cavity in each carpel is called a locule (Latin locus, a place). In these locules the seeds are borne.

Conformation of the Flower.—A flower that has calyx, corolla, stamens, and pistils is said to be complete (Fig. 173); all others are incomplete. In some flowers both the floral envelopes are wanting: such are naked. When one of the floral envelope series is wanting, the remaining series is said to be calyx, and the flower is therefore apetalous (without petals). The knot-weed (Fig. 179), smartweed, buckwheat, elm are examples.

Some flowers lack the pistils: these are staminate, whether the envelopes are missing or not. Others lack the stamens: these are pistillate. Others have neither stamens nor pistils: these are sterile (snowball and hydrangea). Those that have both stamens and pistils are perfect, whether or not the envelopes are missing. Those that lack either stamens or pistils are imperfect or diclinous. Staminate and pistillate flowers are imperfect or diclinous.

When staminate and pistillate flowers are borne on the same plant, e.g. oak (Fig. 180), corn, beech, chestnut, hazel, walnut, hickory, pine, begonia (Fig. 181), watermelon, gourd, pumpkin, the plant is monœcious (“in one house”). When they are on different plants, e.g. poplar, cottonwood, bois d’arc, willow (Fig. 182), the plant is diœcious (“in two houses”). Some varieties of strawberry, grape, and mulberry are partly diœcious. Is the rose either monœcious or diœcious?

Flowers in which the parts of each series are alike are said to be regular (as in Figs. 173, 174, 175). Those in which some parts are unlike other parts of the same series are irregular. Their regularity may be in calyx, as in nasturtium (Fig. 183); in corolla (Figs. 184, 185); in the stamens (compare nasturtium, catnip, Fig. 185, sage); in the pistils. Irregularity is most frequent in the corolla.

Various Forms of Corolla.—The corolla often assumes very definite or distinct forms, especially when gamopetalous. It may have a long tube with a wide-flaring limb, when it is said to be funnelform, as in morning-glory and pumpkin. If the tube is very narrow and the limb stands at right angles to it, the corolla is salverform, as in phlox. If the tube is very short and the limb wide-spreading and nearly circular in outline, the corolla is rotate or wheel-shaped, as in potato.

A gamopetalous corolla or gamosepalous calyx is often cleft in such way as to make two prominent parts. Such parts are said to be lipped or labiate. Each of the lips or lobes may be notched or toothed. In 5-membered flowers, the lower lip is usually 3-lobed and the upper one 2-lobed. Labiate flowers are characteristic of the mint family (Fig. 185), and the family therefore is called the Labiatæ. (Literally, labiate means merely “lipped,” without specifying the number of lips or lobes; but it is commonly used to designate 2-lipped flowers.) Strongly 2-parted polypetalous flowers may be said to be labiate; but the term is oftenest used for gamopetalous corollas.

Labiate gamopetalous flowers that are closed in the throat (or entrance to the tube) are said to be grinning or personate (personate means masked). Snapdragon is a typical example; also toadflax or butter-and-eggs (Fig. 186), and many related plants. Personate flowers usually have definite relations to insect pollination. Observe how an insect forces his head into the closed throat of the toadflax.

The peculiar flowers of the pea tribes are explained in Figs. 187, 188.

Spathe Flowers.—In many plants, very simple (often naked) flowers are borne in dense, more or less fleshy spikes, and the spike is inclosed in or attended by a leaf, sometimes corolla-like, known as a spathe. The spike of flowers is technically known as a spadix. This type of flower is characteristic of the great arum family, which is chiefly tropical. The commonest wild representatives are Jack-in-the-pulpit, or Indian turnip, and skunk cabbage. In the former the flowers are all diclinous and naked. In the skunk cabbage all the flowers are perfect and have four sepals. The common calla is a good example of this type of inflorescence.

Composite Flowers.—The head (anthodium) or so-called “flower” of sunflower (Fig. 189), thistle, aster, dandelion, daisy, chrysanthemum, goldenrod, is composed of several or many little flowers, or florets. These florets are inclosed in a more or less dense and usually green involucre. In the thistle (Fig. 190) this involucre is prickly. A longitudinal section discloses the florets, all attached at bottom to a common torus, and densely packed in the involucre. The pink tips of these florets constitute the showy part of the head.

Each floret of the thistle (Fig. 190) is a complete flower. At a is the ovary. At b is a much-divided plumy calyx, known as the pappus. The corolla is long-tubed, rising above the pappus, and is enlarged and 5-lobed at the top, c. The style projects at e. The five anthers are united about the style in a ring at d. Such anthers are said to be syngenesious. These are the various parts of the florets of the Compositæ. In some cases the pappus is in the form of barbs, bristles, or scales, and sometimes it is wanting. The pappus, as we shall see later, assists in distributing the seed. Often the florets are not all alike. The corolla of those in the outer circles may be developed into a long, straplike, or tubular part, and the head then has the appearance of being one flower with a border of petals. Of such is the sunflower (Fig. 189), aster, bachelor’s button or cornflower, and field daisy (Fig. 211). These long corolla-limbs are called rays. In some cultivated composites, all the florets may develop rays, as in the dahlia and the chrysanthemum. In some species, as dandelion, all the florets naturally have rays. Syngenesious arrangement of anthers is the most characteristic single feature of the composites.

Double Flowers.—Under the stimulus of cultivation and increased food supply, flowers tend to become double. True doubling arises in two ways, morphologically: (1) stamens or pistils may produce petals (Fig. 191); (2) adventitious or accessory petals may arise in the circle of petals. Both these categories may be present in the same flower. In the full double hollyhock the petals derived from the staminal column are shorter and make a rosette in the centre of the flower. In Fig. 192 is shown the doubling of a daffodil by the modification of stamens. Other modifications of flowers are sometimes known as doubling. For example, double dahlias, chrysanthemums, and sunflowers are forms in which the disk flowers have developed rays. The snowball is another case. In the wild snowball the external flowers of the cluster are large and sterile. In the cultivated plant all the flowers have become large and sterile. Hydrangea is a similar case.