Multiple genetic and environmental factors work together to determine an individual plant's hardiness, but it sounds like you are more concerned with what takes place on a cellular level. Here are three definitions I'm getting ready to add to my own plant related dictionary. I hope they are accurate and will give you a starting point to understanding these primary mechanisms plants use, on a cellular level, to deal with freezing temperatures. (If anyone sees problems or errors, please let me know.) Of the three processes defined here, supercooling is thought to be the primary contributor to plants' ability to withstand extreme temperatures.

Freezing-induced cell dehydration - the process where cellular moisture is reduced when water is frozen in areas between a plant's cells. Ice can form outside cell walls first because of the greater number of freezing nuclei present there and the phenomenon of supercooling. The lower vapor pressure of the intercellular ice draws moisture, with a higher vapor pressure, from inside the cells. Plants utilizing this method must be able to tolerate ice within intercellular spaces and withstand the stress of the resulting cellular dehydration. This process works in conjunction with freezing point depression of the cellular solution. Also see freezing point depression of cellular solutions and supercooling.

Freezing point depression of cellular solutions - reduction of the temperature at which cellular fluid freezes by the presence of certain solutes in the cellular fluid. When water content is reduced by freezing-induced cell dehydration, the higher solute concentration present inside the cells decrease the temperature at which freezing will occur. Also see freezing-induced cell dehydration.

Supercooling - a process where cellular liquid remains in a liquid state at temperatures below its normal freezing point because of the lack of a seed crystal or nucleus around which a crystal structure can form. "Deep supercooling" is believed to be instrumental in keeping plant cells from freezing down to, or below, temperatures of -20C. The supercooling process is also utilized to initiate freezing-induced cell dehydration by allowing water inside cells to remain liquid while intercellular water freezes. Also see freezing-induced cell dehydration.

Plants also use systemic measures to avoid tissue death. I might come back and write more on that later.

ok,thanks.
i kinda knew that they couldent let the ice crytals burst the cell walls,just that i dident know how.