A holding structure for holding a plurality of culture plates in a stacked arrangement includes a feature for de-nesting the bottom culture plate from the plate above it. The de-nesting feature is preferably a structure which urges a plate to move laterally relative to the plate above it. The de-nesting of the bottom culture plate from the adjacent culture plate facilitates withdrawal of the bottom culture plate from the base via a robotic plate transfer mechanism. The de-nesting feature can be entirely passive, such as ramps formed in the base of the holding structure.