Flowering plants (angiosperms) have ovules and seeds completely enclosed within carpels. The term angiosperms is derived from two Greek words angeion, meaning “vessel”, and sperma, meaning “seed”. The vessel is the carpels that become ovaries that become fruits. There is one phylum of flowering plants (Magnoliophyta) and it is divided into two classes which are dicots (Magnoliopsida) and monocots (Liliopsida).
Flowering plants are believed to have evolved from now-extinct gymnosperm ancestor. Most contemporary botanists from pteridosperms and that the first flowers had many separate, flattened parts spirally ordered on an extended receptacle. Flowering plants produce two kinds of spores which are heterosporous. Its gametophytes develop in separate structures. The female gametophyte (megagametophyte) develops in the ovule. Integuments, which later become a seed coat, surround the megagametophyte. Pollen grains developed in anthers become male gametophytes. Pollination is the transfer of pollen grains from an anther to stigma; it is brought about by insects, wind, and other agents. After pollination, a pollen tube may grow from a pollen grain to the female gametophyte; the tube nucleus remains at its tip, and the generative cell divides, producing two sperm nuclei. Following the contents discharge of the pollen tube into the female gametophyte, a sperm unites with the egg, forming a zygote; the other sperm simultaneously unites with the two central cell nuclei, forming 3x endosperm.
The nutritive endosperm tissue may become part of the seed or absorbed by the seed’s embryo. Due to variations in how a female gametophyte develops, some flowering plants produce 5x, 9x, or 15x endosperm tissue. Some artificial grouping of flowers to assist identification does not reflect natural relationships. Sources of evidence used to try to group plants naturally include fossils, which suggest the flowering plants first appeared about 160 million years ago. Primitive flowering plants had simple leaves and numerous, spirally arranged flower parts that were not fused to each other; 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 in color. Beetle-pollinated flowers tend to have stronger odors and are usually white or dull in color. Some fly-pollinated flowers expel foul odors. Moth-pollinated flowers tend to be white or yellow. Bird-pollinated flowers are usually bright red or yellow and have much nectar but little odor. Most orchids produce pollen grains in pollinia that adhere or clamp onto parts of visiting insects. The flowers of some orchid species have developed bizarre pollination mechanisms. Herbaria are fundamentally collection of dried, pressed, or otherwise preserved plants, fungi, and algae arranged so that specific specimens may be readily located. Properly preserved plants may last for hundreds of years. A plant to be pressed is placed in a plant press between sheets of newspaper and absorbent material. Dry specimens are mounted to sheets of high-quality paper, with a label giving collection information.
Plant parts may be pressed for art work, place mats, and so on. Flowers may be preserved three-dimensionally. Because so many plants are now on rare and endangered species lists, collectors should try to limit their future collecting to photographs as much as possible.