Plants, in addition to their role as primary synthesizers of organic com- pounds, have evolved as selective accumulators of inorganic nutrients from the earth's crust. This ability to mine the physical environment is restricted to green plants and some microorganisms, other life forms being direct1y or indirect1y dependent on this process for their supply of mineral nutrients. The initial accumulation of ions by plants is of ten spatially separated from the photosynthetic parts, necessitating the transport to these parts of the inorganic solutes thus acquired. The requirement for energy-rich...