22 - Stomata and Measurement of Stomatal Resistance

Abstract

The two main parts of a plant that control its water status are the roots, where water enters, and the stomata on the leaves, where water exits. The chapter focuses on stomata. The stomata are apertures in the epidermis, each bounded by two guard cells. In Greek, stoma means “mouth,” and the term is often used with reference to the stomatal pore only. Esau uses the term stoma to include the guard cells and the pore between them, and the chapter uses her definition. Stomata occur in vascular plants. Vascular plants include the lower vascular plants such as horsetails (Equisetum), ferns (Class Filicinae), gymnosperms, and angiosperms. The angiosperms are the flowering plants, and the group consists of the two large classes of Monocotyledoneae (monocotyledons) and Dicotyledoneae (dicotyledons). By changes in their shape, the guard cells control the size of the stomatal aperture. The aperture leads into a substomatal intercellular space, the substomatal chamber, which is continuous with the intercellular spaces in the mesophyll. In many plants, two or more of the cells adjacent to the guard cells appear to be associated functionally with them and are morphologically distinct from the other epidermal cells. Such cells are called subsidiary, or accessory cells. The stomata are most common on green aerial parts of plants, particularly the leaves. They also can occur on stems, but less commonly than on leaves.