There is a very great difference between the temperature that will cause injury to a tree tissue when it is in active growth and most tender in the spring and that required when it is most resistant in midwinter. With some trees this difference in temperature is as much as 50 deg. to 60 deg.F. or even more. With woody plants, the tissues are least hardy in spring when they are growing rapidly, and as the season progresses hardiness normally increases provided that second or late growth does not occur. There are many changes that take place in the tissues of a tree as hardiness is developed: the moisture content is reduced; cell walls are thickened; the concentration of sugars, starches, and other carbohydrates becomes greater; there is the formation of pentosans, gums, and waxes; and the respiration and other life processes become slower. However, none of these offer a full and satisfactory explanation of why the plant becomes as resistant to cold as it does. All of these changes and probably many others play a part in developing hardiness in woody plants. Maximum hardiness is developed only by trees that support a large area of normal leaves continuously from the time of foliation in the spring until late fall when they are killed by frost. Attacks by insects or diseases that injure the leave or cause partial or complete defoliation at any time during the spring, summer, or before the occurrence of frost in the fall, not only prevent the development of maximum hardiness of the trees, but such defoliation results in reduced growth of the trees and in poor filling of the nuts. The importance of maintaining a large area of healthy leaves on the trees during the entire growing season can hardly be too strongly stressed. This is because trees that hold their leaves are strong, vigorous trees and are the ones best able to withstand cold, as well as other adversities, without injury. This, however, does not mean that fertilizer applications should be made in late summer or that cultivation should be practiced at that time, which would tend under suitable conditions to stimulate late growth of the trees. This is because some trees like the Persian walnut are slow to go into rest at best and practices that stimulate late growth of the trees cause them to be susceptible to cold injury especially in late fall or early winter. I have seen very severe injury and killing of pecan trees in south Georgia as a result of spring fertilizer applications which, because of drouth, did not become available to the trees until late August and early September and then caused second growth of the trees.