Flowers and sexual reproduction

Some flowers are large and showy. Some are very small. Compare the orchid with the tiny flowers of the lilac plant. Even though these flowers do not look alike, they have similar functions. Flowers contain the male and female reproductive organs of the angiosperms. During sexual reproductions, these organs make the nuclei that join together to form a new angiosperm.

Some flowers contain both male and female reproductive organs. Examples of angiosperms with these perfect flowers are apples, garden peas and cherries. Imperfect flowers contain only male or only female reproductive organs. Com, maple and holly plants have imperfect flowers.

The receptacle is the base of the flower. It is the part of the flower that attaches to the stem. Sepals are usually green, leaflike parts of the flower. They cover and protect the flower bud as the young flower develops inside. The showy petals are found just above the sepal. They surround and protect the reproductive organs of the flower.

The stamens – the male reproductive organs – are found just inside the petals. Stamens have two parts. The thin stalk of the stamen is the filament. An anther is found on the top of each filament. Pollen grains, which contain the mail reproductive nuclei, are made in the anther.

The tall, vaselike pistil is the female reproductive organ. It is found in the center of the flower. At the top of the pistil is the stigma. It is covered with a liquid that pollen grains stick to. The stalk-like style leads from the stigma to the ovary at the base of the pistil. Inside the ovary is an ovule. The egg, which contains the female reproductive nucleus, is found in the ovule. Some flowers have many ovules. Others have only one ovule.

The parts of the flower that produce the gametes and carry out sexual reproduction are called the essential flower parts (stamens, pistils), the delicate essential flower parts are protected and adorned by the nonessential flower parts (sepals, petals, etc).

Complete flowers contain all the essential and nonessential parts. Roses, violets, and mustard blossoms, for example, are complete flowers. Incomplete flowers lack one or more of the essential or nonessential parts. The flowers of most grasses lack developed petals and sepals, and so are incomplete.

Sexual reproduction requires pollination. In animals, sexual reproduction involves the fertilization of a female egg cell by a male sperm cell. Angiosperms make eggs, but they do not make sperm. Instead, a sperm nucleus from a pollen grain fertilizes the nucleus of an egg, which is in an ovule within the ovary.

If fertilization is to occur, pollen grains must move from an anther to a stigma in the process called pollination. Birds and insects, such as bee, are common carriers of pollen grains. They are attracted to the odors, bright colors and sweet liquid of flowers. As they move inside a flower they can earn pollen grains from an anther to a stigma. Wind and rain also help in pollination.

One a pollen grain is on stigma, the pollen grain grows a long tube through the style and ovary wall into the ovule. When the pollen tube enters the ovule, a sperm nucleus from the pollen tube can fertilize the egg nucleus inside the ovule. The fertilized egg inside the ovule then forms an embryo, which is a young plant.

Date: 2014-12-22; view: 980