UNIT 2. THE LIFE CYCLE OF A PLANT

Many plants grow out of seeds and bulbs.

Seeds grow roots and shoots. Roots and shoots then grow leaves above ground.

Many plants make flowers, which turn into fruits. Flowers and fruits make their own seeds. We call this a life cycle.

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.

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.

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. . Even flowers that can self-pollinate benefit from being fertilized by pollen from a different plant, which is called cross pollination, because cross pollination results in stronger plants.

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. Pollen sticks to the fur of animals and even to the clothes of humans. Wind blows pollen which lands on other flowers.

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. Water lilies depend on water to spread their seeds. Humans spread many seeds intentionally by planting gardens. Once the seeds fall to the ground, the plant life cycle starts all over again.