There is hardly any soil or climatic condition found in the world where it is not possible for at least one or more kinds of plants to be grown. This is possible because the plants that can be grown under the most adverse conditions have special structures and adaptations with regard to periods of growth and rest or dormancy. One of the most important adaptations of nearly all trees and shrubs that shed their leaves in autumn and survive freezing weather without injury for a part of the year, is that of rest. This rest in plants is somewhat similar to sleep in animals in that it is a period in which the life process activities take place slowly. In other words, the plant physiologist defines rest in living plants as that period in which their buds will not open and grow even though the temperature, moisture, and other external environmental conditions are highly suitable for growth. [Footnote 1: Division of Fruit and Vegetable Crops and Disease, Bureau of Plant Industry, Soils, and Agricultural Engineering, Agricultural Research Administration, U. S. Department of Agriculture.] Different kinds of deciduous plants have or require rest periods of different lengths, just as some people require more sleep than do others. Two or three weeks may be enough for soft-shelled almonds but three or four months may be required for butternuts, to cite extremes. The Eastern black walnut requires more rest than most Persian walnut clones, and they more than the Southern California black walnut. Even within a species there is considerable difference in the rest period of individual seedling trees and certain clones. For example, it has been found that the varieties of Persian walnut grown in northern California and in Oregon, such as Franquette and Mayette, have the longest rest period; and those grown in Southern California, such as Placentia, Ehrhardt, Chase, and others, have the shortest rest period. It is quite possible that the clones and seedlings of the Persian walnut brought to this country a few years ago by the Rev. Paul Crath from the Carpathian Mountains of Poland may require the longest rest period of all. The question may be asked what causes or brings on this rest period in plants and what breaks it? The scientific answers to these questions are not known at this time, but we do know some of the factors which cause the initiation of rest and how it is broken.