Читать книгу Field and Woodland Plants - William S. Furneaux - Страница 10

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A flower, if complete in all its parts, consists of modified leaves arranged in four distinct whorls, the parts being directly or indirectly attached to a receptacle.

The outer whorl is the calyx, and is composed of parts called sepals, which may be either united or distinct. The calyx is usually green; but, in some cases, is more or less highly coloured. Sometimes the calyx is quite free from the pistil or central part of the flower, the sides of which are thus left naked, and the calyx is then said to be inferior. If, however, it is united to the surface of the pistil it is superior. When it remains after other parts of the flower have decayed, it is said to be persistent.

The second whorl—the corolla—is usually the whorl that gives most beauty to the flower. It is composed of parts, united or distinct, called petals.

Both calyx and corolla vary very considerably in shape. They may be cup-shaped, tubular, bell-shaped, spreading, funnel-shaped, lipped, &c. If the sepals and petals are arranged symmetrically round a common centre, the calyx and corolla, respectively, are said to be regular; if otherwise, they are irregular.

The third whorl consists of the stamens, each of which, in its most perfect form, is made up of a filament or stalk, and an anther which, when mature, splits and sets free the pollen that is formed within it. Sometimes the stamen has no filament, and the anther is then said to be sessile.

The mode of attachment of the stamens is very variable. They may grow from immediately below the pistil, or from its summit; or they may be attached to either the petals or the sepals. The filaments are usually distinct, but sometimes they are united in such a manner as to form a tube, or grow into two or more bundles. The anthers are usually distinct, even when the filaments are united; but these sometimes grow together.

Longitudinal Section Through the Flower of the Buttercup

Showing the calyx, corolla, stamens and pistil. The pistil consists of several distinct carpels, one of which is represented in section to show its single ovule.

The central part of the flower is the pistil, and this is made up of one or more parts called carpels. Each carpel, when distinct, is a hollow case or ovary, prolonged above into one or more stalks or styles, tipped by a viscid secreting surface called the stigma. The ovary contains the ovules, attached to a surface called the placenta; and these ovules, after having been impregnated by the pollen, develop into seeds which are plants in embryo. The ovary may have no style, and the stigma is then sessile.

Where the pistil consists of more than one carpel, these carpels may unite in such a manner as to form a single cell, or an ovary of two or more cells. In other cases the carpels remain quite distinct, thus forming a number of distinct ovaries, each with its own stigma. For purposes of identification it is often necessary to note the position of the placenta. This may be at the side of the ovary, in which case it is said to be parietal; or it may stand up in the centre of the ovary, without any attachment to the sides, when it is described as free central. If, however, it occupies the centre of the ovary, but is attached by means of radiating partitions to the sides, it is termed axile.

If the ovary is quite free in the centre of the flower, the surrounding parts being attached below it, it is said to be superior; but if the perianth (p. 11) adheres to it, it is inferior.

A leaf or scale will often be observed at the foot of a flower stalk or at the base of a sessile flower. This is termed a bract, and a flower possessing a bract is said to be bracteate. The bract is sometimes so large that it almost completely encloses the flower, or even a cluster of flowers.

Inferior (1) and Superior (2) Ovary.

The flower is the reproductive part of the plant, being concerned in the production of the seeds; but the organs directly connected with the seed-formation are the pistil and the stamens, the former containing the ovules, and the latter producing the pollen cells by means of which the ovules are impregnated. Thus the stamens and the pistil are the essential parts of the flower, though the corolla and the calyx may perform some subsidiary function in connexion with the reproduction of the species.

This being the case, a flower may be described as perfect if it consists of stamens and pistil only, without any surrounding calyx or corolla; and imperfect if it possesses no pistil or no stamens, regardless of the presence or absence of calyx and corolla.

Unisex Flowers of the Nettle

1. Pistillate. 2. Staminate.

The two outer whorls of a well-developed perfect flower (calyx and corolla) together form the perianth. Some flowers, however have only one whorl outside the anthers, representing both the calyx and corolla of the more highly organised flower. This one whorl, therefore, is the perianth, and its parts are not correctly termed either petals or sepals, since they represent both.

A perfect flower is sometimes spoken of as bisexual, for it includes the two sexual organs of the plant—the ovary or female part, producing the ovules; and the stamens or male part, which is concerned in the impregnation or fertilisation of the ovules.

Many plants produce only unisexual (and therefore imperfect) flowers, which contain either no stamens or no pistil. If such possess stamens and no pistil, they are called staminate or male flowers; and if pistil and no stamens, pistillate or female flowers. These two kinds are sometimes borne on the same plant, when they are said to be monœcious; but often on separate plants (diœcious), as in some of the Nettleworts and the Willow Tree. Spikes of unisexual flowers, such as are common among our forest trees, are called catkins.