Citrus is a common term and genus of flowering plants in the family Rutaceae, originating in tropical and subtropical southeast regions of the world. The most well known examples are the orange, the lemon, the grapefruit, and the lime. The Latin word citrus was borrowed from ancient Greek kedros "cedar, juniper" probably through Etruscan. The Romans applied the word to several different trees with fragrant foliage or wood (compare the completely unrelated cedars). The taxonomy and systematics of the genus are complex and the precise number of natural species is unclear, as many of the named species are clonally propagated hybrids, and there is genetic evidence that even some wild, true-breeding species are of hybrid origin. Cultivated Citrus may be derived from as few as four ancestral species. Natural and cultivated origin hybrids include commercially important fruit such as the oranges, grapefruit, lemons, some limes, and some tangerines. Research suggests that the closely related genus Fortunella (kumquats), and perhaps also Poncirus and the Australian Microcitrus and Eremocitrus, should be included in Citrus; most botanists now classify Microcitrus and Eremocitrus as part of the genus Citrus. Fiber, also spelled fibre, is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread. They are very important in the biology of both plants and animals, for holding tissues together. Human uses for fibers are diverse. They can be spun into filaments, string or rope, used as a component of composite materials, or matted into sheets to make products such as paper or felt. Fibers are often used in the manufacture of other materials. Synthetic fibers can be produced very cheaply and in large amounts compared to natural fibers, but natural fibers enjoy some benefits, such as comfort, over their man-made counterparts.