In the cells of most plants, crystals consisting of calcium oxalate are found which are formed by a process called biomineralisation. It is thought that these crystals act as a storage reservoir for calcium and also provide for the storage of toxic waste (detoxification). The crystals come in a variety of shapes and some are characteristic of certain plant groups.
Stomata consist of specialised cells that mainly occur on the underside of leaves. They regulate the gas exchange between the plant and it’s environment, the plant is 'breathing' through them, as it were. Stomata are very recognizable by the two bean-shaped guard cells that regulate the size of the opening.