Why are plates incubated agar side uppermost?
If you are referring to why we incubate agar plates upside-down, it is all due to gravity. When agar is made, it is a hot liquid. At this time, nutrients, chemicals, etc. can be added before the agar sets up into its gelatin-like consistency in the plate. When we plate organisms, we spread them on the surface of the agar, whereas the nutrients, etc. are mixed throughout the agar. If we incubate the plates right-side up, over time gravity will pull the nutrients to the bottom of the agar, away from the organisms on the surface. By keeping the plates upside-down, we ensure that the organisms can get to the nutrients they need. In essence, gravity will pull the nutrients TOWARDS the organisms instead of away from them.
