The usual reproductive part of a plant is the flower. Flowers contain the reproductive organs of the plant, including the male organs called stamens, which produce pollen, and the female organ, called the pistil, which contains the ovary where seeds are formed. Flowers are responsible for the process of pollination, where pollen is transferred from the stamens to the pistil, allowing for fertilization and the production of seeds. This is why flowers are often brightly colored and fragrant, to attract pollinators such as bees, butterflies, and birds. The other options - leaf, stem, and root - are important parts of a plant, but they are not directly involved in reproduction.