Cactus at the Theosophical Society

Spiny, succulent with pretty red flowers. This one is probably of the Subfamily Opuntioideae.
Did you know that all cacti are succulent but not all succulent plants are cacti? I also found out just today that the spiky things on cacti are not "thorns", they are "spines". Whats the difference? Thorns are modified branches - they grow off branches. Seen the ones on a lemon tree? - thorns. Spines on the other hand are modified leaves and they grow off leaves. Thorns are made of the same material as the branch they grow off but spines are made of a different material than leaves. The spines on a cactus are made of calcium carbonate and pectin (unlike the leaf which is full of water and chlorophyll). And the purpose of the spine on a cactus is to protect the leaf from the scorching rays of the sun! Yes, really! It thus reduces the loss of moisture from the leaf. A revelation isnt it? I always thought they were to protect the plant from predators.