I have a VERY tall growing cactus. It has 4 to 3 sides with VERY hard and sharp neeedles that grow in small clusters on the ridges of each edge, some on the oldest part of the original plant are about an inch long. The thorns do not come off as easy as many cacti that stick and remain in your skin. The only time I've known of one coming off was when a friend swung his arm backwards into the plant. It stuck into his elbow and most has remained. It still troubles him from time to time.

I've had it about 15 years, when it was about 2 feet tall. I have about 10 ft. ceilings and have needed to top it about 4 times.The original base is about 4 inches wide.

It grows completely vertically. When I top it, several new growths appear from just below the cut, and they continue growing straight up.

It reminds me of a desert cactus, but isn't so full of spines and desert cacti seems to grow out, away from the center "trunk" and then shoot shoot up. This one doesn't grow horizontally at all. Right now the tallest shoot is of my first topping of the original. I'm 6'1 and this has grown taller than me!

It starts easily just by sticking it into a sandy soil mixture. I've never seen any flowers on it at all.

Does anyone have any idea of what kind of cactus this is.

Any suggestions/advice will be appreciated.