Just as most animals have a head, body and limbs, most plants share certain characteristics in common with each other. These essential components are responsible for helping the plant secure adequate nourishment both from the sun and soil, as well as to help it grow and potentially out-compete the plants surrounding it for resources.
Although we don't typically see them, roots are perhaps the most expansive part of a plant. Not only do they account for as much as a third of the plant's mass, but they spread both deep and wide. For instance, if all the branching segments of a rye plant's root system were cut and arranged into a straight line, it would extend about 380 miles. The plant invests a lot of resources into developing such intricate root systems, and this is because the roots are the plant's sole source of minerals and water, which it leaches directly from the ground. At the same time, the root system stabilizes the plant so that it cannot be easily blown over by winds. This is a problem for some trees, which might have an extensive but shallow root system. If the roots don't dig deeply enough, then not only will they lack access to the water table, they'll also risk getting uprooted in a storm, killing themselves and damaging anything crushed by their descent.
Growing upward from the root system is the stem, which in some species will branch out into multiple limbs. The stem has two purposes. The first is structural. The stem supports upward and outward growth. The second is for transportation. Resources such as water and minerals are gathered by the roots, while energy in the form of sunlight is captured and processed by the leaves. Energy is needed by the roots to survive, while water and minerals are needed by the leaves. Stems convey the resources to each area, similar to how veins and arteries transport oxygen and nutrients throughout an animal's body.
Leaves are perhaps the most important structures on Earth. Through a process known as photosynthesis, they harness energy from the sun. This energy not only helps plants grow and reproduce, but when animals eat a plant, the animals get this energy as well, which ultimately supports the whole food chain. Photosynthesis is processed through molecules called chlorophyll, which uses sunlight to convert carbon dioxide and water into sugar, which fuels both plant and animal activities. Photosynthesis demands large quantities of water, and even more is lost through structural inefficiency known as transpiration. This is why the relationship between roots and leaves is harmonious, as each provides something the other needs.