Here are three plants from my collection:
Plant number 1
Plant number 2
Plant number 3
Which are cacti?
A little more information. At least one is a cactus.
The first plant grows in warm semi arid regions in South America. Some members of this group of plants get to be mid sized trees but most are shrubby.
The second plant has reduced leaves that function as thorns and it is found in deserts in the SW United States and Mexico.
The third plant lives in the jungles of Southern Mexico and into Central America.
All three plants have very similar looking and distinctive complex flowers, all organized in the same basic way.
So which are cacti? How do you know?