Early dormancy involves a number of phenomena: cessation of active growth, formation of terminal buds, formation of abscission layers in leaves, development of cold resistance, development of winter rest (a chilling requirement), and leaf fall. The buds of some tree species (or some races of a species) have a true rest or winter chilling requirement; usually the roots do not; perhaps the cambium does not. There is a wide range of genetic variation within and between species in response to photoperiod, winter chilling, water nutrients, and other environmental factors which affect the dormancy condition of plants. During the period of winter rest there are many metabolic and developmental processes going on in the buds and twigs. These processes include respiration, photosynthesis, some cell division, enzyme synthesis, production of growth stimulators, and dissipation of growth inhibitors. Dormancy of trees can be divided arbitrarily into three phases: early rest, winter rest, and after-rest. Each of these phases is marked by a distinct set of physiological processes. The transition between the three phases is gradual. Some processes of after-rest can proceed even though the winter chilling requirement (winter rest) of a tree has not been completed fully. Evidence from grafting experiments and chemical treatments to break winter rest, and studies of genetic variability indicate that the processes and phenomena of dormancy are at least partially independent of each other. Different buds and branches and other parts of the same plant may initiate dormancy, break dormancy, and renew vegetative growth independently. Initiation and cessation of dormancy can be triggered by a number of environmental variables: photoperiod, temperature, nutrition, water, an array of chemicals, and shock treatments. Dormancy regulation must either involve a variety of receptors or involve receptors that are responsive to a variety of stimuli. Unless dormancy is defined in a highly restricted sense (that is, possession of chilling requirement), it is hard to conceive of a single receptor or regulator that controls all of the phenomena of dormancy. A large number of genes are definitely involved and hence a large number of enzymes. The kinds of enzymes, their numbers, and their concentrations can be regulated by manipulating the environment.