Life Cycle of Plants and Animals
There are over 375,000 different species of plants on the earth. They range from tiny, single-celled algae to huge sequoia trees. Life on earth would not be possible without plants because they are the only living things that are capable of converting sunlight into energy. That energy fuels the other processes of life on earth.
Because plants make their own food, they are able to live almost everywhere on earth in a wide range of habitats. Many plants have developed special adaptations to help them survive.
The life cycle of an organism refers to the sequence of developmental stages that it passes through on its way to adulthood. Mammals, reptiles, amphibians, birds, fish, insects and other invertebrates--they each have their own unique way of reproducing life. There is an amazing variety of life cycles within the animal world. Surprisingly, only about 3% of all animal species give birth to live young as part of their life cycle. Most animals lay eggs.
Animals need to eat, to grow, to be safe, and to reproduce. This is all part of the life cycle. Their bodies are adapted in a wonderful range of ways to solve these problems of survival.
All of the water that is on the earth has always been here. Earth never gets water added to it--nor does water disappear from the earth. Water is constantly recycled in a process known as the hydrologic or water cycle.
Fresh water is more scarce than you might think. 97% of all the water on the earth is in the oceans, and so only 3% is fresh water. About 2.4% of the water on earth is permanently frozen in glaciers and at the polar ice caps. About 1/2 of 1% of the water on earth is groundwater. Only about 1/100 of 1% of the water on earth is in the rivers and lakes. Water is essential to life on earth, so it is important that we protect our water resources.
Nature has a way of keeping the amount of water on the earth relatively constant. A large amount of water evaporates from the surfaces of oceans, rivers, and lakes every day. It forms water vapor that rises into the air until it cools, condenses, and forms water droplets. Millions of these droplets come together to form clouds. When clouds get heavy enough, gravity tugs on the droplets, and the clouds release their water as rain or snow. This precipitation falls into streams and rivers, which flow back to the oceans, seas, and lakes, where the water cycle can begin again.