In most broadleaf plants, stomata sit on the lower (abaxial) surface of the leaf, the side facing the ground. This arrangement, called hypostomaty, keeps the pores sheltered from direct sunlight and wind, which slows water loss. But the lower surface is not the only option. Grasses, many crop species, and aquatic plants break this pattern in ways that reveal how tightly stomatal placement is tuned to a plant’s environment and lifestyle.
The Default Position and Why It Makes Sense
When you flip a typical broadleaf plant’s leaf upside down, the underside looks duller and sometimes fuzzier than the glossy top. That underside is where you’ll find the stomata in the vast majority of dicot species. Each stoma is a microscopic pore flanked by a pair of guard cells that swell or shrink to open and close the gap. In many plants, the guard cells sit alongside subsidiary cells that help regulate this opening and closing. The number and shape of subsidiary cells varies widely: some species have none at all, while others have six or more arranged around the guard cells.1PubMed Central. Flanking Support: How Subsidiary Cells Contribute to Stomatal Form and Function
Why the bottom? The upper leaf surface is exposed to the sun, wind, and open air. Placing stomata there would increase evaporative water loss because free convection (warm air rising off the sun-heated surface) is stronger on top. Research on leaves with stomata on only one surface versus both has confirmed this: leaves with stomata exclusively on their upper surface lost more water through transpiration than otherwise identical leaves with stomata underneath, specifically because convective airflow was greater on the top surface.2Plant, Cell & Environment. Influence of stomatal distribution on transpiration in low‐wind environments By tucking stomata below, plants maintain a thicker, more humid boundary layer of still air around the pores, slowing the diffusion of water vapor outward.
When Stomata Appear on Both Sides
Plenty of plants do not follow the bottom-only rule. Species with stomata on both the upper and lower leaf surfaces are called amphistomatous. This pattern is especially common in fast-growing herbaceous annuals with thin leaves. Because both the upper and lower pathways for gas movement are already short in a thin leaf, adding stomata to the top surface boosts the leaf’s overall capacity for exchanging carbon dioxide and water vapor with the atmosphere.3PubMed. Two sides to every leaf: water and CO(2) transport in hypostomatous and amphistomatous leaves
Wheat is a clear example. Although stomata are commonly described as being more numerous on the lower surface, wheat flag leaves actually carry greater stomatal densities on their upper (adaxial) surface. The balance between upper and lower densities also varies among wheat cultivars. In a comparison of several cultivars, the differences in total stomatal count were driven by higher upper-surface densities, while the lower-surface densities stayed relatively consistent across cultivars. By contrast, a common bean variety tested alongside those wheats had far more stomata on the lower surface and very few on top.4PubMed Central. Stomata on the abaxial and adaxial leaf surfaces contribute differently to leaf gas exchange and photosynthesis in wheat So the “stomata on the bottom” rule, while a solid generalization for broadleaf plants, misleads when applied to grasses and cereals.
Grasses and Monocots Are Built Differently
Grasses form long, narrow leaves with parallel veins rather than the branching vein networks of most broadleaf plants. Their stomata are also distinctive: instead of the kidney-shaped guard cells found in ferns and dicots, grasses have dumbbell-shaped guard cells flanked by a pair of subsidiary cells running parallel to the long axis of the guard cells.5Annals of Botany. Stomatal cell wall composition: distinctive structural patterns associated with different phylogenetic groups These specialized complexes sit in neat rows between the leaf veins.
In monocots more broadly, the diversity of stomatal complex types is impressive. A classic survey of nearly 200 monocot species across 49 families identified four major categories, ranging from complexes surrounded by four or more subsidiary cells down to those with none at all.6American Journal of Botany. Variation in the Organization of the Stomatal Complex in the Leaf Epidermis of Monocotyledons and Its Bearing on Their Phylogeny Because grasses tend to hold their leaves more vertically or at angles, both surfaces receive roughly similar light exposure. Having stomata on both sides makes sense: neither surface is strongly shaded.
Floating Leaves Flip the Script
Aquatic plants with floating leaves, like water lilies, face a unique constraint: the lower surface of the leaf rests directly on the water. A stoma submerged in water cannot exchange gases with the air. So these plants place their functional stomata on the upper (adaxial) surface, the one exposed to the atmosphere. Some floating leaves also retain stomata on the lower surface, but the active gas exchange happens on top.7Aquatic Botany. Anatomical observations on floating leaves This is the clearest case of the environment dictating stomatal position: once the underside is underwater, there is simply no other option.
Sunken Stomata and Stomatal Crypts
Not all stomata sit flush with the leaf surface. In many species, the pores are recessed into pits, surrounded by overhanging cells, or hidden deep within epidermal depressions called stomatal crypts. Hairs (trichomes) often crowd around these recessed pores, adding another layer of protection from the outside air.8PubMed Central. The why and how of sunken stomata: does the behaviour of encrypted stomata and the leaf cuticle matter?
The intuitive explanation is that sinking stomata into pits reduces water loss, and in desert plants with thick, leathery leaves, that reasoning holds. But the picture is more complicated. Stomatal crypts also appear in species from humid environments, which suggests they serve another function beyond simple drought resistance. One hypothesis is that crypts help shuttle carbon dioxide deeper into the leaf’s interior tissue. In thick-leaved species where the internal path for CO₂ is long, crypts positioned on the underside may deliver gas more efficiently to the photosynthetic cells in the interior.9PubMed. Stomatal crypts may facilitate diffusion of CO(2) to adaxial mesophyll cells in thick sclerophylls
How Veins and Stomata Coordinate Their Positions
Stomata are not randomly scattered across the leaf surface. Their placement is spatially coordinated with the network of veins that supplies water and carries away sugars. This makes sense mechanically: a stoma that opens to release water vapor needs a reliable water supply from a nearby vein, and the distance water travels from the vein ending to the evaporating surface matters enormously. Research has shown that an optimal spatial arrangement of veins and stomata exists, and that this arrangement helps maintain a consistent flow of water across the entire leaf surface.10PubMed. Transport efficiency through uniformity: organization of veins and stomata in angiosperm leaves
Vein density has a powerful effect on how easily water moves from the xylem to the evaporating cells near the stomata. Doubling the density of veins in a leaf more than doubled the efficiency of water transport outside the xylem, partly by increasing the surface area where water exits the vein bundle and partly by shortening the horizontal distance water must travel to reach the epidermis. The position of the vein bundle relative to the upper versus lower epidermis also mattered: when veins sat closer to the lower surface, water reached the stomata-bearing epidermis more readily.11Plant Physiology. How Does Leaf Anatomy Influence Water Transport outside the Xylem? So where the stomata sit is not just about air exposure; it is tightly linked to the plumbing underneath.
What Decides Where Stomata Form During Development
Stomatal position is not inherited as a fixed blueprint. Instead, it emerges through a signaling conversation among neighboring cells as the leaf grows. Cells on the developing leaf epidermis receive positional cues from small signaling peptides and transmembrane receptors. These signals activate transcription factors that push certain cells down the path toward becoming guard cells while preventing their immediate neighbors from doing the same thing, which is why stomata are almost always separated by at least one non-stomatal cell.12PubMed. Mechanisms of stomatal development This “one-cell spacing” rule keeps stomata from forming in clusters that would interfere with each other’s ability to open and close.
Environmental conditions modify this developmental process. Leaves that develop in full sun tend to produce more stomata than leaves of the same species that develop in shade. In silver fir, sun leaves were larger, structurally more complex, and had more stomata than shade leaves from the same tree.13PubMed. Morpho-anatomical and physiological differences between sun and shade leaves in Abies alba Mill. (Pinaceae, Coniferales): a combined approach The ratio between the two leaf surfaces can also shift. A leaf that develops in a bright, exposed position may put more stomata on the top surface than a shaded leaf of the same plant. This plasticity means that asking “where are the stomata” on a given species can yield different answers depending on where on the canopy you sample.
Trichomes and the Boundary Around the Pore
Stomatal location does not tell the whole story of how a leaf manages its water. The hairs, or trichomes, that grow from the epidermis interact directly with stomatal function. Trichomes thicken the boundary layer of still air around the leaf surface, slowing the diffusion of water vapor away from the stomata. They also reflect ultraviolet radiation, absorb light through pigmented compounds, and prevent leaves from overheating. In tomato, the ratio of trichomes to stomata on the leaf surface is linked to how efficiently the plant uses water.14PubMed Central. The ratio of trichomes to stomata is associated with water use efficiency in Solanum lycopersicum tomato A leaf with many trichomes surrounding its stomata effectively adds an extra layer of insulation, even if the stomata themselves are flush with the surface rather than sunken.
Many of the plants famous for fuzzy or silvery leaves live in hot, dry habitats. Think of lavender or lamb’s ear. Their abundant trichomes work alongside stomata that are often restricted to the lower surface and sometimes recessed into crypts, creating a multi-layered defense against water loss.
Stomata Beyond the Leaf
Although leaves are the primary location, stomata are not exclusive to them. Stomata appear on the aerial surfaces of land plants more broadly, including stems, sepals, and the outer surfaces of green fruits. Young green stems, for instance, often carry stomata that contribute to gas exchange before bark develops. On fruits like green tomatoes, stomata allow gas exchange during early development and tend to become non-functional as the fruit ripens and the cuticle thickens. The developmental machinery that builds stomata on leaves, the same peptide signals and transcription factors described earlier, operates on these other organs as well.
Engineering Stomatal Density for Agriculture
Understanding where stomata sit and how many a leaf makes has become a practical concern in crop science. Researchers have learned to manipulate the genes controlling stomatal patterning to produce crops with fewer stomata per unit of leaf area. By adjusting the expression level of an epidermal patterning factor, scientists significantly reduced stomatal density in experimental plants, cutting water use and improving drought tolerance without significantly reducing the plant’s ability to take up nitrogen, a critical nutrient.15PubMed Central. Manipulating stomatal density enhances drought tolerance without deleterious effect on nutrient uptake
The potential payoff is large. Crops bred or engineered to have abnormally low stomatal densities need less water to produce the same yield and also show improved tolerance to salt stress. Because many plant pathogens enter leaves through open stomata, reduced stomatal density may even offer some disease resistance.16PubMed Central. Optimizing Crop Plant Stomatal Density to Mitigate and Adapt to Climate Change These “water-saver” crops represent one of the more promising avenues for adapting agriculture to drier, hotter climates without requiring the kind of large-scale irrigation expansion that many regions cannot support.
How to See Stomata for Yourself
If you want to check stomatal placement on an actual leaf, the classic technique is a nail-polish impression. Paint a thin layer of clear nail polish on the leaf surface, let it dry, peel it off with tape, and examine it under a light microscope. The impression captures the outline of the epidermal cells, the guard cells, and the pore. Try both the upper and lower surfaces to see which one has stomata, and in what density.
More advanced methods exist for researchers who need precise data. Scanning electron microscopy produces detailed images of the leaf surface where the outlines of guard cells and epidermal cells are clearly distinguishable, allowing accurate counts of stomatal density.17PLoS ONE. Novel perspectives on stomatal impressions: Rapid and non-invasive surface characterization of plant leaves by scanning electron microscopy A less common but clever technique exploits the fact that guard cells contain compounds that fluoresce blue under ultraviolet light after treatment with a mild alkali. This fluorescence method works across a wide range of plant species and is well suited for rapid, non-destructive counts of stomatal number and distribution.18Annals of Botany. Epicoricular Phenolics Over Guard Cells: Exploitation for in situ Stomatal Counting by Fluorescence Microscopy and Combined Image Analysis
Stomatal Density as a Climate Record
Stomatal location and density are not just matters of plant physiology. Because plants tend to produce fewer stomata when atmospheric CO₂ levels are high (they need fewer pores to absorb enough carbon), fossilized leaves carry a record of past atmospheric composition. Paleobotanists count stomatal densities on fossil leaf cuticles and compare them to densities on modern relatives to estimate CO₂ concentrations hundreds of thousands or millions of years ago.19The Holocene. Stomatal Density as an Indicator of Atmospheric CO2 Concentration This inverse relationship between CO₂ and stomatal density feeds into both water-use efficiency and carbon uptake rates, making fossil stomata a surprisingly useful proxy for reconstructing ancient climates. It is one of the tidier examples of plant anatomy doubling as a scientific instrument.