In plants, the substomatal cavity is the cavity located immediately proximal to the stoma. It acts as a diffusion chamber connected with intercellular air spaces and allows rapid diffusion of carbon dioxide and other gases (such as plant pheromones) in and out of plant cells.
References Graham LE, Graham JM, Wilcox LW (2006) Plant Biology (Second Edition). Pearsons Education, USA. ISBN 0-13-146906-1
See also Stoma Transpiration stream