There are seven parts to a plant: the stem, leaf, bud, root, flower, fruit, and seed. Each one serves a particular function. Plant parts work together to propagate the species. Individual plant parts have been designed through evolution to have the whole plant be self-sufficient.
Stems have three parts: the xylem, the phloem and the cambium. These support leaves and buds and they provide food and water transportation through the plant. The phloem transports food. The xylem transports water. The cambium provides growth and cell division.
Leaves are typically the biggest part on the plant, with a petiole supporting the blade of the leaf. Leaves will take in sunlight and, through photosynthesis, turn that sunlight into plant sugars. They have different colors, sizes and shapes.
Buds are shoots that haven't developed that will form flowers or leaves. Leaf buds are more pointed than a flower bud. Buds are formed to protect and grow the leaf or flower it will become. Some plants have buds that will be food, such as broccoli and cabbage.
Roots are the lowest portion of a plant. They do not have nodes, leaves or flowers. This is the plant's anchor into the soil, and a means to take in moisture and nutrients from the ground. It can be used as food (turnips) or as propagation means.
Flowers are made up of male pollen with/without a female ovule, petals, nectar glands and sepals. It is a reproductive part of the plant and functions solely for reproduction of the plant. It can be brightly colored, ornamental and/or fragrant.
Fruits are sections of ovules (seeds) and the ovary wall. Seeds can be internal (apple, for example) or external (like strawberries). Fruits are designed for food or for reproduction of the plant.
Seeds are matured ovules and are the embryo (tiny plant), endosperm (protein, fat, or carbohydrates), and seed coat (outer coating). Seeds are for the reproduction of the plant and designed to fall to the ground and mature into another plant. They can also be used as food -- sunflower seeds, for example.