This pattern is thought to re- flect the inefficiency with which these nutrients are mobilized from older tissue to young tissue. This notion is supported by evidence that these metals move inefficiently in phloem (Bu- kovac and Wittwer 1957). However, analysis of deficiency symptoms following nutrient withdrawal and the approach used in this paper (analysis of nutrient mobilization from se- nescent leaves under conditions where nutrients are not lim- iting) represent fundamentally different experiments.