I am very frequently asked what the difference between cacti and succulents is. The short answer is that all cacti are succulents but not all succulents are cacti. The long answer is somewhat more complicated. People are often shocked when I tell them that African succulents are completely unrelated to cacti even if they look similar. Aloes and Haworthia,for instance, are monocot plants and are more closely related to lilies and grasses than they are to dicot cacti. A good analogy is to think of the Arid environments of the world like oceans. In the ocean both fish and dolphins have evolved body plans that look superficially similar, but dolphins are mammals more closely related to humans than to fish. Similarly, many unrelated plants found in the desert look similar because they have evolved to survive in similar habitats. This is known as convergent evolution.
Euphorbia obesa (right) looks similar to cacti like Trichocereus pachanoi (left) but is completely unrelated
