15.2.1 Stomatal structure and function
Stomata (Figure 15.6) comprise a pair of highly specialised guard cells that are encompassed by a pair of larger and thinner subsidiary cells. In nearly all vascular plants, guard cells differ signiﬁcantly from other epidermal cells in having chloroplasts. These differ from mesophyll chloroplasts (Section 1.2) in lacking grana. Stomata also lack plasmodesmata, although a full range of other subcellular organelles is present, including nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus and ribosomes.
Two distinct types of guard cells exist, kidney shaped (Figure 15.6a) and dumb-bell shaped (Figure 15.6b). Kidney-shaped guard cells are found in dicotyledons whereas dumb-bell-shaped guard cells are found in grasses. Dumb-bell shaped guard cells are more advanced in evolutionary terms and more efﬁcient physiologically because guard cells of grasses require fewer solutes and less water to achieve a given unit increase in aperture. Directly beneath each pair of guard cells inside the leaf is a substomatal cavity. Air in this cavity in living leaves is virtually saturated with water vapour because of evaporation from adjacent wet cell walls.
Cell walls of guard cells have two distinctive features: an uneven thickening of walls forming the pore in either case (Figures 15.6a, b) and a radial micellation of microﬁbrils. These two features ensure that an uneven expansion occurs as guard cells inflate. The two ends of the guard cells push against each other to generate an opening. The thickened region lining the edge of the pore cannot stretch lengthways and therefore bends, generating an aperture. Guard cells of grasses are more rigid with thickened regions appressed when pores are closed (Figure 15.6b). Inflating chambers at each end force these sections apart while retaining overall shape.
Stomata of many xerophytic species are sunken below adjacent epidermal cells (Figure 15.7). This adaptive feature produces a microenvironment that protects stomata from wind and atmospheric vapour pressure deﬁcit, and so alleviates transpirational demand on hot dry windy days. Hairs and trichomes on leaf surfaces act similarly (Section 1.1).