Flowering plants (angiosperms) have ovules and seeds completely enclosed within carpels. The term angiosperms comes from two Greek words angeion, meaning "vessel", and sperma, meaning "seed". The vessel is the carpels that become ovaries that become fruits. There is a phylum of flowering plants (Magnoliophyta) and it is divided into two classes which are dicotyledons (Magnoliopsida) and monocotyledons (Liliopsida). Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Flowering plants are believed to have evolved from a now-extinct gymnosperm ancestor. Most contemporary botanists talk about pteridosperms and that the first flowers had many separate, flattened parts arranged spirally over an extended receptacle. Flowering plants produce two types of spores which are heterospores. Its gametophytes develop into separate structures. The female gametophyte (megagametophyte) develops in the ovule. The seed coats, which later become the seed coat, surround the megagametophyte. Pollen grains developed in the anthers become male gametophytes. Pollination is the transfer of pollen grains from an anther to the stigma; it is caused by insects, wind and other agents. After pollination, a pollen tube can grow from a pollen grain to the female gametophyte; the nucleus of the tube remains at its tip and the generative cell divides producing two sperm nuclei. Following the discharge of the contents of the pollen tube into the female gametophyte, a sperm unites with the egg forming a zygote; the other sperm joins simultaneously with the two central cell nuclei, forming 3x endosperm. The nutritional tissue of the endosperm can become part of the seed or be absorbed by the seed embryo. Due to variations in how a female gametophyte develops, some flowering plants produce 5x, 9x, or 15x endosperm tissue. Some artificial groupings of flowers to aid identification do not reflect natural relationships. Sources of evidence used to try to group plants naturally include fossils, which suggest that flowering plants first appeared around 160 million years ago. Primitive flowering plants had simple leaves and numerous spirally arranged floral parts that were not fused together; they possessed both stamens and pistils and were radially symmetrical (regular). Specializations include a decrease in the number of parts, fusion of parts, appearance of compound pistils composed of several individual carpels, inferior ovaries, bilateral symmetry (irregular flowers), and unisexual flowers. Monoecious species have both male and female flowers on the same plants; dioecious species have male and female flowers on separate plants. Bee-pollinated flowers are delicately sweet and fragrant and tend to be blue or yellow. Flowers pollinated by beetles tend to have stronger odors and are usually white or dull in color. Some flowers pollinated by flies expel bad odors. Moth-pollinated flowers tend to be white or yellow. Bird-pollinated flowers are generally bright red or yellow and have lots of nectar but little odor. Most orchids produce pollen grains in pollen masses that adhere or attach to parts of visiting insects. The flowers of some orchid species have developed bizarre pollination mechanisms. Herbaria are basically collections of dried, pressed, or otherwise preserved plants, fungi, and algae, arranged so that specific specimens can be easily spotted. Properly maintained plants can last for hundreds of years. A plant to be pressed is placed.