A′era′tion (ā′ēr-a′shǔn) (in plants). Plants, like animals, respire (see Respiration); therefore, air (oxygen) must reach all the living cells, and carbon dioxid must be got rid of. Green plants also need to absorb carbon dioxid and to get rid of oxygen in the process of food-making (see Photosynthesis). To permit these gaseous exchanges in the larger plants the cells partly separate as they mature, leaving irregular passages, which usually open to the outside by numerous slits, each bounded by two guard cells and called stomata. The air does not flow in mass through these orifices and passages, but the insensible movements of diffusion suffice. This aerating system also permits the evaporation of water by land plants (see Transpiration). Naturally the aerating system is best developed in the larger water-plants, where the great canals can be seen with the naked eye.