Student pilot with instructor on left base experiences NMAC with aircraft on left base for the same runway. Tower was in contact with both aircraft but traffic was not called.

Narrative

I had just completed a go around for Runway 32L and had requested a full stop landing on the parallel Runway 32R. My aircraft entered a left downwind leg for Runway 32L and was preparing to turn a left base leg for Runway 32R. I started to turn on the base leg; but my Instructor corrected me and had me continue on the downwind. On an extended left base leg for Runway 32L was another aircraft; coming in from the practice area. The problem is; both aircraft were in contact with the Control Tower and neither aircraft was informed that the other was in the same area. As a result; the aircraft already on base passed directly under my aircraft within roughly 400 FT. The aircraft that was higher up (mine) proceeded to abruptly climb another 200 FT to avoid any possible accident. The Control Tower indicated only that it appeared my aircraft was turning base (which had happened but was corrected); which in terms of traffic updates shouldn't matter. Regardless of the fact that my aircraft was above the other aircraft; the Control Tower should have informed us of where the traffic was located so we could have found them before passing directly over them.