Without the presence of green plants on Earth, the planet would be a very different place. Green plants have an impact on every living thing on the planet, and have shaped the world for as long as they have existed. As long as they continue to exist, they will have an undeniable impact on the world around them--and they will continue to sustain it.
Photosynthesis & oxygen
The leaves of green plants contain chlorophyll, which is the chemical responsible for photosynthesis. The plants take carbon dioxide from the atmosphere and nutrients from the soil, then convert it into a form that is usable by the plant. In the process, oxygen is released into the air. Photosynthesis--and green plants--is the largest and most thorough way that sunlight is captured and changed into a form that can be used by the other inhabitants of the planet.
Cellulose & energy
In addition to oxygen, photosynthesis also produces cellulose--another chemical that is essential to life on the planet. Cellulose is one of the most important components in the wood of trees and the heavy stems of other green plants; when the wood is burned, the cellulose is released and turned into heat energy. In addition to burning the plant material containing cellulose, it's also used in the production of some common materials. Paper is almost pure cellulose, and without the cellulose compound there would be no cotton production.
Similar energy sources such as natural gas and coal were formed after centuries of compression and decay of plant matter, and the conversion of the cellulose within the dead material. Ethanol is another energy source, made from grain and the chemicals within it.
Green plants are the primary source of energy in another way--nutrition. Insects, birds and herbivores all consume plants directly, but green plants also provide the basis of nutrition for carnivores that never directly consume the plants themselves. Without these green plants, larger animals higher up the food chain would have no food sources.
This doesn't just apply on land--marine life is also dependent on algae, plankton, seaweeds and other, even microscopic, green plant life.
Green leaves are an important source of a number of the vitamins and minerals that are essential to life on earth. Some greens are higher in calcium than even milk, while others contain high levels of vitamin C. They are also an excellent source of vitamins A, D and E, as well as B and retinol--essential for maintaining eyesight. Bioflavonoids are found in green plants in conjunction with chlorophyll; these are used by the human body in breaking down and using other vitamins. Green leaves are high in proteins and fibers, also crucial to the survival of other life forms on earth.
Plants have been used as natural remedies as far back in history as the written word exists. Documents in India, China, the Middle East, Egypt and ancient Greece as far back as 7 B.C. detail the use of various herbs and plants as remedies for everything from burns and fevers to stomach disorders. While some herbal medicine has been dismissed as little more than folklore, modern medicine has discovered the value in many herbal remedies. Breaking an aloe plant to get to the cooling center has long been a treatment for burns and rashes, cooling and soothing the skin on contact. More than 120 drugs on the market have a plant base, and still more are undergoing laboratory testing to determine their potential; many researchers hope that finding a cure for cancer lies in a natural remedy.