Unit 3: How Plants Work

How Plants Work

Plants are made up of roots, stems, and leaves, and most produce flowers, fruit, and seeds. Plants comprise a large part of the ecosystem. In this section, we will learn how plants are built, how they reproduce, how they spread their seeds, and how they are organized into groups based on similarity of their structures.

What makes plants special is that they produce their own food. Leaves capture the sunlight in order to do this. They contain a pigment called chlorophyll, which makes the leaves green. Using carbon dioxide, water, nutrients, and energy from sunlight, the chlorophyll makes the food that the plant needs. This process is called photosynthesis. During this process, plants release oxygen into the air. People and animals need oxygen to breathe.

The roots hold a plant in place, and they absorb water and minerals that plants need to grow. The stem supports the plant and carries the water and minerals from the roots to other parts of the plant. The flower is the reproductive part of most plants. It contains pollen and tiny eggs or ovules. Pollination is the transfer of pollen from a stamen to a pistil. This starts the production of seeds. Pollen is produced in the stamen and gets transferred to the pistil, which is often sticky. Plants are usually pollinated by animals or by the wind. Animals that are good pollinators are bees, butterflies, moths, flies, and sunbirds. They are attracted to the flower by the colorful petals. Actually, this pollination is accidental. The animal is just trying to get food—the sticky pollen or sweet nectar—at the base of the petals. While doing this, the animal accidentally rubs against the stamens (the part of the flower that contains the pollen) and gets pollen stuck on itself. It then moves to another flower and transfers some of the pollen to this flower’s stigma. Plants that are pollinated by birds and insects often have brightly colored petals and a strong smell to attract the animals. Another way plants can be pollinated is by the wind. The wind carries the pollen from one plant to another. Plants that are pollinated this way usually have long stamens and pistils to increase their chance of catching the pollen. They don’t need to be brightly colored or strongly scented, and they don’t need petals because insects don’t need to land on them.

After the flower is pollinated and the egg is fertilized, the ovule develops into a fruit. The fruit is a covering for the seeds. It can be fleshy, like a mango, or hard, like a nut. The seeds contain new plants, and they need the right conditions to grow: water, warmth, and a good location, usually in soil.

Seeds can be dispersed by wind, water, or animals. Plants have developed several adaptations for seed dispersal. For example, some seeds have feathery hairs that help the seed to be carried long distances by the wind. Others are hidden inside the fruit of the plant. Animals eat the fruit, and then the seeds pass through the animal’s gut and are expelled. Still others have structures that will attach to an animal’s fur so the seed gets carried by the animal to another location. And some seeds float in water.

Some plants reproduce themselves in a different way. They send out runners underground or tillers above the ground. Each new runner or tiller becomes a new plant.

Both methods of reproduction allow plants to spread out and grow in new places. This is important so that the new plants don’t grow too close to the parent, thus competing for sun, water, nutrients, and space.