Ca ions are an essential component of plant cell walls and cell membranes, and are used as cations to balance organic anions in the plant vacuole.  The Ca concentration of the vacuole may reach millimolar levels.  The most striking use of Ca ions as a structural element in algae occurs in the marine coccolithophores, which use Ca to form the calcium carbonate plates, with which they are covered.
Calcium is needed to form the pectin in the middle lamella of newly formed cells.
Calcium is needed to stabilize the permeability of cell membranes. Without calcium, the cell walls are unable to stabilize and hold their contents. This is particularly important in developing fruits. Without calcium, the cell walls are weak and unable to hold the contents of the fruit.
Some plants accumulate Ca in their tissues, thus making them more firm. Calcium is stored as Ca-oxalate crystals in plastids.