The name "flower" is a popular one. We know well enough what we mean when we use it, though it might be difficult for us to define. We know it as the first stage of the process by which the plant produces fruit and seed. However beautiful a flower may be, or however sweetly it may smell, we know it is not here simply to please us; it would still bloom if man did not exist. A plant is a distinct living being, and a flower is one of its organs. It has its work to do, just as the leaf and root have theirs, and the ultimate product is seed, and the seed is the embryo of a new generation, just as an egg is the embryo of a new generation in bird life. Examine a flower, and in doing so avoid two sorts: the double flowers so common in the gardens, which are monstrosities produced by cultivation; also such as the Daisy, Aster, and Chrysanthemum, which are compounded of very numerous minute flowers. These are composite flowers, and will be treated in a separate chapter. Choose a simple garden flower or a wild one. It will be seen to consist of outer, usually flat, parts, obviously for protection or adornment, within which are two sets of organs, a circle of little pin-like objects, each with a minute head, surrounding a central body, which little experience teaches us will produce seeds. These two inner organs are the essential parts, and are both necessary for the production of young; the other parts are useful when present, but may be absent without the function of the flower being interfered with. Flowers are of most varied forms; not only do the parts themselves vary according to their kind, but some may be absent. A flower may consist of a single stamen, or only of the innermost organ, but most of the flowers we meet with are what are called perfect; that is, they consist of all the parts of which a flower may be made up.
A perfect flower is formed of a double perianth, a circle of stamens, and a central pistil.
The outer circle of the perianth is the calyx; it is commonly green like the leaves, and made up of three, four, or five parts, each of which is called a sepal. These sepals may be all free from one another or variously united. When the calyx is green and of coarse structure, its duty is evidently that of an outer covering to the bud, while the more delicate organs are being formed within. Sometimes the calyx is not so structured, but may be coloured and delicate when it assumes the duty of the next circle, and may be mistaken for it.
The inner circle of the perianth is the corolla. It is usually coloured and delicate, and made up of three, four, or five parts, called petals, which may be all free or variously united. They may be all equal and similarly constructed, or unequal. When the petals are not all similar the coralla is said to be irregular. As a rule the petals are so inserted that they alternate with the sepals. In most flowers there is a clear distinction between the calyx and corolla, but not in all. Sometimes the passage from one to the other is gradual, and not clearly marked. When the corolla is delicate and brightly coloured its work is evidently to attract attention. It is of first importance to many plants that insects or, in some, birds shall be induced to visit their flowers.
Within the perianth is a circle of stamens. A typical stamen may be likened to a small pin. The shaft is slender and delicate, and is called the filament; the head is round or long, yellow or dark, and is called the anther. A stamen does not look like a leaf, but it is one. Stamens of some flowers are very leaf-like, and in deformed flowers this condition is often reverted to.
The anther, though placed on the staminal leaf, is a separate organ developed there for convenience. It is the direct descendant of the small spore-bearing sacks of lower plants. The anther is made up of four sacks, which may all remain till maturity or they may coalesce in pairs, so that when mature the anther consists of two sacks, or further, all four may blend into one. The anther when mature consists of one or more sacks containing coloured dust. Each particle of this dust is a spore, but in a flower we call it a pollen grain. A pollen grain is a beautiful object under the microscope. It has two coats: the outer one is variously marked, with ridges or spines according to the species; the inner one is very thin, and contains a fluid of the consistency of the white of an uncooked egg. This fluid and what it contains is a necessary substance required to assist in the production of fertile seed. Stamens vary in number more than do petals or sepals. They may be very few or many, but they are very often of the same number, or twice as many as the petals. They may be all free one from the other, or variously united.
In the centre of the flower is the pistil. It is usually dull or green, leaving the lighter duties of protection and attraction to the outer circles. It attends to the important duty of rearing the young. The pistils of flowers exhibit a greater variety of forms than do either of the other circles. Like them it is made up of few or many units, and just as the unit of a corolla is named a petal, so that of a pistil is called a carpel. A carpel is simply a leaf modified to better fit it to perform its special work. In its simplest condition in flowering plants it is a leaf folded on itself longitudinally till the edges come together and join to form a little bag. On the joined edges inside the bag are developed one or more little round bodies called ovules, which will eventually become seeds. These ovules are the direct descendants of the large-spored sacks found in Pines, some Ferns, and some Lycopods. In the Pines and Flowering plants these large spores are not free and shed as they may be in the others, but are retained within the sack where their presence is obscure.
The upper end of a carpel is usually elongated into a slender column called the style, and at the apex of the style is a variously-shaped receptive surface named the stigma.
The pistil is subject to most varied modifications, according to the species of flower. It may consist of a single carpel, as in Peaflowers, Wattles, Waratah, and its allies; it may consist of many carpels, all free from one another, as in Buttercup, Clematis, Strawberry, or Raspberry; or the carpels may be variously united. When such is the case the union may take place in various degrees, from a simple attachment to one another to a condition where they are so blended that all superficial sign of the separate carpels is lost and the ovarian cavities are merged in one, as in Primula and Pimpernel. Most brightly-coloured flowers produce honey or similar substance of use to insects. This is formed from organs called nectaries, which are variously placed in the flowers according to its kind.
A flower may be considered to be a branch whose leaves have become in the course of time very much modified to suit its special functions, and at the same time the branch from which these leaves arise has been so shortened that they appear to be inserted upon a very short space. This portion of the branch is of such importance, and is so variable in shape that it is necessary it should have a name. It is called the thalamus. In many flowers the thalamus is small and of no noticeable development, but the floral circles arise from it one above the other. In the Buttercup, above the stamens, the thalamus forms a conical projection, upon which are placed the numerous carpels. The Raspberry has a similar projection, but here the base of the thalamus is also developed into a small cup upon the edge of which arise the calyx, corolla, and stamens. In Strawberry we find the same condition, only in fruit the terminal projection enlarges enormously, bearing the little carpels on its surface. In the Rose the thalamus cup is greatly enlarged, and constricted above to form a hollow chamber, at the apex of which are the calyx, corolla, and stamens. The pistil here consists of numerous free carpels growing within and at the base of the cup. They have long slender styles, bearing small round stigmas, which protrude in the centre of the flower. In Gumtrees, Ivy and Composites the cup is well developed, but intimately blended with the pistil, so that the other circles of the flower arise above the fruiting portion.
In the ovules are produced the large spores; in the anthers the small spores. An embryo is formed only when the contents of a small spore become mingled with that of a large spore. The pollen is blown or otherwise carried from the anther to the stigma; here it adheres. It grows in an elongating delicate tube through the course of the style, and enters the ovarian cavity. When it meets an ovule it enters it and blends with the contained large spore in which now develops an embryo. The ovule undergoes certain changes, which end in what we call the seed. A seed is an embryo plant enclosed in a tough coat, which enables it to remain dormant, still retaining the power of starting into life when placed in a suitable medium. Some seeds retain this power only for a short period; others for many years. The retention of this power appears to depend largely on the seed coat's resistance to the diffusion of moisture. It appears that in all cases where the embryo loses its contained water beyond a certain degree death ensues.
Many flowering plants have only stamens or pistil in a flower, not both, and the staminate and pistillate flowers may be on one plant or separate ones. This obviously necessitates the arrival of pollen from another flower or plant, in order to fertilise the ovules. This is called cross-fertilisation, to distinguish it from self-fertilisation. When both organs are formed in the same flower it is very common for special contrivance to exist, to prevent self and compel cross-fertilisation.
When flowers are small, dingy, and odourless, cross-fertilisation is effected by pollen being carried by currents of air from the anther to the stigma. This may be observed in grasses and Sheoak. When flowers are brightly coloured or scented, insects visit them in quest of honey, or to gather pollen for food. An insect cannot travel about an open flower without getting dusted with pollen, some of which it will probably leave on the stigma of the next flower it visits.