Flowers are reproductive structures; they contain the reproductive organs of the plant. The male organs are the stamens, which produce pollen. The female organs are the carpels. After fertilisation, part of the carpel becomes the fruit of the plant and contains the seeds. In the flowers of most plants there are both stamens and carpels. These flowers are, therefore, both male and female, a condition known as bisexual or hermaphrodite.

Some species of plants have unisexual flowers, i.e any one flower will contain either stamens or carpels but not both. Sometimes both male and female flowers are present on the same plant, e.g. the hazel, which has male and female catkins on the same tree. In the willow tree, on the other hand, the male and female catkins are on different trees.

The male gamete is a cell in the pollen grain. The female gamete is an egg cell in the ovule. The process that brings the male gamete within reach of the female gamete (i.e. from stamen to stigma) is called pollination. The pollen grains grows a microscopic tube, which carries the male gamete the last few millimetres to reach the female gamete for fertilisation. The zygote then grows to form the seed.

Flower structure

The basic structure of flower is shown in Figure below

Petals

Petals are usually bright coloured and sometimes scented. They are arranged in a circle or a cylinder. Most flowers have from four to ten petals. Sometimes they are joined together to form a tube and the individual petals can no longer be distinguished. The colour and scent of the petals attract insects to the flower; the insects may bring about pollination.

The flowers of grasses and many trees do not have petals but small, leaf-like structures that enclose the reproductive organs.

Sepal

Outside the petals is a ring of sepals. They are often green and much smaller than the petals. They may protect the flower when it is in the bud.

Stamens

The stamens are the male reproductive organs of a flower. Each stamen has a stalk called the filament with an anther on the end. Flowers such as the buttercup and blackberry have many stamens; others such as the tulip have a small number, often the same as, or double, the number of petals or sepals. Each anther consists of four pollen sacs in which the pollen grains are produced by cell division. When the anthers are ripe, the pollen sacs split open and release their pollen.

Pollen

Insect-pollinated flowers tend to produce smaller amounts of pollen grains, which are often round and sticky, or covered in tiny spikes to attach to the furry bodies of insects.

Wind-pollinated flowers tend to produce larger amounts of smooth, light pollen grains, which are easily carried by the wind. Large amounts are needed because much of the pollen is lost, there is a low chance of it reaching another flower of the same species.

Insect-borne pollen grains
Wind-borne pollen grains

Pollen grains

Carpels

These are the female reproductive organs. Flowers such as the buttercup and blackberry have a large number of carpels while others, such as the lupin, have a single carpel. Each carpel consists of an ovary, bearing a style and stigma.

Inside the ovary there are one or more ovules. Each blackberry ovary contains one ovule but the wallflower ovary contains several. The ovule will become a seed, and the whole ovary will become a fruit.

The style and stigma project from the top of the ovary. The stigma has a sticky surface and pollen grains will stick to it during pollination. The style may be quite short or very long.

Receptacle

The flower structures just described are all attached to the expanded end of a flower stalk. This is called the receptacle and, in a few cases after fertilisation, it becomes fleshy and edible.

Lupin

The lupin flower is shown in Figure across. There are five sepals but these are joined together forming a short tube. The five petals are of different shapes and sizes. The uppermost, called the standard, is held vertically. Two petals at the sides are called wings and are partly joined together. Inside the wings are two more petals joined together to from a boat-shaped keel.

The single carpel is long, narrow and pod shaped, with about ten ovules in the ovary. The long style ends in a stigma just inside the pointed end of the keel. There are ten stamens, five long ones and five short ones. Their filaments are joined together at the base to form a sheath around the ovary.

Half-flower of lupin

The flowers of peas and beans are very similar to those of lupins. The shoots or branches of a plant carrying groups of flowers are called inflorescences. The flowering shoots of the lupin are inflorescences, each one carrying about a hundred individual flowers.

Intact
One wing removed
One side of keel removed

Lupin flower dissected

Functions of parts of flower

PartFunction
PetalOften large and coloured, to attract insects
SepalProtects the flower while in bud
Petiole (stalk)Supports the flower to make it easily seen by insects and to be able to withstand wind
NectaryProduces nectar to attract insects
StamenThe male reproductive part of the flower, made up of anther and filament
AntherContains pollen sacs, in which pollen grains are formed. Pollen contains male sex cells
FilamentSupport the anther
Carpel The female reproductive part of the flower made up of stigma, style and ovary
StigmaA sticky surface to the ovary through which pollen tubes grow
StyleLinks the stigma to the ovary, through which pollen tubes grow
Ovary Contains ovules, which develop into seeds when fertilised

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma, enabling fertilisation. Mechanism of pollination include insect pollination and wind pollination. Insect-pollinated flowers contain nectar and have nectar guides which are lines that are visible to insects, guiding them to the location of the nectar. When the insect enters the flower, pollen grains from the anthers stick onto the insect. If pollen grains from a previously-visited flower are present on the insect, they will be transferred to the sticky stigma. Wind pollinated flowers have their pollen carried away by the wind when the exposed anthers shake in the wind. When the pollen grains come into contact with the large feathery stigmas of another flower, they would be trapped. There are two types of pollination self pollination and cross pollination.

Self-pollination and cross-pollination

In self-pollinating plants, the pollen that reaches the stigma comes from the same flower or another flower on the same plant. In cross-pollination the pollen is carried from the anthers of one flower to the stigma in a flower of another plant of the same species.

Self pollination

If a bee carried pollen from one of the younger flowers near the middle of a lupin plant to an older flower near the bottom, this would be self pollination. If however, the bee visited a separate lupin plant and pollinated its flowers, this would be cross-pollination.

The term cross-pollination strictly speaking, should be applied only of there are genetic differences between the two plants involved. The flowers on a single plant all have the same genetic constitution. The flowers on plants growing from the same rhizome or rootstock will also have the same genetic constitution. Pollination between such flowers is little different from self-pollination in the same flower.

Cross-pollination

If a plant relies on self-pollination, the disadvantage will be that variation will not occur in subsequent generations. Those plants may not, therefore be able to adapt to changing environmental conditions. However, self-pollination can happen even if there are no pollinators, since the flower’s own pollen may drop onto its stigma. This means that even if pollinators are scarce (perhaps because of the reckless use of insecticides) the plant can produce seeds and prevent extinction.

Cross-pollination, on the other hand, will guarantee variation and give the plant species a better chance of adapting to changing conditions. Some plants maintain cross-pollination by producing stamens (male reproductive parts) at a different time to the carpels (female reproductive parts). However, cross-pollinated plants do have a reliance on pollinators to carry the pollen to other plants.

Insect pollination

Lupin flowers have no nectar. The bees that visit them come to collect pollen, which they take back to the hive for food. Other members of the lupin family (Leguminosae, e.g. clover) do produce nectar.

The weight of the bee, when it lands on the flower’s wings, pushes down these two petals and the petals of the keel. The pollen form the anthers has collected in the tip of the keel and, as the petals are pressed down, the stigma and long stamens push the pollen out from the keel on to the underside of the bee.

The bee with pollen grains sticking to its body then flies to another flower. If this flower is older than the first one, it will already have lost its pollen. When the bee’s weight pushes the keel down, only the stigma comes out and touches the insect’s body, picking up pollen grains on its sticky surface.

Lupin and wallflower are examples of insect-pollinated flowers

Wind pollination

Grasses, cereals and many trees are pollinated not by insects but by wind currents. The flowers are often quite small with inconspicuous, green, leaf like bracts, rather than petals. They produce no nectar. The anthers and stigma are not enclosed by the bracts but are exposed to air. The pollen grains, being light and smooth may be carried long distances by the moving air and some of them will be trapped on the stigmas of other flowers.

In the grasses, 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. This sequence varies between species.

If the branches of a birch or hazel tree with ripe male catkins, or the flowers of the ornamental pampas grass, are shaken, a shower of pollen can easily be seen.

Wind-pollinated grass flower

Features of wind and insect-pollinated flowers

FeaturesInsect-pollinatedWind-pollinated
Petals– large, coloured, scented,
– with guidelines to guide insects into flowers
– absent/small
Nectar – produced by nectarines
– attract insects
– absent/small and green
Stamen– inside flower– long filaments: anther hang
freely outside flower → pollen exposed to wind
Stigmas– small, sticky
– inside flower → insects rub against
– large, feathery
– hang outside flower → catch pollen
Pollen– smaller amount
– grain round and sticky or covered in spikes to attract insects
– larger amount
– grain smooth, light, easily carried by wind
Bracts
(modified leaves)
– Absent– sometime present

Adaptation

Insect-pollinated flowers are considered to be adapted in various ways to their method of pollination. The term ‘adaptation’ implies that, in the course of evolution, the structure and physiology of a flower have been modified in ways that improve the chances of successful pollination by insects.

Most insect-pollinated flowers have brightly coloured petals and scent, which attract a variety of insects. Some flowers produce nectar, which is also attractive to many insects. The dark lines on petals are believed to help direct the insects to the nectar source and thus bring them into contact with the stamens and stigma.

These features are adaptations to insect pollination in general, but are not necessarily associated with any particular insect species. The various petal colours and the nectaries of the wallflower attract a variety of insects. Many flowers, however have modifications that adapt them to pollination by only one type or species of insect. Flowers such as the honeysuckle, with narrow, deep petal tubes, are likely to be pollinated only by moths or butterflies, whose long ‘tongues’ can reach down the tube to the nectar.

Tube-like flowers such as foxgloves need to be visited by fairly large insects to effect pollination. The petal tube is often lined with dense hairs, which impede small insects that would take the nectar without pollinating the flower. A large bumble-bee, however pushing into the petal tube, is forced to rub against the anthers and stigma.

Many tropical and sub-tropical flowers are adapted to pollination by birds, or even by mammals such as bats and mice.

Wind-pollinated flowers are adapted to their method of pollination by producing large quantities of light pollen, and having anthers and stigmas that project outside the flower. Many grasses have anthers that are not rigidly attached to the filaments and can be shaken by the wind. The stigma of grasses are feathery, providing a large surface area, and act as a net that traps passing pollen grains.

Fertilisation

The pollen grain absorbs liquid from the stigma and a microscopic pollen tube grows out of the grain. This tube grows down the style and into the ovary, where it enters a small hole, the micropyle, in an ovule. The nucleus of the pollen grain travels down the pollen tube and enters the ovule see Figure below.

Fertilisation showing pollen tube

Here it combines with the nucleus of the egg cell. Each ovule in an ovary needs to be fertilised by a separate pollen grain. Although pollination must occur before the ovule can be fertilised, pollination does not necessarily result in fertilisation. A bee may visit many flowers on a Bramley apple tree, transferring pollen from one flower to another. The Bramley however is ‘self-sterile’; pollination with its own pollen will not result in fertilisation. Pollination with pollen from a different variety of apple tree, for example a Worcester, can result in successful fertilisation and fruit formation.

Seeds

Fertilised ovules develop over time into seeds, which protect the developing embryo inside. Seeds are held within a seed pod, fruit or nut, which can be dispersed to new locations so that when they germinate, the new plants that develop do not compete with their parents.

Plants have evolved many ingenious means by which to bring about seed dispersal. A few are listed below:

  • Some seed pods such as those in the pea family mature and dry out, so they eventually snap, causing the seed pod to open quite suddenly, ejecting the seeds some distance from the parent plant.
  • Nuts are collected by animals like squirrels which bury them as a reserve of food for the winter. They may bury several groups of nuts and fail to dig them all up during the winter. The nuts remain in the soil ready to germinate when conditions are favourable.
  • Wind-dispersed seeds

Fruits contain seeds, and usually have a parachute or a wing to help them be carried away from the parent plant by the wind examples dandelion, sycamore.

The dandelion fruit has a group of fine hairs called a pappus which catches the wind and acts like a parachute. The fruit counterbalances the pappus.

Wind-dispersed seeds

The sycamore has a wing with a large surface area. When the fruit drops off the tree it spins, slowing down in descent. If caught by the wind the seed will be carried away from the parent plant, reducing competition for nutrients, water and light.

  • Animal-dispersed seeds

There are two main modification of fruits for animal dispersal succulent fruits and hooked fruits.

Succulent fruits attract animals because they are brightly coloured, juicy and nutritious. When eaten, the seed pass through animals faeces, which may be a long way from the parent plant. The faeces provides nutrients when the seeds germinate.

Hooked fruits catch on to an animal’s fur as it brushes past the parent plant. Eventually the seed drops off, or the animal grooms itself to remove them. This disperses the seeds away from the parent plant.

Animal-dispersed seeds

Seeds have all the necessary components to ensure successful germination and the growth of a new plant. Within every seed is a embryo root and shoot ready to develop when the time is right. Once a seed has been formed in the ovary, it loses water so that it can enter a dormant phase and not develop further until conditions for growth are favourable.

Inside their seeds, dicotyledonous plants have two seed leaves, or cotyledons, which store food reserves needed for germination see Figure below

The main parts of a dicotyledonous plant seed

The cotyledons are surrounded by a hard protective seed coat called the testa. Many seeds have to endure quite harsh envirionmental conditions, so the testa protects the delicate tissues inside. In the wall of the testa is a pore called the micropyle through which water is absorbed to begin the process of germination.

Germination

The stages of germination of a French bean are shown in Figure across. A seed just shed from its parent plant contains only 5-20% water, compared with 80-90% in mature plant tissues. Once in the soil, some seeds will absorb water and swell up, but will not necessarily start to germinate until other conditions are suitable.

The radicle grows first and bursts through the testa Figure (a) . The radicle continues to grow down into the soil, pushing its way between soil particles and small stones. Its tip is protected by the root cap. Branches, called lateral roots, grow out from the side of the main root and help to anchor it firmly in the soil. On the main root and the lateral roots, microscopic root hairs grow out. These are fine outgrowths from some of the outer cells. They make close contact with the soil particles and absorb water from the spaces between them.

In the French bean a region of the embryo’s stem, the hypocotyl, just above the radicle. In the French bean a region of the embryo’s stem, the hypocotyl, just above the radicle, Figure (b), now starts to elongate. The radicle is by now firmly anchored in the soil, so the rapidly growing hypocotyl arches upwards through the soil, pulling the cotyledons with Figure (c). Sometimes the cotyledons are pulled out of the testa, leaving it below the soil, and sometimes the cotyledons remain enclosed in the testa for a time. In either case, the plumule is well protected from damage while it is being pulled through the soil, because it is enclosed between the cotyledons Figure (d).

Germination of French bean

Once the cotyledons are above the soil, the hypocotyl straightens up and the leaves of the plumule open out Figure (e). Up to this point, all the food needed for making new cells and producing energy has come from the cotyledons.

The main type of food stored in the cotyledons is starch. Before this can be used by the growing shoot and root, the starch has to be turned into soluble sugar. In this form, it can be transported by the phloem cells. The change from starch to sugar in the cotyledons is brought about by enzymes, which become active as soon as the seed starts to germinate. The cotyledons shrivel as their food reserve is used up, and they fall off altogether soon after they have been brought above the soil.

By now the plumule leaves have grown much larger, turned green and started to absorb sunlight and make their own food by photosynthesis. Between the plumule leaves is a growing point, which continues the upward growth of the stem and the production of new leaves. The embryo has now become an independent plant, absorbing water and mineral salts from the soil, carbondioxide from the air and making food in its leaves.

Environmental conditions affecting germination

  1. Water:
    • absorbed through microphyle until radicle is forced out of testa
    • activate enzymes for converting soluble food stores in the cotyledons down to soluble food ⤏ for growth + energy production of baby plant.
  2. Oxygen:
    • respiration ⤏ release energy ⤏ growth
  3. Warmth/Temperature:
    • enzymes present in the seed get activated and work best at optimum temperature (20-40C) which trigger growth in the baby plant.
  4. Light intensity:
    • high or very low light intensity does not allow enzymes to function normally

Metabolism and germination

Germination begins as water is absorbed by the seed in a process known as imbibition. Water enters through the micropyle of the testa.

Water rehydrates stored food reserves in the seed and, in a starchy seed such as a barley grain, it triggers the embryo plant to release a plant growth hormone called gibberellin see Figure below.

Longitudinal section through a barley seed, showing how secretion of gibberellin by the embryo results in the mobilisation of starch reserves during germination

The gibberellin in turn stimulates the synthesis of amylase by the cells in the outer aleurone layer of the seed. The amylase hydrolyses starch molecules in the endosperm (food store), converting them to soluble maltose molecules. These are converted to glucose and are transported to the embryo, providing a source of carbohydrate that can be respired to provide energy as the radicle (embryo root) and plumule (embryo shoot) begin to grow, or used to produce other material needed for the growth, such as cellulose.

Absorption of water by the seed splits the testa, so that the radicle and plumule can emerge and grow. When the leaves of the seedling have grown above ground, they can begin to photosynthesise and take over from the food store in the seed in supplying the needs of the growing plant.