Monvillea spegazzinii, Weberocereus tunilla and Pterocereus foetidus, among
others, have square stems and they are genuine cactus. There are many
Cactaceae with square (quadrangular) stems.

Euphorbia horrida does not have a square stem.

Many Euphorbias have spines on specialized spine shields not arising
directly from the stem.

All Cactaceae have areoles (axillary buds filled with hairs not scales) but
so do Didiereaceae and Portulaca!

Please provide link to picture again.

..
Iris Cohen wrote in message
...
It is not a cactus at all. It is a Euphorbia, possibly Euphorbia horrida.
Do a
search on Euphorbia & you will undoubtedly find it. Take a close look. The
thorns are growing directly out of the stem, not from an areole. That is
how
you tell it is not a cactus. Also, I don't know of any cactus with such a
square stem. There are several Euphorbias that do that.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)