flower

Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants (angiosperms). Typically, they are structured in four circular levels around the end of a stalk. These include: modified leaves that support the flower; petals, often designed to attract pollinators; male parts, where pollen is presented; and female parts, where pollen is received and its movement is facilitated to the egg. When flowers are arranged in a group, they are known collectively as an inflorescence. Floral growth originates at stem tips and is controlled by specialised genes. In most plant species, flowers are able to 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 animals, such as birds and insects, or non-living factors such as wind and water. The colour and structure of flowers—such as nectaries and nectar guides—assist in pollination. After pollination, both sex cells (excluding their cell walls) and their nuclei are fused together in the process of fertilisation. 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 plant's survival and growth. 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 may be biological, like animals and insects, or abiotic, like wind and water. Flowers evolved between 150 and 190 million years ago, in the Jurassic. Angiosperms replaced non-flowering seed plants in many ecosystems, as a result of flowers' greater reproductive effectiveness. In plant taxonomy, which is the study of plant classification, flowers are a key feature used to differentiate plants. For thousands of years humans have used flowers for a variety of other purposes, including: decoration, medicine, food, and perfumes. In human cultures, flowers are used symbolically and feature in art, literature, religious practices, ritual, and festivals.