Epidermis of Stem Leaf and Sepal

The epidermis is the outermost cell layer of a plant and serves as protection against uncontrolled evaporation and attack by microbes and herbivores. On stems and roots, the primary epidermis is replaced by cork during secondary growth. The epidermis of roots differs from that of shoots (stems, leaves, reproductive organs) in terms of both developmental origin and structure and is therefore treated separately. Root epidermis is not often seen in medicinal plant material because older roots in which cork has developed are most often used.

Ordinary epidermal cells form a compact layer without intercellular spaces. They may appear isodiametric (leaves of dicots) or elongated and somewhat parallel to each other (leaves of monocots, all stems). The anticlinal cell walls may be sinuous or straight. Epidermal cells contain no chloroplasts and generally have thin primary walls. However, some have thickened walls, and lignified primary walls may be found in the leaf epidermal cells of some medicinal plants (Eucalyptus spp., Laurus nobilis).

The hydrophobic lipid cutin is deposited in the outer wall of epidermal cells and on top of the outer wall to provide a waterproof surface and aid in water retention. Cutin plus the wall material it is embedded in form the cuticular layer (Figure 8.1a-d), and the layer of pure cutin on the outside of the cells forms the cuticle. The cuticle may have a flat surface or form characteristic ridges or bumps called papillae (singular: papilla; adjective: papillate) that are useful for diagnostic purposes. Wax is deposited on top of and in the cuticle. This wax melts during slide preparation using heat, and if it is present in sufficient amounts, may solidify into visible birefractive wax crystals upon cooling. These epicuticular crystals must not be confused with calcium oxalate crystals in the tissues.

Stomata (singular: stoma) are pores in the epidermis that regulate gas exchange and water balance in the plant. They may occur on both sides of a leaf or on one side only, in which case it is generally the lower surface (abaxial surface, excepting some aquatic plants). Each stoma consists of two kidney-shaped cells (dumbbell shaped in grasses)—called guard cells—that regulate stomatal opening and closing. If the epidermal cells adjacent to the guard cells differ in shape or size from other epidermal cells, they are called subsidiary cells or accessory cells. These cells are thought to assist, reinforce, or protect the stomatal cells.

According to the arrangement of the surrounding epidermal or subsidiary cells, several diagnostic stomatal types are distinguished. Plant families often contain a number of stomatal types. Some families are associated more closely with a certain stomatal type than others and are sometimes referred to by the corresponding plant families (e.g., rubiaceous or carophyllaceous type).

Stomatal type can therefore be helpful in narrowing the possible identity of unknown plant material. The primary types of stomata are described and shown in Table 8.1. Similarly, as seen in the example of the paracytic stomata in Table 8.1 and Figure 8.2c (Senna alexandrina), different types of stomata can occur on the same leaf. Within a given species, the type of stoma designated is derived from the appearance of the majority of the stomata present.

Trichomes (or hairs) are cells or groups of cells that project markedly as hairs from the epidermal surface and give leaves such as marshmallow (Althaea officinalis) their soft, hairy (hirsute) quality. They are generally composed only of epidermal cells, but may include subepider-mal tissue as well (the term emergence has been applied to the latter group). Trichomes provide a wealth of diagnostic characters for the identification of plant material; some types are diagnostic of certain plant families. The collective trichomes found on a plant surface are termed the indumentum. As the plant ages or during processing, the trichomes often will break off the plant surface, leaving a characteristic scar, or cicatrice. The epidermis of

FIGURE 8.1 Examples of cuticular tissues. (a) Upper and lower epidermis of Arctostaphylos uva-ursi leaf, both with a thick cuticle (transverse section); (b) cuticular striation of Illicium verum follicle (surface view); (c) upper surface epidermis cuticular striation of Tussilago farfara leaf (surface view); (d) upper surface epidermis cuticular striation of Tussilago farfara leaf (surface view). (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

FIGURE 8.1 Examples of cuticular tissues. (a) Upper and lower epidermis of Arctostaphylos uva-ursi leaf, both with a thick cuticle (transverse section); (b) cuticular striation of Illicium verum follicle (surface view); (c) upper surface epidermis cuticular striation of Tussilago farfara leaf (surface view); (d) upper surface epidermis cuticular striation of Tussilago farfara leaf (surface view). (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

some species will have small bumps or projections called papillae rather than trichomes. Papillose describes a surface having minute papillae. The two primary types of trichomes are nonglandular and glandular.

Nonglandular trichomes (also known as covering trichomes) are characterized by an acute or rounded terminal tip that is never spherical or swollen. Glandular trichomes have terminal cells that are modified to secrete or store substances such as essential oil, salt solution, nectar, or polysaccharides. The glandular head, which can be single or multicelled, is attached to the leaf epidermal surface directly (sessile) or by an elongated single- or mul-ticell stalk. The secretions of these trichomes may in some cases be responsible for the therapeutic value of the plant medicine, but for diagnostic purposes the recognition of their structure is often essential. The trichomes listed in Table 8.2 and shown in Figure 8.3 are relevant for botanical identification.

Table 8.1 Illustrative Examples of the Most Common Types of Stomata

Stomata Type

Characteristics

Structural characteristics of a stoma

Consists of the stomatal pore (center) surrounded by guard cells and epidermal cells

Guard cells Stomatal pore

Guard cells Stomatal pore

Epidermal cell

Anomocytic stomata (irregular-celled) (Ranunculaceae type)

Cells adjacent to the guard cells do not differ in size or shape from other epidermal cells. This is the most common type of stomatal complex

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Anisocytic stomata (unequal-celled) (Cruciferae type)

Stomata with three or four subsidiary cells that resemble other epidermal cells, except that one of them is considerably smaller than the other two (e.g., Brassicaceae, Solanaceae).

Diacytic stomata (cross-celled) (Caryophyllaceae type)

Stomata have two subsidiary cells surrounding the guard cells and these subsidiary cells have a common wall at right angles to the longitudinal axis of the guard cells (e.g., Acanthaceae, Caryophyllaceae, Lamiaceae)

Table 8.1 Illustrative Examples of the Most Common Types of Stomata (continued)

Stomata Type

Characteristics

Cyclocytic stomata (wheel-celled)

Stomata surrounded by four or more subsidiary cells, which form a wheel-like ring around each stoma

Rubiaceous Stomata

Paracytic stomata (parallel-celled) (Rubiaceae type)

Two subsidiary cells are parallel to the pore of the stomatal aperture (e.g., Convolvulaceae, Fabaceae, Magnoliaceae, Rubiaceae)

Tetracytic stomata (four-celled)

Four subsidiary cells are present: two lateral and two terminal

Note: Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria, and Wasicky, R. 1929. Lehrbuch der Physiopharmakognosie für Pharmazeuten. Wien und Leipzig: Guter Zustand.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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