FLOWER STRUCTURE & DISPERSAL OF FRUITS AND SEEDS

  






FLOWER STRUCTURE

A flower is a reproductive structure of a plant. Most fiowers have both male and female reproductive organs, though some are of a single sex. The floral parts are arranged in whorls or spirals with short internodes, often borne 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 gynoecium.

   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 nectaries within the flower. The calyx and corolla are collectively known as the perianth. The term is important in those cases, e.g. many monocotyle- dons, where there is no obvious distinction between petals and sepals or simply one whorl of members.

  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 contaun the male reproductive cells or gametes.

   The gynoecium is the female part of the flower. It consists of carpels, which may be either 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, the ovary. Extending from the ovary is a style, expanded or divided at one end into a stigma, which will receive pollen from another flower. The ovules when fertilized will become seeds, while the whole ovary will become 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.


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STRUCTURE OF THE FLOWERING PLANT


Number of parts

In many species of flowering plant, 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 repeated; for example, there may be two whorls of five petals or two whorls of five stamens. In some genera of the Malvaceae, e.g. Hibiscus, and of the Mimosaceae, e.g. Acacia, the stamens are numerous, the numbers varyıng from one plant tö another. The floral parts usually alternate so that petals do not come opposite sepals but between them. Likewise, 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 their length, forming tubular structures as in Thunbergia and Tecoma. In flowers like Cassia 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 from others, as in Caesalpinia and the bean family, e.g. Crotalaria.


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 longitudinal section shows only the cut surfaces.

Caesalpinia puleherrima: pride of Barbados. Five sepals, not joined; five petals, one being much narrower than the rest and having a nectary at its base; a single whorl of ten stamens with very long filaments; a single pod- shaped carpel making the ovary which contains a row of ovules, and has a long style projecting from it.

Crotalaria retusa: rattlebox. Five fused sepals; 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, five with rounded anthers and five with elongated anthers. The filaments are fused to form a sheath round the ovary. The ovary 1s 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.

Aspilia latifolia: wild marigold. The flowers of the Composite family such as marigolds and sunflowers 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. The inner florets of Aspilia have five sepals fused to make a tube surrounding the base of the corolla tube, similarly formed from five joined petals. Inside the base of the corolla tube is an ovary with a single ovule and a long style ending in a forked stigma. Five long anthers are grouped together round the style. The outer florets are sterile, i.e. have no stamens or gynoecium but their fused petals are much larger than those of the inner florets and make a flat, conspicuous ribbon that resembles the Single petal of a non- composite flower.

   Zea mays: maize. The flowers are unisexual, containing either stamens or ovary but not both, and the long, crowded inflorescences bear flowers of only one sex, the male flowers on terminal branches and the female flowers lower down on axillary branches. The male flowers are small With no petals or sepals in the usual sense, but green, leaf-like bracts. The flowers are in pairs; each pair is called a spikelet and 1S enclosed by two of these bracts called glumes. Two smaller bracts, pales, enclose each androecium of three stamens. When mature, the anthers hang outside the bracts.

    The female inflorescence is completely wrapped in a small number of leaves and forms the cob. The female spikelets are arranged spirally on the inflorescence stalk. Each fertile tlower is enclosed by thin, transparent glumes and pales. The flower has a gynoecium consisting of an ovary with a single Ovule, anda long style which protrudes from the top of the cob. Also inside the spikelet is a small sterile female.

   Maize is a cereal of considerable economic importance. It is, however, closely related to wild grasses whose flowers differ from it by having both male and female organs.




  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. There, 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 away from the parent plant. This helps to reduce overcrowding among and competition between members of the same species for light, air, water and mineral salts, and results in the colonization of new areas.


Wind dispersal

(a) Parachute" fruits and seeds. 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 air currents. The hairs of the cotton seed (Gossypium) are outgrowths from the seed coat. In some members of the Compositae, e.g. Tridax the pappus of hairs on the fruit is fornmed from the calyx.

b) Winged fruits and seeds. Seeds of Jacaranda, Tecoma and Spathodea have papery extensions formed from fragments of the placenta making wing-like structures. The extra surface area of these wings offers increased air resistance, so delaying the fall of the seed and increasing its chances of being blown away from its parent plant by the wind. The wings" of Combre tum fruits are extensions of the pericarp.


Animal dispersal 

(a) Mammals: hooked fruits. In Acanthospermum and Desmodium, hooks develop on the ovary wall. In Bidens there are hooks on the calyx. These hooks catch in the fur of passing mammals or in the clothing of people and later, at some distance from the parent, they fall off or are brushed or scratched off. The mud adhering to hooves or feet may also carry seeds which are thus dispersed by wandering birds or mammals, including man.

(b) Mammals and birds: succulent fruits. The succulent texture and in some cases the bright colour of these fruits may be regarded as an adaptation to this method of dispersal. Sometimes, e.g. guava (Psidium), the fleshy part of the fruit is eaten and the seeds with their resistant seed coats pass be undigested through the animal's alimentary canal to be dropped with the faeces some distance away from the parent plant. In the case of the mango, the fruit may be carried away from the parent tree by animals such as rats, the flesh eaten and the seed discarded. Fruit bats and birds will often carry off succulent fruits and discard the seeds some distance from the parent plant.


Self-dispersal

There are many, quite different methods of self-dispersal which depend on some mechanism in the fruit or seed forcibly ejecting the seeds from the fruit. A springy outgrowth on each of the two seeds in a Thunbergia fruit flicks them out when the fruit opens. In some pod-bearing plants, the fruit e dries in the sun and the tough, diagonal fibres in the pericarp e shrink and set up a tension. When the carpel splits in half down two lines of weakness, the two halves curl back suddenly and flick out the seeds. In the ripe fruits of Impatiens (balsam), Osmotic forces are set up which cause the intake of water and the swelling of the fruit. The pericarp finally splits into five lobes which curl up suddenly and flick out the seeds.


FLOWER STRUCTURE & DISPERSAL OF FRUITS AND SEEDS FLOWER STRUCTURE & DISPERSAL OF FRUITS AND SEEDS Reviewed by Admin on August 21, 2021 Rating: 5

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