Many deciduous trees enter a stage each year when their visible growth ceases. This is not always associated with cold weather or lack of water, and may occur with many species in mid-to-late summer. Trees entering this phase are said to be in rest. Rest is caused when internal factors are unfavorable for growth, while dormancy is defined as external factors being adverse for growth.

Studies were conducted under greenhouse conditions to investigate the relative efficiency of urea absorption by 1-month-old peach and apple leaves. A 4 percent solution of urea containing .1 percent Colloidal X-77 was applied to the leaves in the form of a fine spray. To aid in this procedure, an improved micros prayer with a l milliliter capacity was developed. Accuracy of the sprayer was ± l percent. Under greenhouse conditions, the upper and lower surface of peach and apple leaves absorbed urea. More urea was absorbed through the lower than the upper surface. Peach lower surface absorbed nearly as much as apples after 48 hours. In another experiment using a controlled environmental growth chamber, the effect of temperature, humidity and surfactant (Colloidal X-77) on absorption of 1 percent 14C urea solution by apple and peach leaves were studied. Uptake was again much greater from the lower surface of the leaves as compared to the upper surface. Low relative humidity (25 percent) reduced absorption substantially. High temperature (24 centigrade) under low humidity (25 percent) decreased absorption. Uptake was increased substantially with the high temperature (24 centigrade) and relative humidity (85 percent). Peach leaves were more sensitive to temperature than apple, in regard to the amount of absorption that occurred. In peach, a 5 to 10 fold decrease in absorption was observed when the temperature was lowered from 24 to 10 centigrade. Surfactant increased absorption through the lower surface within a short period after application but decreased it afterwards. Urea absorption through 45-day-old leaves at 85 percent relative humidity and 24 centigrade indicated that within 48 hours over 90 percent of the urea applied to lower surfaces was absorbed by both species of leaves. A cuticular permeability experiment indicated that upper cuticles from both species of leaves were permeable to urea. It seemed that permeability of peach cuticle increased with time at the higher temperature. After 48 hours, the amount of urea, which penetrated through the peach cuticle at 24 centigrade, was 2.7 fold as much as at 10 centigrade. Urea absorption within 1 hour and translocation after 4 hours were observed under favorable conditions (24 centigrade and 85 percent relative humidity). Radioautograms of 14C urea treated apple and peach leaves indicated that the 14C urea and/ or its metabolites had been translocated within a large portion of the leaf within 8 hours after application. Studies were also performed on these species utilizing microradioautography and histochemistry techniques. Microradioautograms prepared from treated leaf sections demonstrated that adsorption and absorption of radioactive urea occurred on the epidermal hairs of apple leaves. Urea entry occurred in both apple and peach leaves as evidenced by high activity of 14C compounds within the leaf tissue. Microscopic observations of freshly sectioned leaves of both apple and peach demonstrated a relatively high amount of pectinaceous substances between the cell walls and especially the bundle sheath and bundle-sheath extension cells. Pectinaceous substances were present more in apple cuticle than in peach cuticle.