This lifeform is found widely in Africa. This lifeform is found widely in the Indo-Australian region. This non-native lifeform is now locally established in greater North America. This lifeform is found widely in the New World tropics. This lifeform is grown commercially for human food.

Lemon (Citrus limon) is probably a hybrid of some sort of the species Citrus medica. The lemon is grown in most tropical countries of the world. It is used for flavoring beverages and is an excellent source of vitamin C.

Citrus genus (citrus fruits)  appears to have originated in southeast Asia and  various islands in the Pacific Ocean.  There are about 16 original species in this genus.  One characteristic of the fruits is their multiple chambers filled with seeds and liquids.  Several of these species have been domesticated and developed into an enormous number of hybrids and subspecies that produce a wide variety of fruits including oranges, grapefruits, limes, tangerines, and lemons.  Eight of the original species and two hybrids are established in greater North America.

Rue Family (Rutaceae) has worldwide tropical locations with many species found in South Africa, Indonesia, and Australia. There are about 1,700 species. The family is of considerable economic importance because it contains the various citrus fruits, including the orange, lemon, lime, and grapefruit. There are 104 species in 23 genera now growing in greater North America.

Geranium Order (Geraniales) is a large order composed of many  different families. Included here are the Geranium, Oxalis, and Citrus groups.

Dicots (Dicotyledoneae Class) are the predominant group of vascular plants on earth. With the exception of the grasses (Monocots) and the Conifers (Gymnosperms), most of the larger plants that one encounters are  Dicots. Dicots are characterized by having a seed with two outer shell coverings. Some of the more primitive Dicots are the typical hardwood trees (oaks, birches, hickories, etc). The more advanced Dicots include many of the Composite Family flowers like the  Dandelion, Aster, Thistles, and Sunflowers. Although many Monocots reach a very high degree of specialization, most botanists feel that the Dicots represent the most advanced group of plants.

Seed plants (Phylum Embryophyta) are generally grouped into one large phylum containing three major classes: the Gymnosperms, the Monocots, and the Dicots. (Some scientists separate the Gymnosperms into a separate phylum and refer to the remaining plants as flowering plants or Angiospermae.)

For North American counts of the number of species in each genus and family, the primary reference has been John T. Kartesz, author of A Synonymized Checklist of the Vascular Flora of the United States, Canada, and Greenland (1994). The geographical scope of his lists include, as part of greater North America, Hawaii, Alaska, Greenland, Puerto Rico, and the Virgin Islands.

Kartesz lists 21,757 species of vascular plants comprising the ferns, gymnosperms and flowering plants as being found in greater North America (including Alaska, Hawaii, Greenland, Puerto Rico and the Virgin Islands).

There are estimates within the scientific world that about half of the listed North American seed plants were originally native with the balance being comprised of Eurasian and tropical plants that have become established.

Plant kingdom contains a large variety of different organisms including mosses, ferns, and seed plants. Most plants manufacture their energy from sunlight and water. Identification of many species is difficult in that most individual plants have characteristics that have variables based on soil moisture, soil chemistry, and sunlight.

Because of the difficulty in learning and identifying different plant groups, specialists have emerged that study only a limited group of plants. These specialists revise the taxonomy and give us detailed descriptions and ranges of the various species.  Their results are published in technical journals and written with highly specialized words that apply to a specific group.

On the other hand, there are the nature publishers. These people and companies undertake the challenging task of trying to provide easy to use pictures and descriptions to identify those species.