[from the text] Transitions from one sort of vegetation to another-tension zones they have been called-are places of unusual interest to botanists. For along these lines, if they be in fact under tension from the struggles of diverse plants for ascendency, are afforded our best opportunities for unravelling the complex of factors which condition the success of the types concerned. Sometimes such vegetation lines, like the zones around a pond, are evidently due to transitions in physical or climatic conditions, each type of vegetation being clearly restricted to the habitat suitable to its own growth. In other cases the cause of the change in vegetation is not so readily apparent. Some of these obscure transitions are due to less obvious physical factors which may be made out by study or experiment. Others, conceivably at least, might represent actually moving migration fronts in no way related to physical factors. It was an attempt to gain an understanding of one of these more obscure boundaries that led to the present paper.