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.

4.4.2. Angiosperm life cycle

Within the anther of the flower, the male gametophyte develops from microspore mother cells, which divide meiotically, each giving rise to four haploid microspores. The nucleus in each microspore then divides mitotically, developing into a pollen grain containing two nuclei. One of the nuclei subsequently divides again, usually upon germination, resulting in three nuclei per pollen grain: two sperm nuclei and one vegetative nucleus.

Within the ovule, the female gametophyte develops from a megaspore mother cell, which divides meiotically to produce four haploid megaspores. Three of the megaspore degenerate; the fourth divides mitotically, developing into an embryo sac consisting of seven cells with a total of eight haploid nuclei (the large central cell contains two nuclei, the polar nuclei). One of the smaller cells, containing a single haploid nucleus, is the egg cell.

The pollen germinates on the stigma, producing a pollen tube that grows down the style into the ovary. The two sperm nuclei enter the embryo sac through the tube; one nucleus fertilizes the egg cell, the other merges with the polar nuclei, forming the triploid endosperm. The embryo undergoes its first stages of development while still within the ovary of the flower and the ovary itself mature to become a fruit. The seed, released from the mother sporophyte in a dormant form, eventually germinates forming a seedling.

Seeds and fruits come from flowers.

When you eat a lima bean, you are eating an embryo and other parts of a seed. A seed is a mature ovule. A new plant grows from a seed.

After fertilization, the fertilized egg develops rapidly and becomes the embryo of a seed. Two cotyledons develop, which often contain food for the growing embryo. Sometimes food is also stored outside the cotyledons. Meanwhile, the wall of the ovule becomes the seed coat. Dicot seeds, such as bean, have an embryo, two cotyledons, and a seed coat. Monocot seeds, such as corn, have one cotyledon, an embryo, and a seed coat.

Some seeds are made so that they can move easily from place to place. This helps in the spreading of new plants. Catalpa seeds are shaped like wings, and milkweed seeds have hairs that help the wind more them.

When you eat a peach, tomato or olive, you are eating a fruit. Following fertilization in the angiosperm, the ovary develops into the fruit. As the gymnosperms, the ovules develop into the seeds, each containing its embryo sporophyte. Fruits may develop from a single carpel or from fused carpels, and each carpel may contain one or more ovules. In the peach, for instance, the skin, the fleshy, edible portion of the fruit, and the stone are three layers of the matured ovary wall. The seed is inside the stone. This is an example of a simple seed formed from a single ovule. In the pea, the pod is the mature ovary wall, and the peas themselves are seeds. The raspberry is an aggregate of many tiny, fleshy fruits, each formed from a separate carpel.

Fruits provide new and ingenious way for seed dispersal. In some plants, the fruit itself carries wings, as in many of our common trees; in others the fruit bursts open, shooting out the seeds. Some species of geranium send forth their seeds by a sort of slingshot. Often, the fruits are edible and brightly colored, tempting birds and mammals to eat them. The seeds within the fruit pass unharmed through the digestive tract hours later and often miles away. Burrs adhere to fur, feathers or one’s trouser legs, to be carried by unwilling messenger to far-off fields and meadows. In the tumbleweed, the whole plant is blown across the open country, scattering seeds as it goes. Angiosperm seeds travel much father and faster than simpler seeds of the gymnosperm.

4.4.3. Reproduction from roots, leaves and stems

Any reproduction that occurs without the combining of male and female sex cells or sex nuclei is called asexual reproduction. Some angiosperms have special kinds of organs which they use to reproduce asexually in nature. Most often, these are roots, leaves and stems.

You can grow whole new plants from the sweet potatoes you buy in a grocery store. Sweet potatoes are the roots of flowering plants. New shoots will readily grow from a sweet potato.

The leaves of some plants can also grow into new plants. Suppose you bump into a jade plant and knock off a leaf. You can grow a whole new jade plant by placing the leaf in the soil or water. The same is true for the African violet plants you see to the right.

Any plants have special stems that allow them to reproduce asexually. Onion plants can reproduce asexually by using bulbs which grow underground. The bulb is the part of the onion plant that you eat. Bulbs are really short, fat stems with roots. The stem begins to grow when the bulb is planted. Most of the bulb is made up of tightly packed leaves that store food. You can see these leaves when an onion is cut open. Other common plants with bulbs are tulips, lilies and daffodils.

A potato is a tuber that grows underground. A tuber is a thick stem with buds that can grow into a new plant. The buds of the tuber are the "eyes" you cut out of potato before you cook it. New plants grow from the buds when a potato is planted. Dahlias and peonies can also reproduce by tubers.

The strawberry plants are connected by a runner, which is a stem that grows across the top of the soil. New strawberry plants grow at points where the runner touches the ground. Strawberries spread out in a field because these runners stretch out in all directions.

Table 4.1

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