After the rest in trees has become deep or profound a certain amount of chilling temperature must prevail before the rest period is broken so as to permit the buds to open and grow normally on the approach of warm weather. This is often spoken of as the chilling requirement. If the rest period is not broken by a suitable amount of chilling, tree growth is very slow to start in the spring, and then only certain of the longer and stronger twigs may force into growth; water sprouts may develop on the trunks and main limbs; flower buds may not open but fall off; and even though the trees may flower the flowering period is long and few or no fruits or nuts may be set. The most effective chilling temperature is not known but we can be reasonably certain that temperatures of 45 deg.F. to 32 deg.F. are just as effective in breaking the winter rest period as are those well below freezing, if not more so. This chilling requirement is essentially the same as the rest period. Almonds have a short rest period and require 2 to 3 weeks of chilling, while butternuts, with a long rest period, may require 3 or 4 months. When the tree has been subjected to adequate chilling the rest period is broken and with the oncoming of warm weather growth, blossoming and fruit setting is normal. A distinction of great importance from a physiological and a practical point of view is made between rest and dormancy in plants. This difference can be simply stated: plants, trees, or seeds that will not grow when external environmental conditions are favorable for growth are in rest, but after the rest period has been broken and they do not grow because of unfavorable conditions they are said to be dormant.