Abstract: According to the inhibition extent of oxygon released from leaves which are in artificial solution wite a series of low water potentials, the paper proposed a method for measuring the relative drought hardiness of plants. It has been found out that there is a common law for the changes in drought hardiness of evergreen trees in process of hibernation, which is a tendency with "M", i. e. drought hardiness using at the stage of acclimation to cold in the primary period of hibernation, falling drown at severe cold stage, rising again at the stage of air temperature going up, and falling down again at the beginning of spring. With the method incroduced in this paper, the changes in drought hardiness of various tree species or one tree in the process of growth can be compared vertically and horizontally, which provides the bases for the study on environmental physiology and the disposition of various tree species in planlation. Some corresponding water running principles are proposed from the law summarized in this paper.

Key words: Evergreen tree, Oxygen released from photosynthesis, Drought hardiness