Flower

Clockwise from top left: Magnolia grandiflora, Phalaenopsis bellina, Passiflora caerulea, Protea cynaroides, Zea mays and Helianthus annuus.

Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants (angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, modified leaves; corolla, the petals; androecium, the male reproductive unit consisting of stamens and pollen; and gynoecium, the female part, containing style and stigma, which receives the pollen, and ovary, which contains the ovules. When flowers are arranged in groups, they are known collectively as inflorescences.

Floral growth originates at stem tips and is controlled by the presence of MADS-box genes. Simple models are used to describe this development. In most plant species flowers are heterosporous, and so can produce sex cells of both sexes. Pollination mediates the transport of pollen to the ovules in the ovaries, to facilitate sexual reproduction. It can occur between different plants, as in cross-pollination, or between flowers on the same plant—or even the same flower, as in self-pollination. Vectors transport the pollen between stamen and stigma. They may be living animals, such as birds and insects, or non-living factors such as wind and water.

After pollination, fertilisation occurs. Both the nuclei and the cells (excluding the cell walls) are fused together in this process. The result is a cell called a zygote, which has two copies of each chromosome. Through cellular and nuclear division, the zygote grows into a seed, which contains structures to assist in the future plants survival and success. At the same time, the ovary forms into a fruit, and the other floral structures die. The function of fruit is to protect the seed and aid in dispersal. This dispersal is divided into vectors originating either from external sources, or from the plant itself. External vectors include both living things, such as animals and insects, and non-living things, such as wind and water.

Flowers evolved between 150 and 190 million years ago, during the later part of the Jurassic era and early Cretaceous. As a subgroup of seed plants, angiosperms used the flower to outcompete other members, as a result of greater efficiency. The colour of flowers assists in pollination. In plant taxonomy, which is the study of plant classification, flowers are a key tool used to differentiate plants. For thousands of years humans have used flowers for a variety of purposes including: decoration, medicine, food, and perfumes. In human cultures, flowers are used symbolically and feature in art, literature, religious practices, ritual, and festivals.