I can't say for sure. It looks a lot like what is called a Christmas Tree cactus, but it also has the characteristics of a euphorbia. If the stem is very green and looks like a catus and the leaves mostly on top...then it is the catus. If the stem is more woody it may be a type of euphorbia. The crown of thorns is a type of euphorbia: http://www.google.com/search?q=crow ... visions_inline&ct=broad-revision I have both. Both usually make branches. The Christmas Tree Cactus sometimes it just shoots up like that one is doing. http://www.cactusshop.com/library/ImagePopup.aspx?id=1726 That is what they look like when they branch. I have two I took off a branching one to start new ones and they look much like yours right now...going up with leaves on top. Note on the site that leaves are the "blooms". The euphorbia usually have colored flowers and the large stem is more woody. You could take it to a nursery and they should be able to identify it for you.
It looks like my crown of thorns. If so it will have beautiful red flowers that are very small. Mine was that size when I first got it. Now it stands 4 ft tall and at least 2 ft wide.
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