~James Joyce
There are 2000 different types of cacti categorized. There is no consensus on the correct way to classify them. Cacti vary greatly in appearance and behavior. It is common to refer to any succulent plant as a cactus, but that is not correct. While all cacti are succulent, not all succulent plants are cacti.
All the cacti belong to the Cactaceae plant family. There are certain characteristics used to group these plants. Features unique to cacti are fleshy stems that perform the functions of leaves, and the spines which are believed to be leaves modified to protect. The occurrence of photosynthesis in the stem, not leaves also differentiates cacti from other plants. Another defining characteristic of cacti is that they produce aereoles. The areolas can appear in different places on the cactus, depending on the specie. Aereoles are composed of two connected perpendicular buds from which new arms, flowers or spikes grow.
Cactus plants were classified amateurs and professionals alike until around 1984 when the International Cactaceae Systematics Group (ICSG) began to attempt a standardization of cacti classification. The group published a (now out-of-print) book: The New Cactus Lexicon, Volumes I and II: Descriptions and Illustrations of the Cactus Family (Major new handbook on cacti, looks set to become the standard reference for all naturalists and scientists with an interest in the Cactaceae), and as far as I can tell, has never been heard from again.
The process of classifying cacti remains an inexact science. Cacti are plentiful, varied, interesting and often beautiful to behold. That is not disputed.
Note: This much is established: Cactus is classified in the division Magnoliophyta, class Magnoliopsida, order Caryophyllales, family Cactaceae
Something else I learned: All cacti originated in the Americas except Rhipsalis baccifera which is native to Madascar.
Madagascar? Wonder how that came about. That would be an interesting tale, no doubt.
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