Plants (Plantae) are one of six kingdoms of living things. Plants are producers, in other words, they take energy from the environment in the form of sunlight and use it to make organic molecules for various biological functions, such as cell growth and repairing broken tissue. This process of capturing sunlight is called photosynthesis. A plant needs sunlight, carbon dioxide, minerals and water to make food by photosynthesis. A green substance in plants, called chlorophyll, traps the energy from the sun needed to make food. Chlorophyll is mostly found in leaves. The leaf can be thought of as a food factory. Leaves of plants vary in shape and size, but they are usually the plant organ best suited to capture solar energy. Once the food is made in the leaf or other parts, it is transported to the other parts of the plant such as stems and roots.
There are always exceptions, one is the Palo Verde (Parkinsonia sp.) which has chlorophyll in the bark which enables the plant to conduct photosynthesis within the bark. Another exception are plants in the Cactus family which photosynthesize through the epidermis.
Most plants grow in the ground, with stems above the soil surface, and roots below. Some float on water. Water and some nutrients come from the roots. The evaporation of water from pores in the leaves pulls water up through the plant. This is called transpiration.
Plants and trees that photosynthesize produce oxygen as a waste product of making sugar using sunlight, carbon dioxide, and water. As humans and animals are oxygen breathers, a decrease in plants means a decrease in oxygen, therefore we could not exist without plants. Plants are food sources and are also used for medicinal purposes. Many plants have healing properties, and one-quarter of prescription drugs come from plants. Plants also store carbon, which helps keep carbon dioxide out of the atmosphere.