Pollination and Fertilization in Flowers
Investigating Pollen Transfer and Fusion of Gametes in Angiosperms
Aug 18, 2009
A living seed may well be the most remarkable plant structure on the planet. These amazing packages of potential plant life begin with pollination which then leads to fertilization. Fertilization in turn leads to the development of seeds.
The Structure of Pollen
Pollen grains are truly extraordinary pieces of life. They are microscopic (the size of dust particles) and are produced in numbers that defy imagination. A single birch catkin can contain five and half million of them. Since a birch tree may carry several thousand catkins, the amount of pollen produced by several thousand trees in whole birch woods boggles the mind.
Pollen grains vary in shape. Some are round, others ovoid. Some are ribbed, others spiked. Yet others look like plump cushions while some are dimpled into a bowl shape. Furthermore, their surface is often intricately patterned with meandering lines, pocked with tiny circles, or in some inscribed with intricate geometric designs. These patterns are so individual and characteristic that they can be used to identify the particular species of plant they came from.
The outer rind that covers pollen grains is composed of a substance so stable and so resistant to decay that it may survive for tens of thousands of years and still be recognizable. Thus by sampling successive layers of peat in a bog, for example, and extracting, identifying, and counting the pollen grains under the microscope, it is possible to chart the ecological history of an area and detail the arrival, flourishing, and departure of different plant species.
Pollen Transfer
Once pollen has been transferred either by the breath of the wind or any one of a host of animal couriers to a receptive stigma, the first step in seed formation – pollination has been accomplished.
A stigma will not react with pollen from another species. It might be tempting to imagine this is some sort of geometric recognition much like a lock and key, and this may be partially true, but biochemical signals and responses play a crucial part as well.
The consequence is that plants are not only able to ignore pollen that comes from an alien species, but in some cases, they can even distinguish between a pollen grain that has been produced by themselves and one from another individual of their own kind and thus avoid self-fertilization.
The Process of Fertilization
Once pollination is achieved, the next step is fertilization. But how is the pollen, stuck to the top of the stigma, to get to the egg sealed within the ovary so far below?
The answer lies with a tube-like growth known as the pollen tube. When a pollen grain germinates, the tube nucleus within directs the formation and growth of the pollen tube down through the style and into the ovary and eventually an ovule. As the pollen tube grows, its generative cell divides mitotically to form two haploid sperm cells, which move down the pollen tube as it grows.
The pollen tube penetrates the ovule through the tiny micropyle opening. At least one pollen grain must germinate on the stigma for each ovule that develops into a seed. In watermelon and corn, for example, hundreds of pollen grains are required to pollinate one flower. The sperm being carried in the tube can now reach the egg through the passageway that has been formed. The union of male and female sex cells known as fertilization is now imminent.
The time elapsing between the germination of the pollen grain and fertilization is usually short, but it may take days, weeks, or even months. In barley, it is less than 1 hour; in corn, about 24 hours; in tomato, approximately 50 hours; in cabbage, about 5 days. In Witch-hazel, a shrub of the eastern United States, pollination and pollen tube formation take place in the fall but fertilization is delayed until spring, five to seven months later.
In a sense, two fertilizations occur within the ovule. One of the sperm fuses with the egg, forming a diploid zygote (full set of genes). This zygote will eventually develop into an embryo – a tiny, miniature plant complete with embryonic root, stem, and leaves.
The second sperm fuses with the two polar nuclei, producing a primary endosperm nucleus. This nucleus then develops into a tissue called endosperm. The endosperm grows by means of food supplied by the parent plant and it, in turn, nourishes the embryo. The endosperm grows faster than the embryo, and the young seed, supplied with abundant food, enlarges rapidly. This double fertilization must occur within every ovule if seed formation is to follow.
Once everything comes to maturity within what was the ovary, the plant accomplishes the purpose for which the flowers formed in the first place, the formation of seeds – a perfect package of potential plant life.
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