Plants of this group are highly adapted to
survival in dry
environments. They are so
advanced that for the most part they look nothing like most other
plants. Cactus spines evolved for
protection, and possibly also as
shade for the plants in some cases. The spines are not
leaves; in fact, there are some '
primitive' varieties of cactus which still have small leaves wedged between the spines. Alternatively, in
Africa and other areas which lack cactus, there are other plants which have evolved to look very much like
cactus.
Many of these, some from the diverse Euphorbiaceae family, have the same form as a cactus, right down to the ribs, the cylindrical shape, and the spines. However, they are genetically unrelated to cactus. This is a case of convergent evolution. One way to tell a cactus from a non-cactus is to look where the spines connect to the plant. A cactus will usually have a small pad there, which the spines are attatched to.
Many succulents which have cactus-like forms are mistaken as cactus. The 'living rock' (Lithops) or 'baby toes' type succulents are not actually related to cactus, although phylogenically they are similar. All cacti are succulents; however not all succulents are cacti