Question

Using the information you have gathered, address the Focus Question: What is the life cycle of a flowering plant? Draw a circular diagram of the life cycle of a flowering plant. Be sure to include the five structures, listed in Table 1, in the proper order (gametophyte, sporophyte, zygote, spore, gamete). Indicate whether each structure is haploid or diploid and the process by which each structure is produced by the previous structure. You should draw male and female gametophytes and other structures separately.

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Answer 1

The plant life cycle starts when a seed falls on the ground. There are many different kinds of plant life, but the flowering plants, or angiosperms, are the most advanced and widespread due to their amazing ability to attract pollinators and spread seeds. The major stages of the flower life cycle are the seed, germination, growth, reproduction, pollination, and seed spreading stages.

Seed Stage

The plant life cycle starts with a seed; every seed holds a miniature plant called the embryo. There are two types of flowering plant seeds: dicots and monocots. An example of a dicot is a bean seed. It has two parts called cotyledons in addition to the embryo. The cotyledons store food for the plant. Cotyledons are also the first leaves that a plant has-they emerge from the ground during germination. Monocots have only one cotyledon-the corn seed is an example. Both kinds of seeds have the beginnings of a root system as well. The hard outside of the seed is called the seed coat and it protects the embryo. Some seeds are capable of growing even after many years if they are kept cool and dry.

Germination

When a seed falls on the ground, it needs warmth and water in order to germinate; some seeds also need light. Dicots have seed coats that soften with moisture. After being planted in the soil for a few days, the seed absorbs water and swells until the seed coat splits. Monocots have harder seed coats that do not split, but stay in one piece. The stem, called the hypocotyl, pushes through the soil along with the cotyledons, or seed leaves; this is called germination, or sprouting. The tiny root pushes down and grows, looking for water and nutrients. Soon the cotyledons fall off and the first true leaves emerge. It is important that the seed is planted in the right place at the right time in order for it to germinate. Some seeds need to go through a fire in order to sprout, such as prairie grasses. Some need to go through the stomachs of animals, or be scraped.

Growth

In order to complete the flower life cycle stage of growth, plants have to produce their own food. This process is called photosynthesis. As soon as the leaves emerge, they start the process of photosynthesis. Plants contain chloroplasts in the leaves which convert the energy from sunlight, carbon dioxide, and water into sugars, which they use as food. The plants store the sugars in the roots and stem. The root system continues to develop, anchoring the plant into the ground and growing root hairs which help the plant to better absorb water and nutrients. The stem grows longer towards the sun and transports water and food between the roots and leaves. Sugars and starches are changed into energy used to make new plant growth. New leaves grow from the top of the stem, or meristem. After a while, flower buds develop. Some plants flower within days while it takes others months or even years.

Reproduction

Inside the bud, a tiny but complete flower forms. The sepals protect the bud before it opens. Over time, the bud opens and blossoms into a mature flower and the sepals look like little green leaves at the base of the flower. The flower is the sexually reproductive part of the plant. The petals of the flower are often very noticeable, brightly colored, and strongly scented in order to attract pollinators. This is a very exciting stage of the plant life cycle!

The female part of the flower is called the pistil and it has four parts-- the stigma, style, ovary, and ovules. The male part of the flower is called the stamen and it consists of the long filament and the anther, where pollen is made. In the center of the flower, there is a long slender tube that ends in a rounded oval. The tube is called the style. On the top of the style is the stigma-its job is to catch pollen. It may be sticky, hairy, or shaped in a way that helps it to better trap pollen. Sometimes several stamens surround the pistil. Once the pollen is trapped it travels down the style to the rounded part at the end, called the ovary, where eggs are waiting to be fertilized. The fertilized eggs become seeds in this stage of the flower life cycle. In fruit producing plants, the ovary ripens and becomes fruit.

Pollination

Some flowers have only male parts, and some have only female parts. In others, the male and female structures are far apart. These plants depend on insects, birds, animals, wind, water, or other pollinators to carry pollen from the male flowers or male parts to the female flowers or female parts. Without pollinators, there would be no seeds or new plants in these plant species.

Brightly colored petals, strong smell, nectar, and pollen attract pollinators. Flowers are specially adapted to attract their specific pollinators. For example, the corpse flower smells like rotting flesh in order to attract flies. Pollen sticks to the legs and wings of insects that go from flower to flower for nectar and pollen, which they use as a food.

Spreading Seeds

Seed spreading, or dispersal, is the final stage of the flower life cycle. Seeds are spread in many ways. Some, like dandelion seeds, are scattered by the wind. Others rely on animals-an example of this is the cockleburs that get stuck in the fur of animals and hitchhike to new locations. Water lilies depend on water to spread their seeds.

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The plant exhibit alternation of generations in their life cycle. The plant is the diploid sporophytic phase that produces the haploid spores through meiosis which develop to form the gametophyte. The haploid gametophyte produces haploid gametes which fuse during fertilization to form the diploid zygote which develops into the sporophyte.

Thus the current sequence will be Sporophyte, Meiosis, Spores, Gametophyte, Gametes, Fertilisation, Diploid zygote.

In terms of chromosomes, the gametophyte is haploid (has a single set of chromosomes), and the sporophyte is diploid (has a double set). In bryophytes, such as mosses and liverworts, the gametophyte is the dominant life phase, whereas in angiosperms and gymnosperms the sporophyte is dominant.

The main difference between haploid and diploid cells is the number of chromosome sets found in the nucleus. Ploidy is the area of biology that refers to the number of chromosomes in a cell. Therefore, cells with two sets are diploid, and those with one set are haploid. In diploid organisms, such as humans, the haploid cells are used only for the sex cells for reproduction, while the rest of the cells are diploid. Another difference between haploid and diploid cells is how they reproduce. Haploid cells are reproduced using meiosis, while diploid cells go through mitosis. Most mammals are diploid organisms, and their somatic cells will typically be diploid and their gamete cells will be haploid.

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