Tree growth is initiated in the spring with coming of warm weather and other suitable conditions. At first the rate of growth is slow; but the rate increases and goes through a maximum and then slows up again and finally ceases. On the cessation of growth in length, a terminal bud is formed and the tree begins to go into rest. This period of growth is determined by the age of the tree, the suitability of moisture and nutrient supply. Young trees grow longer during the spring and summer than do old ones. Deficiencies of soil moisture or nutrients or both cause the cessation of growth and the beginning of rest. In some trees, such as tung, cessation of growth and the initiation of rest is caused by the change from long to short day-lengths. After rest has begun, the longer it continues the more profound or deeper it becomes until a maximum is reached, i.e., it becomes increasingly difficult, up to a certain time, to make the trees start growth again even though optimum conditions are provided. Some trees such as Persian walnuts and pecans, for example, are slow to go into deep or profound rest in late summer or fall. For this reason, there may be several cycles or periods of growth during the summer and early fall, depending on weather conditions and whether the leaves on the trees have remained in a healthy condition. Under conditions of dry weather growth stops on the Persian walnut and pecan and when this is followed by a rainy period and warm weather growth begins again. In fact in early summer a walnut or pecan tree may form terminal buds on all the shoots and remain without growth long enough for an apple or pear tree to go into complete or profound rest; then later, new shoot growth may be made from all or nearly all of the walnut or pecan shoots. Not only is this an important factor in promoting susceptibility to cold injury but in the case of bearing trees more often than not this late growth prevents the proper development of the kernels in the nuts and they are poorly filled or shriveled at harvest. Should the leaves of these trees in midsummer or later be so seriously damaged by disease or insects as to result in partial or complete defoliation, new growth is generally sure to follow even in late fall if growing conditions are suitable. This habit permits such trees to grow so late that there is much greater danger of severe injury from late fall or early winter than is the case with most other deciduous fruit trees. Furthermore, it explains why we see so much cold injury in the shoots and limbs of trees; they had grown late and had no chance to develop hardiness before killing temperatures occurred.