BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.5. Sexual reproduction in flowering plants

21.5.2. Flower structure

The term "flowering plants" reflects the unique defining feature of this group: the presence of flowers. Flowers are reproductive Organs whose evolutionary origin remains unclear, although some authors suggest they are composed of highly specialized leaves. A generalized diagram of a flower is shown in Fig. 21.16, and some of the terms used to describe flower parts are explained below. A cluster of flowers arranged on the end of a single stem is called an inflorescence. Presumably, numerous flowers gathered together are more effective at attracting insect pollinators than a small solitary flower.

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Fig. 21.16. Generalized diagram of flower Structure (longitudinal section).

The distal part of the pedicel that bears the floral organs is called the receptacle. Situated on the receptacle is the perianth, which consists of two whorls of leaf-like structures. In monocots, both whorls are usually similar, as seen, for example, in daffodils, tulips, or squills. In dicots, the whorls frequently differ from one another: the outer whorl consists of sepals and is called the calyx, while the inner whorl consists of petals and is called the corolla.

Sepals are typically green and leaf-like; they enclose and protect the flower buds. Occasionally, sepals are brightly colored and petal-like, in which case they also assist in attracting insect pollinators.

In insect-pollinated plants, petals are generally large and brightly colored to attract insects. In wind-pollinated flowers, petals are usually small and green, or absent altogether.

The androecium refers to the collective group of stamens, which form the Male reproductive organs of the flower. Each stamen consists of an anther and a filament. The anther contains pollen sacs in which pollen is produced. A vascular strand runs through the filament, supplying nutrients and Water to the anther.

The gynoecium, or pistil, is the collective term for the carpels that form the FEMALE REPRODUCTIVE ORGANS of the flower. Carpels consist of a stigma, a style, and an Ovary. The style positions the stigma in such a way that it can receive pollen during pollination. The ovary is the swollen, hollow Base of the carpel containing one or more ovules. Ovules are structures in which embryo sacs develop and which mature into seeds after Fertilization. Each ovule is attached to the ovary wall by a short stalk called a funiculus, and the site of attachment is known as the Placenta.

The carpels of a flower may remain free—that is, separate from one another—as in buttercups, or they may be fused into a single structure, as in dead-nettles. The styles may also be fused or remain distinct despite fused carpels.

The receptacle and flower are described as hypogynous if the stamens and perianth are situated below the gynoecium, as shown in Fig. 21.16; epigynous if they are located above the ovary; and perigynous if the receptacle is flattened or cup-shaped, with the gynoecium positioned in the center and surrounded by the stamens and perianth. The ovary is termed superior if it is positioned on the receptacle above the other floral parts, as in a hypogynous flower. An inferior ovary lies on the receptacle beneath the other floral parts, as in an epigynous flower.

Nectaries are glandular structures that secrete nectar—a sugary fluid that attracts animal pollinators, typically insects, though in tropical regions pollinators may also include birds and bats.

The terms listed below apply to the plant as a whole as well as to its flowers.

1. Plants are termed hermaphroditic (bisexual) when both male and female reproductive organs are produced on the same plant.

2. In dioecious plants, male and female reproductive organs are borne on separate plants; that is, any given individual plant is either exclusively male or exclusively female, as found in yew, willow, poplar, and holly.

Monoecious plants bear separate male and female flowers on the same individual plant; Examples of such plants include oak, hazel, beech, and sycamore. These plants are hermaphroditic.

Figures 21.17–21.19 depict typical insect-pollinated flowers. The Morphology of a typical grass flower—a wind-pollinated monocot—is illustrated in Fig. 21.20.

The structure of a flower is most clearly demonstrated by cutting it vertically into two equal halves and illustrating one half, outlining the cut surface with a continuous line.

Fig. 21.17. Schematic Cytology/practical/54.html">Longitudinal section of the flower of meadow buttercup (Ranunculus acris), a representative of dicotyledonous plants. This is a perennial herbaceous plant commonly found in damp meadows and pastures. It flowers from April to September. Pollinators include dipterans and small hymenopterans. Each carpel contains a single seed. Buttercups possess no specialized mechanisms to aid in seed dispersal.

Fig. 21.18. STRUCTURE OF THE sweet pea flower (Lathyrus odoratus), a dicotyledonous plant belonging to the family Papilionaceae. Flowers appear in July. The calyx consists of five sepals. The corolla is formed by five petals: one positioned at the top—the standard (banner); two at the sides—the wings; and two fused petals forming the so-called keel, located between the wings. The corolla is white or colored. It is pollinated by bees, which are attracted by color, scent, and nectar. Among the petals, the standard is particularly prominent. The wings serve as a landing platform for the insect. Under the weight of the bee's body, the wings move downward together with the keel to which they are connected. The style and stigma are thereby pushed outward, coming into contact with the underside of the bee's body, which may carry pollen gathered from another flower. While the bee collects nectar at the base of the ovary using its long proboscis, pollen from the anthers is deposited onto the lower surface of the style, from where it can also be transferred to the bee. Self-pollination is also possible. Closely related to the sweet pea is the garden pea (Pisum sativum), though it more frequently undergoes self-pollination. The fruit is a legume, consisting of a single carpel containing multiple seeds.

Fig. 21.19. Longitudinal section through the flower of the white dead-nettle (Lamium album), a representative of dicotyledons. It is a perennial herbaceous plant commonly found along hedges and on wasteland. It flowers from April to June and again in autumn. Pollination is carried out mainly by bumblebees. As a bumblebee lands on the lower lip of the corolla and pushes its way into the flower, its back—which may carry pollen brought from another flower—comes into contact with the stigma, resulting in cross-pollination, although self-pollination is also possible in this species.

Fig. 21.20. Meadow fescue (Festuca pratensis), a wind-pollinated lawn grass.