Calcium (Ca) plays a key role in cell formation, and in maintaining the cell's membranes and structure. Ca affects the permeability to cell membranes. Ca is an activator of many enzyme systems, such as calmodulins. In plants, Ca is not mobile nor translocated; its deficiency symptoms first appear on younger leaves and the growing point. For example, young leaves fail to unfurl in corn, and have dead spots (necrosis) in celery. The growing tips die in both plants.