Melocactus maxonii (to Melocactus curvispinus) is found widely in the northern New World tropics including Mexico, Colombia, Venezuela, and the Caribbean.

The Melocactus genus is found widely in the New World. This genus can be found from Mexico south to southern Peru, and south through Colombia and Venzuela to eastern Brazil. This genus is also found in the Caribbean. There are about thirty-three species and several hybrids in this genus. Melocactus intortus is a widespread Caribbean species and appears to occur in greater North America.

Cereinae Group (Subtribe Cereinae) contains some large and giant plants.  This group has been divided into two tribes.  The Cereus and Melocactus and related genera have been moved to the Cereeae tribe.  The Bergerocactus, Carnegia, Cephalocereus, Echinocereus, Lemeaireocereus, and Pachycerus and related genera have been moved into the Pachycereea tribe.

Cactus Family (Cactaceae) is found in the New World only except where introduced. It reaches its greatest development in the tropical deserts of the New World with large numbers of species found in Mexico, Chile, and Argentina. There are probably less than 2,000 species in the family. (In the Anderson book, he describes 1,810 species.) To facilitate study of this fascinating family, it is usually broken into various subfamilies and tribes. Kartesz lists 203 species as growing in greater North America.

Opuntiales Order contains only the Cactus family. Because a large number of amateur hobbyists have collected, named, and grown these species over the years, the taxonomy of the group is difficult to unravel. However, Anderson's 2001 book on the cactus family unravels many of these problems. In this product, the older names have been used in many cases, but the individual species text notes the newer preferred names.

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.