Hydathodes are the same as stomata

Epidermal prints made by applying an acrylic adhesive (Acrifix®) to the leaf surface. After hardening, the plastic film was simply peeled off and examined under an interference contrast microscope. Left: the upper side of the leaf of a palm tree (Hovea belmoreana) with extremely small epidermal cells arranged in rows (typical of monocotyledons). Right: underside of the leaf of a Cracculaceae (Aeonium acuneatum) with pairs of stomata and extremely large epidermal cells.

There are two techniques for getting a clear picture of the structural makeup of epidermal surfaces. On the one hand the scanning electron microscopy, on the other hand the microscopic analysis of plastic prints. The success of both techniques is demonstrated by a number of examples (click on the blue text areas!).

The "real", i.e. the least specialized, epidermal cells make up the bulk of the cells of the terminating tissue. When viewed from above, they are either polygonal (plate or tabular shape) or elongated. The walls formed between them are often corrugated or curved. What induces this form during development is unknown, the present hypotheses explain the facts only unsatisfactorily. Stretched epidermal cells are found on organs or parts of organs that are stretched themselves, e.g. on stems, petioles and leaf veins as well as on the leaves of most monocotyledons. The top and bottom of leaf blades can be covered by differently structured epidermis, whereby the shape of the cells, the thickness of the walls as well as the distribution and number of specialized cells (stomata and / or trichomes) per unit area can vary. Large variations can also be found within individual families, e.g. B. in the Crassulaceae.

Often the outer wall of the cells is thicker than that of the other walls. This is particularly evident in the epidermis of conifer needles, coarse leaves of many species of the rainforest and in xerophytes (plants from dry locations). Thin walls can be found in aquatic plants. In many seeds the wall strengthens in the course of maturation and fills almost the entire lumen of the cell, the protoplast is displaced and degenerated. The epidermal cells of most species are free of chloroplasts. Exceptions are found for ferns, aquatic plants and some shade plants.

Usually the epidermis is single-layered, but species from several families (Moraceae: here most Ficus-Species, Piperaceae: Hot peppers [Peperonie], Begoniaceae, Malvaceae etc.) multi-layered water-storing epidermis have been detected, which emerged from an originally single-layer tissue layer through periclinic divisions. Epidermal cells secrete a cutin layer (cuticle) on the outside, which covers all epidermal surfaces as an uninterrupted film. It can be either smooth or structured by protrusions, ridges, folds and furrows.

However, a fold of the cuticle that is visible by looking at the surface is not always based on the formation of cuticular ridges. There are certainly cases where a cuticle fold is only the expression of the underlying protrusions of the cell wall. To detect such structures, one has to rely on the evaluation of cross sections through the cuticle and the underlying epidermal cells.

In some cases, such as the tomato fruit, the cuticle is pigmented by embedded carotenoids. In addition, waxes, oils, resins, salt crystals and water-soluble (hydrophilic) slimes are often secreted. Mucilage occurs particularly frequently when the seeds are formed. Wax excretions prevent the leaves from being wetted, even more than the cuticle itself. In many cases, these waxes are also structured in themselves. A thick wax coating gives plant surfaces a whitish appearance. It works in two ways. On the one hand it restricts transpiration (the loss of water), on the other hand it causes the light to be reflected (due to the light color) and thus protects the plant from excessive heating. The superimposed epicuticular waxes also cause microturbulence and thus reduce the adhesion of water and particulate contamination

The non-wettability of leaf surfaces has long been known and has been well researched. However, it has been largely overlooked that unwettable surfaces are also virtually unpolluting. This connection has only recently been investigated in detail and proven experimentally. As it looks particularly good on the large, shield-shaped leaves of the sacred lotus plant (Nelumbo nucifera) demonstrates, the symbol for purity in Asian religions, it was called the "lotus effect" by W. BARTHLOTT and C. NEINHUIS.

Cross section through a gap opening of Rhoeo discolor. S. Guard cell, N Adjoining cell, E. non-specialized epidermal cell I. Intercellular space. (R. KAPPLER, 1984)

An important function of the epidermis is performed by the stomata, the stomata. The complete functional unit is called the stomatal complex or stomata apparatus. This includes two guard cells containing chloroplasts, between which there is a pore (a gap), as well as two to four neighboring chloroplast-free secondary cells. In cross-sections it becomes clear that the guard cells have cell walls of different thicknesses and that an intercellular space (a "respiratory cavity") is superimposed on them, which is connected in a communicating manner with the other intercellular spaces of the plant tissue concerned. The gap can be widened or closed as required. The stomata regulate transpiration (water release) and carbon dioxide uptake. Both the water content and the carbon dioxide concentration in the plant tissue act as regulators of the opening state. The guard cells regulate the pore size by changing their shape.

Stomata occur on all parts of the plant above ground. Their number is in the order of 100-300 / square millimeter, but can - depending on the species - also fluctuate within much wider limits. The distribution and the gap diameter can also be just as variable. On the roots, in the epidermis of the chlorophyll-free shoots of the parasitic species Monotropa hypopitys (Spruce asparagus) and Neottia nidus-avis (Nestwurz) as well as in some submerged aquatic plants they are absent, in other aquatic plants, however, they are normal. They are also found in colored and white petals, but they are often functionless here. In the parallel-veined leaves of most monocotyledons, some dicotyledons and in the needles of conifers, they are arranged in parallel rows.

Their formation goes back to guard cell mother cells, which in turn are created in the epidermis at regular intervals through inequitable divisions. The initial of the gap opening is the smaller, plasma-rich of the two daughter cells. The two guard cells arise from it through an equal division, the formation of the secondary cells can take place in very different ways. The development of the stomata in a leaf occurs mostly asynchronously. In parallel-veined leaves, their development follows the successive differentiation of the individual leaf sections. The wave of differentiation is basipetal, i.e. it begins at the tip of the leaf and extends towards the base of the leaf.

With reticulated leaves, various stages of development of the stomata are distributed like a mosaic over the entire leaf surface.

Since there are different pathways for stomata apparatus to develop, it is to be expected that morphological variants will also occur. The guard cells of Gramineae are always mentioned as a typical exception, which are structured in the shape of a dumbbell in many species.

The hydathodes (water crevices), which can often be found at the ends of the ducts, can be derived from the stomata. The guard cells still look like the stomata, but they can no longer be closed. A water separation through them is called guttation. Characteristic hydathodes are found on the leaf margins of nasturtiums (Tropaeolum majus), the woman's coat (Alchemilla vulgaris: see adjacent figure. Out. J. v. SACHS: "Lectures on Plant Physiology", 1877) and on the leaf tips of many grasses. Salts dissolved in the guttation water, as well as sugar and other organic substances, crystallize out at the outlet after the water has evaporated. Typical examples of this are the lime secretions of the stone breakage species (Saxifraga) and the salt glands of the halophytes (salt plants) are called.

Epidermal appendages of various shapes, structures and functions are called trichomes. They appear as protective, supporting and glandular hair in the form of scales, various papillae and, in the case of roots, as absorbent hair. Only epidermal cells are involved in their formation. A trichome often arises from just one such cell, sometimes several are involved in the formation.

The trichomes are to be distinguished from the emergences (e.g. the spines) and the short shoots (e.g. the thorns), because these not only contain cells of the epidermis, but also cells of other tissues.

Hair in various forms occurs on plant surfaces. They can be single or multicellular, branched or unbranched, living or dead. Their walls can be hardened by silicate, calcium carbonate or other deposits, giving them a bristle-like character. Such stiff bristles (e.g. in the Boraginaceae and Cruciferae) protect the plants from animal damage. Much of the hair, especially the heavily branched hair, is used to protect against perspiration. It is known that plants in dry locations are either succulent (and thus also have a thick cuticle) or have a thick, silvery felt felt. At Stachys lanata (Filziger Ziest) 120 hairs / square millimeter were counted. A microscopic analysis shows that hair cells are often heavily branched and dead. This has three advantages for the plant:

  1. The dead cell lumen is filled with air. It gives the cells a whitish-silvery appearance. A large part of the incident light is reflected. The effect is the same as that achieved with thick layers of wax.

  2. A circulation-calmed space is created above the leaf surface. In other words: water losses on the leaf surface are reduced to a minimum.

  3. As the cells die, the surface area that loses water is drastically reduced. If the cells were alive, excessive water loss would be inevitable due to the large surface area caused by the branches.

Hair is regularly arranged on leaf surfaces, they are always placed at approximately the same distance from each other. Similar patterns also occur in the stomata. This raises the question for us of the causes of this regularity. Although we cannot give a final answer today, we can present a mathematical model that was set up according to rules that should also apply in plant tissues.

Aerenchyma with "inner hairs" that show a lignification reaction (Nymphaea spec.): Hand cut through the petiole. Staining with phloroglucinol HCl. (Photo: W. KASPRIK).

Extremely long (1-6 cm), single-celled and unbranched hair, the wall of which consists of almost pure cellulose, surrounds the seeds with Gossypium (Cotton). They are branched e.g. at Lobelia at Arabis alpina and at Malacantha alnifolia (adjacent picture). Multicellular hair can consist of one or more rows of cells. Typical examples are the layered hairs of the plane tree (Platanus hybrida) or that of the mullein (Verbascum nigrum). Hairs on leaves of oak (Quercus robur) are cluster-shaped, those of many Malvaceae are star-shaped. At the olive pasture (Elaeagnus angustifolia) they are built like an umbrella (scale hair). In the Bromeliaceae, such scales have the function of absorption hair. The moisture in the atmosphere is collected by capillary forces and benefits the plant. In addition to hair on surfaces, so-called "inner hair" also occurs, e.g. B. the water lily

Glandular hairs consist of a multicellular stem and a single or multicellular head. Examples of hair development: Origanum (Lamiaceae), Adenocaulon (Compositae), Citrus deliciosa (Rutaceae). Glandular hairs on leaves of tobacco (Nicotiana tabacum) have multicellular heads, in primroses (e.g. Primula sinensis) and pelargoniums (e.g. Pelargonium zonale) they are unicellular. The secretion (from Pelargonium zonale) is an essential oil. The gland cells themselves are rich in plasma, and secretion occurs through the cell wall. The oil collects on the cell surface and therefore appears under the microscope as a highly refractive cap that is covered by a thin film of cuticle and cell wall components. When they burst, the secretion is released. The fruit peel, e.g. of the citrus fruits, contains sunken secretory caverns (glands).

The stamens (filaments) of Tradescantia virginiana are surrounded by multicellular hairs, the individual cells look barrel-shaped. The protoplast is wall-mounted, the vacuole content is colored by a red-violet dye (an anthocyanin). The vacuole is penetrated by numerous strands of plasma of different thicknesses. The nucleus is central and it looks like it is suspended from the plasma strands. Since the plasma strands are constantly changing their shape, the core is constantly pulled back and forth, as if hanging on elastic rubber bands. The plasma flows vigorously in the plasma strands and transports clearly recognizable granules. The current is strictly directed, with oncoming traffic on separate lanes in many strands. Because of the easy recognizability and regularity of the flow, the filamentous hairs are the Tradescantia virginiana has become a classic demonstration object for these cell activities.

For another reason, the knapweed's stamen hairs (Centaurea jacea) and the cornflower (Centaurea cyanus) known. They are sensitive to touch and trigger a stimulating movement of the stamens.

Stinging hairs of the stinging nettle (e.g. Urtica dioica): A stinging hair is actually an emergence, it is in two parts and consists of a multicellular base, in the formation of which also subepidermal cells are involved, and a hair cell embedded in it. The lower (basal) section of the hair cell is called the globe. It is surrounded by the base cells in the shape of a cup. At its upper end, the hair runs out to a point and ends in a small head attached to the side. At the transition point, the cell wall is noticeably thinner than in the other sections. Embedded silicates also make them brittle, so that the head easily breaks off when touched and leaves a point at the predetermined breaking point that resembles a puncture cannula. Because of the rigidity of the wall, the pressure is transferred unbuffered to the globe, the contents of which (sodium formate, acetylcholine and histamine) are squeezed out through the cannula and, if necessary, injected into a wound site.

Papillae are protuberances on the surface of the epidermis. The textbook example for this is the papillae on the flower surfaces of the pansy (Viola tricolor) as well as the leaf surfaces of many species in the tropical rainforest. They give the surface a velvety consistency. Some epidermal cells can be designed to store water. A typical example are the bladder cells on the surfaces of many types of ice flowers and other succulents. In some plants, e.g. in the bellflower (Campanula persicifolia) the outer walls of the epidermis are thickened in the shape of a lens. They act like converging lenses and bundle the light, which in turn is perceived by specific receptors (light perceptors) and converted into a physiological reaction.

Bladder cell in supervision. Epidermal impression.
Monanthes lowei. (Preparation J. THIEDE)