Plants support life by providing food and oxygen, which is needed for energy. All matter and all energy transform in and out of each other. Plants, by photosynthesis, create matter, (food) by cellular factories called chloroplasts. In short, energy from sunlight, plus carbon dioxide from the air, plus water (H2O) allow all life to exist on earth. As they too, are carbon based lifeforms, all plants are distant cousins of human beings.
On the leaves of the plant are structures called stomates. Chloroplasts use the stomates to convert sunlight, carbon and water which flows through the plant's "veins". The veins, or tubes that carry water are called the Xylem. The stomates open and contract, allowing the exchange of the gases. In this way, with the evolution of plants about 500 million years ago (MYA) the vascular systems of plants set the stage for ever more complex life forms to arise. Like human beings, plants are multi-cellular, or Eukaryotic.
There are some new theories which suggest that Earth as we see it today, was formed as much from plant life as from geologic events. This plant landscape theory suggests that the interaction of water and plants, the cooling of the planet and the ongoing collection of CO2 from the air, is not only what forms Earth as we see it, but also allows Earth to be able to support any and all life. Life came about because plants completely transform the very surface of the planet.
As animals need this created oxygen which plants "breathe" out through their stomates, all animals need plants. Even if oxygen was not a by product of living plants that all creatures need, plants would still be vital. Plants are the only known matter that create what is called "food!" As plants are what shape earth, create breathable air, cool the planet, store and convert energy for all human beings and other beings, needless to say they are immensely important.
With the advent of climate change, a vast and untried experiment is occurring. With plant life removed from earth, and events such as carbon stored under the permafrost in Siberia being released, there is a threat of undoing the cooling of the planet that the evolution of CO2 storing plants allow. That is, by cooling the planet, plants made life possible. By heating the planet and removing so many plants, the reverse is happening. Desertification, the process whereby healthy forest and grassland is destroyed, the rising seas, dying ecosystems and marine acidification are just a few of the dangers which threaten plant life, and therefore human life, as well.
A plant then, although often thought of as a passive thing, is actually a tremendously interactive being. Whether one subscribes to the primordial soup theory, the geologic formation theory, the idea that life arrived from other planets, or that plants formed it, Earth today depends entirely upon the health of plants. Most likely is that all origins have some truth to them. It is certainly something to think about when crunching into a carrot, or noticing clothes, desks, walls and even car fuel comes from plants. They are anything but passive "things."