It is a well-known fact that many kinds of non-woody as well as many woody plants develop hardiness or cold resistance on exposure to very gradually falling temperatures. This change, in the case of non-woody plants such as cabbage or wheat, is spoken of as "hardening off." It is not known how important this is in developing cold resistance in flower and leaf buds of woody plants. It is quite possible that buds that have become extremely tender as a result of rapid growth might, if held for some time at temperatures too low for further growth, become quite resistant to low temperatures just as do wheat or cabbage. Generally speaking, the greatest amount of cold injury to the buds or above-ground portions of a tree occurs on a single night. The length of the cold period is of only indirect importance as influencing the rate of temperature fall or the acquiring of cold resistance by the trees. Trees that are subjected to low temperatures over a considerable period of time are not nearly so likely to be injured as are those that are subjected to a low temperature suddenly. That is really why there is so much severe cold injury to woody plants in the South. In the deep South freezing weather may be uncommon but when freezes do occur usually they follow a period of comparatively warm weather and the temperature falls quickly. It is this sudden change in temperature that causes the severe injury. Two different places may have had the same mean monthly temperature yet at one place severe injury may have occurred and no injury at the other place with plants normally having equal hardiness. A careful analysis of the situation, however, would probably show that at the place where the injury occurred a period of warm weather had existed which was followed by a rapid drop in temperature to a killing low on a single night, whereas the trees at the place where no injury occurred were not subjected to such changes in temperature. On the other hand, injury to the roots usually occurs only after prolonged periods of cold weather. This is largely because the soil cools slowly and it requires a long period of cold weather to reduce the soil temperature sufficiently and to such depths as to cause injury to the roots.