|MadSci Network: Microbiology|
An interesting question. A plant would not grow from this system as there are no viable seeds. I think some of your questions will also be answered as you take additional biology courses in coming years.
For now, think about what bread is. It is a mixture mostly consisting of flour and water that has been heated. It is high in nutrients (its a food after all). After baking, the moisture content is low on the crust making it inhospitible for most microorganisms to grow.
All living organisms need water to grow, even if other nutrients are available. Some molds can grow with very little water, so these tend to be the organisms we compete with for that food source. As long as there is enough water in the bread, the mold will continue to spread and consume the bread.
One way to keep the moisture level high enough is to store the bread in a container or bag that prevents it from drying out. However, a new limitation sets in if the container prevents gas exchange. Molds are aerobic, which means they need oxygen to grow. In a sealed system they could consume all the available oxygen and stop growing even if there is enough water.
The microbial population could shift from the aerobic mold to anaerobic bacteria. The mold will have produced more water in the system as a result of respiration. This will increase the moisture content in the closed system. As the oxygen is depleted by the mold, anaerobic bacteria can start to grow if there is enough moisture. So, eventually the bread could conceivably be converted completely into microbial biomass plus metabolic byproducts.
I hope that answers your question. It would be interesting to try it and see, but anaerobic bacteria are very stinky.
Try the links in the MadSci Library for more information on Microbiology.