Can I do serious injury to my hand like this ?

Just recently I bought a pair of boxing gloves. I don't have a punching bag so I've tied a piece of styrofoam wrapped in leather to a tree and am hitting that. I had only punched it for a few minutes when my knuckles started to bruise. The next day I went out and hit the pad on the tree for another few minutes and now my knuckles are bruised even more. Are my knuckles meant to bruis ? Will they just get tougher, like when your hands get callouses from doing a lot of digging, or will I end up seriously injuring my hands ? Is it alright to keep hitting this pad on the tree ? Any information about the affect this could have on my hand would be appreciated. I'm a curious guy, so be sure to incorporate why I should stop, or why it's alright for me to keep going in your answer. Thanks a bundle.