What is a Cactus or Succulent? ... 2/2

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ALL Cacti are Succulents BUT NOT all succulents are cacti

The term "Cacti and other succulents" is often used and is similar to saying "Sheep and other farm animals".

Succulents are found mainly in the following: Agavaceae, Aizoaceae (mesembs), Aloaceae, Asclepiadaceae (stapeliads), Cactaceae, Crassulaceae, Euphorbiaceae and Portulacaceae. See succulents for a complete listing of succulent families and genera.

All Cacti are classified as succulent plants, since they (all) exhibit (usually) stem (and sometimes root) succulence. The very primitive cacti have leaves that are not usually succulent - a link in evolution.
Not all succulents are Cacti, however, because they do not all belong to the Cactaceae family. Similarly not all farm animals are sheep.

The cactacae family is exclusive to the new world (Americas) and exclusively have areoles - special organs from which spines, flowers, new branches originate. Cacti comprise the single largest group of succulent plants

Succulent species are present in most plant families and spread throughout the world, and has evolved many times independently in the families.

Among the plants usually referred to as 'The Other Succulents' are Euphorbias, a large, diverse group from the family Euphorbiaceae. Euphorbias are sometimes mistaken for Cacti, and similarities do exist amongst the two groups. Cacti, however, are 'New World' or from the Americas, whereas Euphorbias are found primarily in Africa or the 'Old World' and are not related to each other. One way to differentiate is Euphorbias contain a toxic thick white sap (don't get it in your eyes or mouth) whereas most cacti (exception - mammillaria) don't contain any white sap. The Euphorbia genus is also represented by, amongst others, Pointsettas.

So there are approximately 10,000 separate species of plants, in nearly all plant families, that fall into the succulent category ... but they are not all Cacti - just 1600 of them !