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Sexual Reproduction In Flowering Plants

Flower Structure

A flower is a reproductive structure of a plant. Many flowers have both male and female reproductive organs, though some are of single sex.. The floral parts are borne in whorls or spirals with short internodes, often at the end of a flower stalk whose end is expanded to form a receptacle. The outer whorl is called the calyx, the next, the corolla. Within the corolla is the androecium and, finally, the gynaecium. An inflorescence is a group of flowers borne on the same main stalk. The calyx consists of sepals, which are usually green and small. They enclose and protect the rest of the flower while it is in the bud. The corolla consists of petals, which are often coloured and scented. They attract insects which visit the flowers and collect nectar and pollen, pollinating the flowers as they do so. Small grooves or darker lines in the petals, called “honey guides” are thought to direct the insect to the nectarines within the flower. The androecium is the male part of the flower and consists of stamens. The stalk of the stamen is the filament. At the end of the filament is an anther which contains pollen grains in four pollen sacs. The pollen grains contain the male reproductive cells or gametes. The gynaecium is the female part of the flower. It consists of carpels, which may be single and solitary, many and separate from each other, or few and joined together. In all of them, the ovules, which contain the female gametes, are enclosed in a case called the ovary. Extending from the ovary is a style, expanded or divided at one end into a stigma, where pollen from another flower will be received. The ovules when fertilized will become seeds, while the whole ovary will be the fruit. The wall of the ovary develops into the pericarp of the fruit. Nectaries are glandular swellings, often at the base of the ovary or on the receptacle, which produce a sugary solution called nectar. Insects visit the flower and drink or collect this nectar.

The Number of parts in a plant or flower

In many species of flower plants, the structures described above occur in definite numbers. For example, if there are five sepals there are likely to be five petals and five or ten stamens. Whorls may be repeater; for example, there may be two whorls of five petals or two whorls of five stamens. In the buttercup and rose families there are numerous stamens and carpels, and the numbers vary from one plant to another. The floral parts usually alternate so that petals do not come opposite sepals but between them, and stamens are borne between petals, and so on.

Variations

In many flowers, petals or sepals are joined, or fused, for part of or all of the way along their length, forming tubular structures as in the foxglove and deadnettle families. In flowers like the buttercup all the petals are the same size and are not joined, but in many others which have joined or free petals, some petals differ in size and shape form others, as in the pea family and deadnettles.

The half flower

A drawing of a half flower is a convenient method of representing flower structure. The flower is cut in halves with a razor blade, the outline of the cut surfaces drawn, and the structures visible behind these filled in. A longitude section shows only the cut surfaces.

Buttercup

Five sepals, not joined, but curled back in the bulbous buttercup: five petals, with nectarines at their base, not joined up: about sixty stamens, thirty or forty carpels separate from each other with a short style, each containing a single ovule.

White Deadnettle

Five spiky sepals, joined at the base to make a cup, surrounding a tube of five fused petals of which the uppermost is particularly well developed. Only four stamens, with their black anthers close together upon the top petal and their filaments joined to the petal tube. A forked stigma, with a long style joined to the petals for part of its length and leading to a four lobed ovary.

Lupin Plant

Five fused sepals, with a two lobed appearance; five petals, not all joined but of different shapes and sizes. The uppermost petal is called the standard, and the two partly joined petals at the side are the wings. Within the wings are two partly joined petals forming a boat shaped keel. Inside the keel are ten stamens the filaments of which are fused to form a sheath round the ovary. The ovary is long, narrow and pod shaped, and consists of one carpel with about ten ovules. The style ends in a stigma, just within the pointed end of the keel.

Composite flowers

The flowers of the composite family, daisies, dandelions, hawkweeds, etc., are arranged in dense inflorescences. What at first appears to be a petal in the flower head is actually a complete flower, often called a floret. In some compositae, the outer florets with conspicuous petals have no reproductive organs and are called sterile: the inner florets with corollas of tiny , fused petals carry the reproductive organs.

Grasses

The flowers of the grasses are tiny, inconspicuous and in dense inflorescences. There are no petals or sepals in the usual sense, but the reproductive organs are enclosed in two green, leaf like structures called bracts. The ovary contains one ovule and has two styles with feathery stigmas. There are three stamens the anthers of which, when ripe, hang outside the bracts. The cereals, wheat, oats, barley, maize, etc., are grasses especially bred and cultivated by man for the sake of the food stored in the fruits or seeds of their flowers.

Pollination

The transfer of pollen from anthers to stigma is called pollination. Cross-pollination is the transfer of pollen from the anthers of one flower to the stigma of another flower of the same species. In some species self-pollination occurs either regularly or when cross-pollination has failed to take place, as in the willow herb. In cross pollination pollen is usually transferred on the bodies of insects which enter the flowers, or by chance, air currents carrying the pollen form one flower to the next. The structures of many flowers are closely adapted to the method of insect or wind pollination.

Insect Pollination Mechanisms

Essentially, these mechanics involve an insect’s visiting one flower, becoming dusted with pollen form the ripe stamens, and then visiting another flower where some of the pollen on its body adheres to the stigma. When ripe, the pollen sacs of the anther split open and expose the pollen, which can then be dislodged. In many flowers the anthers and stigma do not ripen at the same time, so that a visiting insect is not likely to affect self-pollination. Before the stigma is ripe the lobes may not be expanded or the chemicals needed for the growth of the pollen tube my not be present.

The Buttercup

Some flowers are elaborately adapted to pollination by a particular group of insects like moths or bees. In the buttercup, however, it seems likely that the general wanderings of various insects, e.g. bees, beetles and ants, round the petals and nectaries bring the insects’ bodies into contact with the reproductive organs resulting in the transfer of pollen from anthers to stigmas, either in the same flower (self-pollination), or from one flower to another (cross-pollination).

Wind Pollination

1. They are small, inconspicuous flowers; petals often green. No scent or nectar. 2. The anthers are large and loosely attached to filament so that the slightest air movement shake them. The whole inflorescence often dangles loosely (hazel male catkins), and the stamens hang out of the flower exposed to the wind. 3. Large quantities of smooth, light pollen grains produced by the anthers. 4. Feathery stigmas hanging outside the flower.

Insect Pollinated

1. Relatively large flowers or conspicuous inflorescences. Petals brightly coloured and scented; mostly with nectaries. 2. Anthers not so large and firmly attached to the filament. They are not usually carried outside the flower but are in a position within the petals where insects are likely to brush against them. 3. Smaller quantities of pollen produced. The grains often have spiky patterns or stick together in clumps. 4. Flat or lobed, sticky stigmas inside the flower.

Insects respond to the stimulus of colour and are “attracted” to the flowers. When in the flower, they collect or eat the nectar from the nectaries, or pollen from then anthers.

The wind is more likely to dislodge pollen form exposed, dangling anthers than from those enclosed inside the petals.

With wind pollination, only a very small proportion of pollen grains is likely to land on a ripe stigma. If large quantities of pollen are not shed, the chances of successful pollination become very poor. Smooth, light grains are readily carried in air currents and do not stick together.

The feathery stigmas of grasses form a “net” of relatively large area in which flying pollen grains may be trapped.

Grasses Continued

White deadnettle

Only long tongued insects like bees can reach the nectaries. When these insects alight on the lower petals and push their heads into the corolla tube, their backs come into contact with the anthers or stigma under the top petal. In related species the anthers at first hang lower and the stigma forks are closed. When the anthers have shed their pollen the stigma opens and bends lower than the anthers.

Lupins

Lupins have no nectar, and the bees that fisit them collect only pollen form the flowers. Other members of this family, e.g. the clovers, do produce nectar. The weight of the insect when it alights on the “wings” of the flower depressed them. Near their base, the wings are linked to the petals of the keel so that these too are forced down, and the stamens and stigma protrude form a hole at the end of the keel and touch the underside of the insects’ body. In the lupin, the anthers push out the pollen which has collected in the point tip of the keel, and it emerges rather like toothpaste from a tube, much of it adhering to the insect’s body. When the insect alights on an older flower which has shed all its pollen, the style and stigma protrude form the keel and pollen from the insect’s body will stick to the stigma.

At first, the feathery stigmas protrude from the flower, and pollen grains floating in the air are trapped by them. Later, the anthers hang outside the flower, the pollen sacs split, and the wind blows the pollen away. The sequence varies with species. If the branches of a hazel tree with ripe male catkins, or the flowers of the ornamental pampas grass, are shaken, a shower of pollen can easily be seen.

Incompatibility

In both wind and insect pollinating flowers, pollen from certain species may reach the stigma of a different species. Usually the chemicals present in the cells of the stigma prevent further development of the “foreign” pollen grains.

Importance of pollination to agriculture

After fertilization the ovary of a plant develops into a fruit. Fruit formation therefore depends on fertilization, which can follow only after pollination. Farmers and fruit growers are well aware that a good yield of fruit will only occur only if most of the available flowers have been pollinated. Many of the cereals are self-pollinated or wind-pollinated, and the fact that they are growing close together makes the latter effective. Insect pollination of field of clover or an orchard of apples, however, needs a fairly dense population of insects, particularly bees.

Fertilization

The following four generalizations apply to both plants and animals. 1. A gamete is a reproductive cell. A male gamete is usually small with a nucleus and little cytoplasm; it is the gamete which leaves the male organ and moves about, either by its own power or by an external agencies like wind or insects. 2. The female gamete is larger, with a nucleus and more protoplasm than the male; it sometimes contains food reserves. Often it never leaves the female organ or body, where it is produced, until after it is fertilized; even when it does do so it is usually immobile. The male gamete in a flowering plant is a nucleus in the pollen grain; in most animals it is the sperm. The female gamete in plants is a large egg-cell in the ovule, while in animals it is the ovum. 3. The product of the fusion of the male and female gametes is called a zygote. 4. Fertilization is the fusion (joining together) of the nuclei of male and female gametes to form a zygote. After fertilization the zygote undergoes cell division and growth, developing into a new individual or a preliminary form, an embryo, a seed or a larva.

Fertilization in Plants

Fertilization follows pollination, but the interval of time between the two events varies in different species form sixteen hours to twelve months. The pollen grain absorbs nutrient secreted by the stigma, and the cytoplasm in the grain grows out as a tube. The tube grows down the style between the cells absorbing a nutritive fluid from them. On reaching the ovary it grows to one of the ovules and enters it through a hole, micropyle. The tip of the pollen tube breaks open in the ovule, and the male nucleus, which has been passed down the tube, enters the ovule and fuses with the female nucleus there. Each egg cell of an ovule can be fertilized only by a male nucleus from a separate pollen grain.

Results of fertilization

Fruit and seed formation

After fertilization the petals, stamens, style and stigma wither and usually fall off. The sepals may persist in a dried and shriveled form. Food made in the leaves reaches the fertilized ovules and the ovary, which grow rapidly. Inside the ovule cell division and growth produces a seed containing a potential plant or embryo. The embryo consists of a miniature root or radicle, a small shoot or plumule, and one or two leaves, the cotyledons which sometimes contain food reserves. The integuments of the ovules become thicker and harder, forming the testas of the seeds, and finally, water is withdrawn form the seeds, making them dry and hard. In this condition they are best able to withstand extremes of temperature and other adverse conditions. The ovary wall may become dry and hard, forming a capsule or pod as in the poppy and lupin, or it may become sweet, succulent and fleshy as in tomatoes, gooseberries and plums.

Fruits

The whole ovary after fertilization is called a fruit. In the strawberry and rose hip, the pips are the fruits and the fleshy part is the receptacle. In the apple and pear the swollen receptacle is fused to the outside of the overy wall. In wither case, the whole structure is often called a “false fruit”. In agriculture such fruits have been specially bred and selected for their large, edible receptacles. The distinction between fruit and vegetable at the green grocer shop does not correspond to the biological definition of fruit; for example runner beans, French beans, cucumber, marrow and tomato are all fruits.

Dispersal of fruits and seeds

When flowering is over and the seeds are mature, the whole ovary or the individual seeds fall from the parent plant to the ground, where, if conditions are suitable, germination will subsequently take place. In many plants the fruits or seeds are adapted in such a way that they are distributed over considerable distances from the parent plant. This helps to reduce overcrowding among, and competition for light and water between members of the same species and results in the colonization of new areas. The principal adaptations are those which favour dispersal by wind and animals. In addition, some plants have “explosive” pods or capsules that scatter the seeds, and others have fruits that are adapted to dispersal by water.

Wind dispersal

Censer Mechanism

Examples are the white campion, poppy and antirrhinum. The flower stalk is usually long and the ovary becomes a dry, hollow capsule with one or more openings. The wind shakes the flower stalk and the seeds are scattered on all sides through the openings in the capsule.

”Parachute” fruits and seeds

Clematis, thistles, willow herb and dandelion all have seeds or fruits of this kind. Feathery hairs projecting from the fruit or seed increase its surface area so much that air resistance to its movements is very great. In consequence, it sinks to the ground very slowly and is likely to be carried great distances from the parent plant by slight turbulent air currents.

Winged Fruits

Fruits of the lime, sycamore, ash and elm trees have extensions from the ovary wall, or leaf like bracts on the flower stalk which make wing like structures. These cause the fruit to spin as it falls from the tree and so prolonging its fall, increasing its chances of being carried away in air currents.

Animal Dispersal

Mammals. Hooked fruits

In herb bennet, hooks develop from the styles of the fruits; in agrimony they grow from the receptacle. The small hooks on goose-grass fruits grow out of the ovary wall and the larger ones on burdock are developed from bracts surrounding the inflorescence. In all these cases, the hooks catch in the fur of passing mammals or in the clothing of people, and later, at some distance from the parent plants, they fall off or are brushed or scratched off and the seeds may germinate where they fall.

Birds. Succulent fruits

Fruits like the blackberry and the elderbury are eaten by birds. The hard pips containing the seed inside are undigested and pass out with the faeces of the bird away from the parent plant. Even if the seeds are not swallowed, the fruit is often carried away before the seeds are dropped, e.g. rose hip. Some seeds with a fleshy, sticky covering, e.g. yew and mistletoe, stick to the bird’s beak and are discarded some distance from the parent plant, in the latter case often being wiped off on to a branch of the tree on which it will grow as a parasite. The succulent texture and conspicuous colour of these types of fruits may be regarded as an adaption to this method of dispersal.

Aside

Pollen on stigma

If the stigmas of a number of flowers are examined dry and by reflected light under the low power of the microscope, pollen grains may be seen adhering to them. If the stigmas are crushed with water between two slides, pollen tubes may be seen growing between the cells.

Pollen tubes

By placing pollen grains in a 10-20 percent solution of cane sugar in a cavity slide and covering them with a class cover slip the growth of pollen tubes may be seen after several hours. Sweet pea pollen has provided satisfactory.

Pollen

Pollen can be examined microscopically by dusting or squashing ripe anthers on to a slide.

Biology


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