I would guess some people would call them Aluminum tubes. The true term is hydraulic lines, and there is a 90% chance a "O" ring(s) is the culprit that is leaking. There's always a chance the hydraulic line got bent somehow and formed a crack. First thing to do is clean all grass clippings and any dust/dirt under the tractor and inspect where the leak is.
I think there is about 13 or 14 "o" rings on the 345 and in various sizes.