The name stomata (sing. stoma) has been applied to the elliptical apertures in the epidermis of leaves and other green parts of plants. The stoma is a modified epidermal cell and consists of a rift and guardian cells (usually two in number). The guardian cells are rightly named for it is their function to regulate the amount of evaporation from the leaf by opening and closing the rift. Unlike ordinary epidermal cells, the guardian cells contain chlorophyll, and for that reason they were once, thought to belong to the parenchyma.
Proceedings of the Iowa Academy of Sciences
©1892 Iowa Academy of Science, Inc.
Stewart, F. C.
"The Stomata and Palisade Cells of Leaves,"
Proceedings of the Iowa Academy of Science, 1(Pt. 3), 80-84.
Available at: https://scholarworks.uni.edu/pias/vol1/iss3/23