Well can it? This came up in our last adventure. My thought was no it cant since it can only fire rays in certain arcs on a level plane but it really isnt clear.
I think that this is addressed in the FAQ, and the answer was "no". Beholders always float right-side up so they cannot bring all of their eyestalks to bear on a target below them.
If you glance back there has been a thread on this issue just within the last couple of weeks, including a response from the Sage saying "No". Worth checking for if you want all the gory details.