The For most of us, flowers are a source of pleasure, with their variety of shapes and vast range of colours. They're used to brighten our homes and gardens, and their bright blooms also bring colour to our city streets. But flowers are also the reproductive organs of the plant. It's for this reason that scientists work with flowers to breed new, better and more valuable varieties of plants.
A flower is therefore not just decorative, but plays an important part in plant reproduction. The role the flower plays in reproduction is easier to see in the larger, simpler flowers of the many monocotyledons. The small number and large size of their flower parts allows us to easily illustrate their structure. Each flower has three sepals, which overlap three petals.
Inside the petals there are six stamens that produce pollen. Each stamen is attached to the flower by a filament. The enlarged sac at the tip of the filament is called the anther.
Inside the anther, there are four microsporangia. Each microsporangium is nourished by a layer of surrounding cells called the tapetum. A microsporangium consists of diploid sporicite cells. The cellulose walls of these cells are soon replaced by a layer of callos.
Each sporocyte now divides twice by meiosis. This creates four haploid microspore cells, each with one set of chromosomes. This group of adhering microspores is known as a tetrad. Soon the callos is removed, and spore of pollen begins to form on the surface of each cell.
The surrounding tapetum now releases additional sporopolinone. One area of each wall, the sulcus, remains smooth, receiving very little sporopolinone. However, the rest of the wall is covered in a thick, ornate coating of this tough, resistant material. Each microspore nucleus now divides by mitosis.
The lower generative cell separates from the wall and moves into the cytoplasm of the larger tube cell. This is the immature male and is known as a pollen grain. Before it is released, the pollen grain accumulates nutrient reserves.
Finally, most of the water is lost, and the pollen grain enters into a resting state. Before the anther releases the pollen, the cells beneath the epidermis develop thickened walls. This is called the endothysium.
When the pollen and the anther mature and dry out, the endothysium shrinks. This tears open the anther between each pair of microsperangia, allowing the pollen to escape. At the center of the flower there are three carpals.
At the base of each carpal there is an enlarged ovary. A slender style extends from the ovary. and at the tip of the style there is a stigma the three carpels are fused together forming one stigma and a style with a central canal the base of the canal leads into each of the three ovaries In the young carpal, small outgrowths appear on the ovary wall.
Each of these is a megasporangium. Two layers of sills, the integuments grow over the megasporangium. A small pore, the micropyle, is the only area that remains uncovered by the integuments.
This entire structure is called an ovule. Inside the sporangium, there is a single diploid sporocyte cell. The diploid sporocyte divides by meiosis to form four haploid nuclei, each with one set of chromosomes.
In some plants, such as lilium, walls are not formed around these nuclei. But in the majority of flowering plants, walls are formed around each nucleus, resulting in four megaspore cells. Three of these then degenerate. The surviving spore enlarges, and the nucleus divides three times, yielding eight haploid nuclei. Two of these nuclei move towards the center.
These are known as the polar nuclei. Seven cells now form. At the micropylar end, there is an egg cell and two synergid cells. In the middle, there is a central cell containing the two polar nuclei.
Above this, the remaining nuclei form three antipodal cells. This is the female gametophyte, also known as the embryo sac. This is contained within the megasporangium. which is covered by the integuments of the ovule. The ovule, in turn, is surrounded by the cartil.
The transfer of pollen to the receptor surface of the stigma is called pollination. This happens by various means such as wind, insects or other animals. The pollen grain now absorbs water and begins to swell.
A pollen tube soon emerges containing the tube nucleus and the generative cell. The generative cell devised to form two sperm cells. In order to reach the ovaries, the pollen tube must now carry the sperm down the full length of the canal.
It does this by first growing through the stigma to reach the surface of the canal directly below. Here it grows along the moist transmitting tissue, which serves as a path to the ovary. Pollen tubes have a very fast rate of growth, often several micrometers per minute.
This growth is localized at its tip. Vesicles containing wall material accumulate at the tip, where they are required for wall growth. Behind this the cytoplasm flows around the cell in regular streams. This is known as cytoplasmic streaming. The two sperm cells and tube nucleus continually move forward as the cell grows.
Plugs of wall material called callos form, which keep the cytoplasm close to the growing tip. When the pollen tube reaches the base of the style, it grows into one of the three ovaries. Inside the ovary, the tube locates the two rows of ovules. Approaching an ovule, the pollen tube grows into the micropyle, where it penetrates the megasporangium.
It then grows into one of the synergid cells, releasing the two sperm. One sperm fertilizes the egg cell, forming a diploid zygote cell. The second sperm fuses with the two polar nuclei, forming a primary endosperm cell.
This double fertilization is characteristic of all angiosperms. The ovary contains hundreds of ovules, and each requires a pollen tube to be fertilized. To accomplish this, the style supports the growth of a large number of pollen tubes. These grow down the style and enter each of the three ovaries.
When the pollen tubes enter an ovary, they each locate and fertilize an ovule. Each ovary now has many fertilized ovules, which will develop into seeds. After the ovules are fertilized, the flower undergoes several changes.
Many parts begin to wither. Eventually these parts are shed. Only the ovary remains healthy and firmly attached to the stem.
Inside the ovary, the ovules are now about to undergo many changes. Each ovule contains two fertilized cells. The nucleus of the primary endosperm cell begins to divide to form endosperm tissue with no cell walls. The zygote divides, forming a large basal cell and a small apical cell.
The basal cell produces a chain of cells called a suspensor, and the apical cell gives rise to an embryo. The suspensor now begins to transport nutrients into the young embryo. In monocotyletons, a single seed leaf or cotyleton forms. As it enlarges, the embryo also receives nutrients directly from the surrounding endosperm.
During this time, walls slowly form within the endosperm tissue. A tiny shoot tip forms on one side of the embryo. Below this, there is a stem called the hypocotyl.
At the base of the hypocotyl, there is a root. This is covered by a protective root cap. In most monocotyletons, nutrients now begin to accumulate within the endosperm.
When nutrient accumulation is complete, much of the water is removed, and the embryo enters a state of dormancy. However, a different sequence of events takes place in the ovules of most dicotyletons. In these ovules, the growing seed leaves accumulate nutrients and absorb all of the endosperm.
As both types of ovules mature, the integuments develop into a tough, protective seed coat. The ovule is now a seed. At the same time that the ovule is changing into a seed, the developing embryo is releasing hormones. These stimulate the ovary to expand and make room for the enlarging ovules. In this way, the ovary wall expands and is transformed into a fruit.
In most lilies, the fruit is a tough, dry capsule that tears open to release the seeds. The seeds are soon dispersed and will germinate to form new plants, thus completing the life cycle. We have seen that flowers capture pollen and ensure the success of fertilization and seed formation. In the plant world, reproduction by flowers is the most efficient.
It's therefore not surprising that a great variety of flowering plants dominate the landscape. Many of these attain great size, but others are tiny and almost microscopic. Some flourish in places where water is scarce. Others thrive in ponds and lakes.
And from this vast variety, we have, by careful breeding, developed all those plants which provide us with food. With the great variety of flowering plants, the world in which we live is indeed a very colourful place.