At which end does polymerization of microtubules occur?
From my Campbell's Biology textbook (Ninth Edition):


  Because of the orientation of tubulin dimers, the two ends of a microtubule are slightly different. One end can accumulate or release tubulin dimers at a much higher rate than the other, thus growing and shrinking significantly during cellular activities. (This is called the "plus end", not because it can only add tubulin proteins but because it's the end where both "on" and "off" rates are much higher.)


So in summary: polymerization and depolymerization occur on both the + and - ends. The "+" and "-" merely designate the speed/efficiency of polymerization and depolymerization.