Plant Anatomy: Practical Course - Paniuta O.O. 2019

Topic 2. Tissues
Laboratory Work No. 6. Dermal Tissues

Theoretical Background. Protective Tissues are those that shield the plant from various adverse environmental factors.

The primary protective tissue is the epidermis (or Skin). As plants grow and develop, it is replaced by the secondary protective tissue—the periderm—and later still by the tertiary tissue, known as the bark or rhytidome.

Epidermis. The epidermis originates in the growth point from the primary meristem. The periderm, on the other hand, can develop from various tissues, even permanent ones.

The epidermis originates from Cells in the Superficial layer of the apical meristem. Epidermal cells form a continuous layer covering young stems, leaves, roots, fruits, and other plant Organs. The epidermis consists of living cells with cellulosic walls, containing a Nucleus and Cytoplasm, but typically lacking METABOLISM/14.html">Chloroplasts. The inner and lateral walls of epidermal cells are thin, whereas the outer wall is thickened and very frequently covered with a cuticle and various outgrowths. The epidermis protects the plant against Water loss, overheating, and other environmental impacts.

The epidermis is classified as a complex tissue because it comprises three groups of cells that differ in morphological and Anatomical Features: the main epidermal cells; guard cells and subsidiary Cells of the Stomata; and trichomes, which are derivatives of epidermal cells in the form of outgrowths or hairs.

Stomata. Stomata consist of two bean-shaped guard cells that fit tightly against each other at their ends. The guard cells contain chloroplasts. Visible between the guard cells is the stomatal pore, which appears as a narrow, through-opening. Flanking the guard cells on the sides are two narrow cells, while wide and short cells located above and below them serve as the subsidiary cells of the stomata.

The primary physiological property of stomata is their ability to open and close the stomatal pore. When guard cells are saturated with water and develop significant turgor, the inner convexity of the guard cells flattens and, due to uneven wall thickening, the stomatal pore opens. Conversely, when water is scarce and turgor within the guard cells decreases, their convexity increases, causing the stomatal pore to close. The opening and closing of stomata can be readily observed in a microscopic preparation. To do this, a preparation must be made from a water-saturated leaf with open stomata. After observing the width of the stomatal pore under a Microscope and measuring it with a micrometer, a drop of concentrated sugar solution is added to the preparation, causing the guard cells to lose a substantial amount of water. Plasmolysis of the cells soon occurs, and the stomata rapidly close. By removing the sugar from beneath the coverslip and adding water again, the reverse phenomenon can be observed: the guard cells take up water, increase in volume, and the stomatal pore opens. This Procedure can be repeated several times. Stomata will continue to open and close as long as the guard cells remain alive.

Chloroplasts residing within guard cells play a major role in the opening and closing mechanism of stomata. Epidermal guard cells are positioned in such a way that they contact only adjacent epidermal cells. They are separated from the palisade parenchyma by air spaces. Because of this arrangement, guard cells can obtain water exclusively from neighboring epidermal cells. The transfer of water from one living Cell to another is governed by the general laws of osmosis: water flows toward the cells with a higher concentration of cell sap. Thus, Photosynthesis takes place in the guard cells, producing sugars and rendering these cells rich in CARBOHYDRATES. Under the action of Enzymes, the synthesized sugar is converted into starch, and starch is converted back into sugar, which significantly increases the osmotic concentration of the guard cell sap and ensures the intake of water supplied by adjacent epidermal cells. As a result, the stomatal pore opens. If a plant is placed in the dark where carbohydrate synthesis ceases, the concentration of cell sap in the guard cells decreases. Under such conditions, they cannot draw water from neighboring epidermal cells, their turgor drops, and the stomatal pore closes. This is why stomata are typically open in the light and closed in the dark.

The primary function of the epidermis is to protect the plant from desiccation and mechanical damage. Its Anatomical Structure is well-suited to this function. The margins of epidermal cells are very often uneven and wavy, with the protrusions of one cell fitting tightly into the recesses of another to form a strong, interlocking bond.

The epidermal cells of many plants frequently form specialized surface outgrowths (hairs) known as trichomes. Their appearance varies widely among different plant species; they may take the form of typical hairs, papillae, bumps, hooks, scales, and so forth. The formation of hairs has important adaptive significance, protecting the plant against excessive Transpiration, overheating by solar radiation, and herbivory.

The various types of structures characteristic of the plant epidermis arise from the outward bulging of the outer cell walls. Regardless of their shape, they always originate from the epidermis and can be easily peeled off along with it, making it convenient to investigate and study their shape and structure.

The simplest form of Hair is the papilla. Papillae appear as cone-shaped outgrowths on the outer walls of epidermal cells and impart a velvety texture to The surface of the perianth in many plants. Papillae are not separated from epidermal cells by a partition; rather, they are direct extensions of these cells. Papillae can be easily studied on the petals of various flowers, where they create a velvet-like texture that is perceptible to the Touch.

An optimal object for observing papillae is the flowers of various plants, particularly pansies (Viola tricolor).

Wax. The surface of many plant leaves exhibits a thin layer of wax secreted by epidermal cells. The best subject for studying this structure is young aloe leaves.

Periderm. The epidermis covers only young plant organs—with the exception of leaves, which retain their epidermal covering throughout their lifespan. As plant organs grow, the epidermis ruptures and dies off. Only in conifers and other evergreens does the epidermis persist for several years. On the perennial organs of all plants, the epidermis is replaced by a secondary protective tissue known as the periderm, which consists of cork (or phellem), cork cambium (or phellogen), and phelloderm. The periderm can originate from various tissues. For example, in willow, viburnum, and oleander, the Components of the periderm develop from epidermal cells that transform into cork cambium; in elder, the cork cambium giving rise to the periderm originates from a layer of primary cortical cells located beneath the epidermis; whereas in currant, it develops from cells in deeper layers. This process unfolds as follows: beneath intact, undamaged epidermal cells, a layer of phellogen cells is established from the parenchyma. Phellogen cells are formed through the division of epidermal cells or the underlying parenchymal cells. Individual phellogen cells subsequently give rise to a continuous ring of phellogen. In herbaceous plants, a single phellogen layer persists throughout their entire life, whereas in many perennial woody plants, new layers of cork cambium are established deep within the stem shortly after the Formation of the first phellogen.

The phellogen ring consists of a layer of tightly packed parenchymal cells rich in cytoplasm. In cross-section, these cells are rectangular with small radial dimensions. In longitudinal tangential sections, phellogen cells appear as polygons with 4 to 6 sides.

Phellogen cells divide by tangential walls, giving rise to new cells. Of the two daughter cells produced by this division, the inner one almost invariably remains a cambial cell. It expands radially to reach the typical size of a phellogen cell and divides again by a tangential wall. Meanwhile, the outer cells initially grow and subsequently differentiate into permanent tissue cells.

As a result of this phellogen activity, cork is formed on its outer side.

During growth and differentiation, cork cells maintain their tangential dimensions, forming radial files in which one cell lies directly beneath another. The cork cambium cell is located directly beneath the innermost layer of cork cells. Cork cells expand radially, and their size is always slightly larger than that of the cork cambium cells.

During the differentiation of cork cells, their contents and cell walls undergo significant alterations. The Cell walls thicken and become impregnated with suberin. Pits are very poorly developed in cork cells.

Pit canals are sparsely distributed and penetrate only the cellulosic layer of The Cell wall rather than the suberized layer.

The protoplast in cork cells dies off very early, and the cell lumen becomes filled with air. Occasionally, the cell lumen contains granular matter rich in Tannins and their degradation products, such as resins. In birch cork cells, a resin-like substance called betulin is frequently found, while cork oak contains cerin in the form of needle-like crystals. Crystals and druses of calcium oxalate can also be very frequently observed within cork cells.

In many plants, such as the birch, the phellogen produces 4 to 6 layers of thin-walled cork and 2 to 3 layers of thick-walled cork during the growing season, making annual rings easily distinguishable within the cork layer.

The thickness of the cork layer varies among different plants. In birch, the cork thickness reaches 3–4 mm and consists of several dozen delicate layers. In the Amur cork tree (Phellodendron amurense), native to the Far East, the cork layer is thicker; it is even thicker in the cork oak.

Acting as a protective tissue, cork performs a defensive role, especially in the aerial parts of trees. It shields tree trunks and shoots from excessive transpiration and sharp Temperature fluctuations. The protective efficacy of cork is attributed to the tight packing of its cells, the impregnation of cell walls with suberin—which has low permeability to water and gases—and the fact that cork cells are filled with air, which is a poor heat conductor.

The cork protects the plant from bacterial and fungal invasion as well as from animals chewing on the bark, thanks to its hard, rigid, and indigestible nature.

The phellogen produces cork outwards and the phelloderm, consisting of living cells, towards the center. In such cases, following the tangential division of the phellogen cells, the outer cell remains cambial, while the inner cell grows slightly and differentiates into phelloderm cells. Typically, no more than 2-3 layers of phelloderm cells are formed.

Phelloderm cells are living, thin-walled, and invariably contain chloroplasts. In their mature state, they are nearly indistinguishable from the cells of the primary cortex.

The periderm isolates internal tissues from the external environment; however, this isolation is not absolute, as lenticels emerge within the periderm—structures that facilitate gas exchange between internal living tissues and the surroundings. They are visible even to the naked eye as small wart-like growths on the smooth surface of perennial branches in various plants. Their cells lie deeper and rupture outward through the periderm and epidermis. Lenticel formation varies among plants. For instance, in cherries and lilacs, they originate beneath stomata. If stomata are numerous, lenticels develop only under some of them; if sparse, under every single one. When stomata are arranged in groups, lenticels form beneath those groups.

Lenticels appear earlier than the cork tissue as small mounds on the green surface of a young SHOOT. Subsequently, the epidermis ruptures above the central part of the mound, forming depressions surrounded by a ring-like rim. Consequently, lenticels acquire an oval or fusiform shape with a longitudinal axis oriented either along or across the organ.

Bark (Rhytidome). In many trees throughout their lifespan, only the outer periderm develops, expanding in volume in tandem with the tree stem. In such species, the bark remains smooth nearly until the end of the plant's existence. For example, in the common beech, it persists for 50 years, and in the cork oak, throughout its entire life. However, such plants are few. In most woody plants, one can periodically observe the repeated formation of inner periderm layers. In these instances, the outer periderm and all bark layers situated outside the newly formed periderm die off and crack. As a result, rhytidome (bark) forms, appearing as a scaly layer. The formation of the inner periderm and scaly bark can begin at varying ages across different plants. For instance, in the grapevine, it emerges as early as the first year; in some trees, by the 3rd–4th vegetation season; in birch, apple, and pear, by the 5th–8th year; in linden, by the 10th–12th year; in alder, by the 15th–20th year; and in certain trees, by the 25th–35th vegetation season. Rhytidome is an exceptionally complex tissue complex. Its formation initiates at the Base of the tree and progressively extends upwards.

Typically, after the formation of the first ring-shaped periderm (due to phellogen activity), new periderms subsequently develop deeper within, which may encircle the stem in a continuous ring or fail to form a continuous layer.

Thus, as a result of the repeated emergence of the phellogen in deeply seated tissues, thick layers of dead tissue accumulate on the organ surfaces. The aggregate of dead tissues resulting from their isolation by new phellogen layers constitutes the rhytidome (bark).

Objective: to study the Structural Features of protective tissues.

Materials and equipment: light microscopes, Glass slides and coverslips, dissecting needles, tweezers, glass rods, filter paper, distilled water, acetic acid, zinc chloriodide, phloroglucinol with Hydrochloric acid, concentrated sugar solution, plant material.

Slide. Epidermis of leek leaf (Allium porrum L.)

Take a small piece of a green leek leaf, stretch it firmly over the outer side of the index finger, strip off the upper epidermal layer using tweezers, and place it in a drop of water on a prepared glass slide. The section should be placed with its outer side facing upward, carefully straightened with dissecting needles, and covered with a coverslip. Place the prepared slide on the microscope stage and examine it under low magnification. Under the microscope, the epidermis is seen to consist of elongated, colorless cells tightly fitted against one another, interspersed with rows of stomata. Having selected the thinnest and clearest area on the slide, center it in the field of view and switch the microscope to high magnification. Not all elements of the slide lie in the same focal plane; therefore, a detailed examination requires continuous adjustment of the fine adjustment knob. First, examine The structure of the epidermal cells, and then the STRUCTURE OF THE stomata. Epidermal cells are elongated. They possess thin, porous walls; the cytoplasm forms a thin inner lining along the cell wall, and a nucleus is visible within the cytoplasm. The central part of the cell is occupied by a vacuole. Draw the observed Structural components of the slide.

Next, examine the structure of the stomata. Stomata consist of two guard cells with an aperture (pore) between them. The outer walls of the guard cells are thickened. Chloroplasts are present within the guard cells. Nuclei are obscured by the chloroplasts. To verify the presence of nuclei, treat the slide with 95% alcohol for a few minutes, then transfer it to a glass slide in a drop of alum carmine solution. After 20 min, rinse the slide with water. The nuclei take up the stain, and microscopic examination reveals that they share the same Kidney-like shape as the guard cells.

Stomata are predominantly located in depressions of the epidermis. To observe pores within the cell walls, lower the objective even further. After examining the slide, sketch the structure of the stomata.

Slide. Transverse section of leek leaf epidermis (Allium porrum L.)

Make a lengthwise slit in a piece of elderberry pith with a scalpel, and insert a piece of leek leaf folded lengthwise in half into this incision. Alternatively, the tissue piece can be clamped within a split soft cork instead of elderberry pith. Level the surface of the tissue so that the section cuts the object at a right angle. Periodically moisten the object and the razor with water to prevent the section from drying out or falling off the blade. Select the best sections for examination from those produced. The size of the section is immaterial, but it must be thin and not skewed. Arrange the sections in a row on a glass slide in a drop of water so that they do not overlap, and cover them with a coverslip. Having examined the slide under low magnification, select the thinnest region where epidermal cells with stomata are visible. Center this area in the microscope's field of view and switch to high magnification. Epidermal cells are clearly discernible on the slide. The outer epidermal wall is thickened, and pores are not visible on it. Adhering closely to the outer thickened epidermal layer is a glossy, highly light-refractive band—the cuticle. The outer contour of the cuticle is rough, while the inner contour is smooth. The cuticle covers the cell wall in a continuous layer, extending down into the stomatal crypts. Above the stomatal pore, the cuticle projects in the form of ridges.

By adjusting the optical distance with the fine adjustment knob, position the microscope objective so that the stomata, located in depressions well below the leaf surface, are clearly visible. Their walls are unevenly thickened. Those bordering adjacent epidermal cells are thinner, whereas the upper and lower walls are thicker. An anterior cavity (substomatal chamber/vestibule) is situated in front of the stomatal pore. A posterior cavity lies beneath the stoma, followed by an air space. Palisade parenchyma, consisting of living, thin-walled cells rich in chloroplasts, abuts the inner walls of the epidermal cells.

Sketch the slide after completing your observation.

Slide. Epidermis of Virginian spiderwort leaf (Tradescantia virginiana L.)

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Fig. 24. Tradescantia leaf epidermis:

1 - guard cells, 2 - leucoplasts, 3 - nucleus, 4 - chloroplasts, 5 - stomatal pore

Snap a fresh Tradescantia leaf from the upper side towards the lower and, gripping it with tweezers, peel off a piece of the epidermis, which separates very readily from the underlying tissues. Place the tissue fragment on a glass slide in a drop of water, outer side up, and cover with a coverslip. First, examine the slide under low magnification to locate the thinnest area, then switch the microscope to high magnification (Fig. 24).

Under these conditions, one can observe that the epidermis primarily comprises rectangular cells whose length is 4–5 times greater than their width. Interspersed among them are narrow, elongated cells whose length is 8–12 times greater than their width (located in the center of the leaf flanking the midrib). Stomata are clearly visible among the rectangular cells. The cells are alive, and their walls fit tightly against one another. They are filled with a watery, transparent, or purple-tinted content—the cell sap within the vacuoles. The Cells also contain nuclei with a granular structure; leucoplasts are situated adjacent to the nuclei. A thin layer of cytoplasm adheres closely to the cell wall and traverses the cell in all directions as protoplasmic strands. The walls of the epidermal cells are uneven, bead-like, perforated by pores, and feature weakly thickened regions. Chloroplasts are absent in the ordinary epidermal cells.

The narrow, elongated epidermal cells contain significantly less cell content, sometimes lacking it entirely. These standard narrow cells overlie and shield the leaf Veins.

Having examined the slide, you should make a drawing of it.

Slide. Cross-section of the leaf epidermis of Virginia spiderwort (Tradescantia virginiana L.)

Clamp a small piece of the leaf in a split elderberry pith or cork. Since stomata are located primarily on the lower side of the leaf, the section must be made to capture the lower epidermis. The surface from which the section is cut must be even so that the cells in the section are not slanted. Examine the slide first under low magnification and then under high magnification.

In the cross-section, locate and draw the cuticle, epidermis, guard cells, anterior and posterior chambers, air cavity, and palisade parenchyma situated beneath the epidermis. Pay special attention to the thickening of the guard cell walls, as it is precisely due to this uneven thickening that the stomata open when the cells become turgid with water.

Slide. Multilayered epidermis of the rubber fig leaf (Ficus elastica Roxb.).

There are plants whose epidermis consists of two or three layers of cells. The common fig is one such plant.

Fig. 25. Botryoidal cystolith of Ficus:

1 - three-layered epidermis; 2 - mesophyll;

3 - cystolith

Prepare a cross-section of a fig leaf. To do this, cut the fig leaf lengthwise into narrow strips, from which cross-sections are then made. When preparing the section, ensure that the upper epidermis is captured. After making several sections, place them side by side on a glass slide in a drop of water and, without covering with a coverslip, examine them under low magnification. Having selected the thinnest sections and discarded the thicker ones, cover the slide with a coverslip, making sure no air bubbles are trapped between the coverslip and the microscope slide. After examining the slide under low magnification, choose an area where both the epidermal cells and the outgrowth—the cystolith—are visible. Center this chosen area in the microscope field of view and switch to high magnification. Under the microscope, it is noticeable that the slide consists of two parts (Fig. 25)—the colorless epidermis and the leaf mesophyll.

The multilayered epidermis of the fig leaf is thicker on the upper side and thinner on the lower side. On the upper side of the leaf, the epidermis consists of three cell layers. In the outer epidermal layer, the cells are small, narrow, and thick-walled; In the second layer, they are larger; and the third layer consists of large, elongated cells that closely adjoin the leaf mesophyll. The epidermal cells of the fig are filled with colorless cell sap. In some cells, a small outgrowth resembling a bunch of grapes can be seen. This formation is an outgrowth of the upper wall of the epidermal cells known as a cystolith. As the cell wall grows, it forms a pouch-like outgrowth whose walls are impregnated with calcium carbonate. To verify this, place a piece of filter paper at one edge of the coverslip to draw out the water from underneath, and apply a few drops of acetic acid to the opposite edge. Observing the slide under the microscope, one can see that under the Influence of the acetic acid drawn under the coverslip by the filter paper, the calcium carbonate dissolves, causing the cystolith to decrease in size and the walls of the pouch to straighten out. Upon careful observation, it is easy to see carbon dioxide bubbles released as a result of the reaction between calcium carbonate and acetic acid. After some time, all the calcium carbonate dissolves, leaving only a narrow, elongated pouch in place of the cystolith.

The upper epidermal layer is covered with a thick cuticle, which gives the fig leaf a smooth and glossy appearance. Beneath the upper multilayered epidermis lies the leaf mesophyll, followed by the epidermis of the lower leaf surface. On the lower side of the leaf, stomata are located within special depressions.

After carefully examining the slide, you should draw it. Pay special attention to the accurate depiction of the cystolith and the correct proportion of cell sizes across the different epidermal layers.

To better differentiate individual structural elements, treat the slide with chlor-zinc-iodine. To do this, prepare a fresh section and place it in a drop of chlor-zinc-iodine. The Cellulose-composed walls of the epidermal cells stain blue-violet with chlor-zinc-iodine. After the dissolution of the calcium carbonate, the cystolith also stains blue-violet, while the cuticle stains yellowish-brown.

Slide. Leaf epidermis of American century plant (Agave americana L.)

Cut a small plate from the leaf surface with a knife and prepare cross-sections from it. Several sections should be made, examined under low magnification, and, after selecting the thinnest ones, studied under high magnification. Under the microscope, it is noticeable that the epidermis consists of fairly large cells elongated perpendicular to the leaf surface. The outer cell walls are thickened, while the lateral and inner walls are thin. The exterior of the epidermis is covered with a thick layer of cuticle, which extends onto the lateral cell walls in the form of sharpened wedges. The epidermal cells are living and contain cytoplasm, a nucleus, and chloroplasts, inside of which starch grains are visible. The stomata are located deep between the epidermal cells.

They consist of small guard cells in which the outer wall is thin, while the inner wall surrounding the pore toward the interior of the leaf gradually increases in thickness. The ends of this wall are heavily cutinized, forming ledges in front of the stomatal pore and at the exit from the pore into the air cavity.

To obtain a clearer, more vibrant preparation, treat it with chlor-zinc-iodine, which stains all cutinized parts brown, making them more visible. The cytoplasm and chloroplasts also turn brown. Meanwhile, the walls of the epidermal and mesophyll cells stain blue-violet, indicating their cellulosic nature. The agave leaf features a thick cuticle, deeply sunken stomata, and cutinized lateral cell walls. Such a structure is characteristic of plants growing in conditions of water scarcity.

Slide. Stem epidermis of cereal rye (Secale cereale L.)

Fig. 26. Wheat leaf epidermis:

1 - cell wall; 2 - pits;

3 - cytoplasm; 4 - nucleus; 5 - stomatal guard cells; 6 - subsidiary cells

Prepare several thin sections from the surface of the rye stem, taking care not to include the mechanical tissue. For better clarification of the slide, place the sections on a microscope slide in a drop of water or glycerin. Cover the slide with a coverslip, select the thinnest area under low magnification, and then examine and draw it in detail under high magnification (Fig. 26).

Epidermal cells are arranged in rows and vary in shape. Some rows consist of elongated, thick-walled cells with straight walls, while others also feature elongated cells, but with porous and wavy walls. Elongated cells with straight and wavy walls typically alternate with small, nearly square cells.

In grasses, stomata are always arranged in regular longitudinal rows among the elongated cells with wavy walls.

Slide. Structure of Stomata in Yellow Water-Lily (Nuphar luteum Smith.)

The most convenient specimen for studying the structure of stomata in floating aquatic plants is the leaf of the yellow water-lily. In this plant, stomata develop on the upper surface of the floating leaf. They are located near the ends of the main veins, where the leaf margin forms a small indentation.

To examine the structure of stomata in aquatic plants, sections are prepared, placed in a drop of water, and initially observed under low magnification. In general terms, the structure of stomata in aquatic plants is similar to that of terrestrial plants, but they are larger in size. The stomatal pore and air cavity in aquatic plants are filled with water rather than air. The guard cells of stomata in these plants die off early and become immobile, while the stomatal pore between them remains wide open.

Slide. Papillae on the Petals of Wild Pansy (Viola tricolor L.)

To study the structure of papillae, use thin scissors to cut a few fine ribbons from the magenta, purple, or yellow flower petals. Place the excised pieces on a dry microscope slide and examine them under low magnification. To get a better view of the papillae, gently scrape the edges of the section with tweezers; this will tear away some papillae, making those on the undamaged part of the section much easier to see. Under the microscope, it is evident that each epidermal cell produces a single such outgrowth. These outgrowths are filled with a cell sap uniformly pigmented with anthocyanin. Near the base of the petal, where it is colored yellow, the outgrowths are conical and resemble unicellular pillars with wavy lateral walls in their upper portions, containing chloroplasts inside the cells. To study the structure of the papillae in more detail, place a drop of water on a microscope slide and use a scalpel to gently scrape some papillae from the petal surface into the water. Then, cover the prepared specimen with a coverslip and examine it under the microscope, first at low magnification and then at high magnification.

Papillae always remain short and are not separated by walls from the epidermal cells from which they originate.

Slide. Structure of a Unicellular Leaf Trichome in Apple (Malus domestica Borkh.)

Using a scalpel, gently scrape some trichomes from a fresh, fixed, or dried apple leaf into a drop of water, tease them apart from one another, and examine them under low and high microscope magnification.

Under the microscope, it is noticeable that apple trichomes are unicellular, dead, and have a curved or bent shape. As a rule, they are either completely devoid of contents or contain only tiny granules. Cytoplasm and nucleus are absent. The cell wall of the trichomes is thin and transparent (Fig. 27, c).

Slide. Trichomes of Potato Leaves (Solanum tuberosum L.)

Trichome slides can be prepared from fresh, fixed, or dried leaves. Take a potato leaf and gently scrape trichomes from it with a scalpel into a drop of water on a microscope slide (Fig. 27, a). Straighten them with a dissecting needle and separate them, then cover with a coverslip and examine under the microscope. Potato trichomes resemble tiny threads, but they are shorter and straighter than those of the apple. In potatoes, trichomes are multicellular. Some cells lack cytoplasm; the cell walls are thin and colorless, with small bumps on the outside.

Slide. Trichomes of Mullein Leaves (Verbascum thapsus L.)

Fig. 27. Trichomes:

a - simple multicellular on a potato leaf; b - stellate on an oleaster leaf; c - simple unicellular on an apple leaf; d - branched multicellular on a mullein leaf

Mullein leaf slides are prepared using the method described above. Both fresh and fixed or dried leaves can be used. The prepared specimens are examined under the microscope (Fig. 27, d).

Mullein trichomes have a complex structure. Each features a slender stalk bearing several whorls of branches. The stalk consists of segments formed by cells whose length decreases from the base to the apex. Thus, the basal cell is the longest, and the apical one is the shortest. Beginning with the second cell from the base, each segment bears three branches arranged in a whorl; on the uppermost segment, there are up to ten. Consequently, under the microscope, one can observe a branching system that essentially constitutes a single trichome. The trichome cells are dead and possess thin, colorless walls. When studying trichomes on a mullein leaf, low microscope magnification must be used because each trichome is extensively branched and quite large.

Slide. Stellate Scales of Oleaster Leaf (Elaeagnus sp.)

Oleaster leaves are densely covered with whitish stellate scales that tightly overlap one another, giving the leaf a silvery appearance. Using a sharp scalpel, scrape some scales from the surface of a fixed or dried leaf, place them in a drop of water on a microscope slide, cover with a coverslip, and examine under the microscope first at low and then at high magnification. Observing the slide under high magnification reveals that the scales look like multi-rayed flat little stars (Fig. 27, b). The rays of the scales are either joined along their entire length or partially divided and converge at the center. Each ray represents a separate cell. These cells lack internal contents and are most frequently filled with air. The scales are attached to the leaf by very short stalks, which, like the scales themselves, are epidermal outgrowths. Preparing transverse sections of oleaster scales is very difficult because they tend to detach or lie flat on their sides. Such scales on oleaster leaves protect the leaf from excessive water evaporation and the harmful effects of sharp temperature fluctuations.

Slide. Stinging Trichomes of Nettle Leaves (Urtica urens L.)

The stinging trichomes of nettles have the most complex structure.

Fig. 28. Stinging trichome of a nettle leaf

Scrape hairs from a leaf placed right next to a drop of water on a glass slide using a scalpel directly into the droplet. Then, cover the specimen with a coverslip and examine it first under low and then under high magnification (Fig. 28).

Under the microscope, it can be observed that each hair is an elongated cell, widened at the base, situated on a group of small living cells known as the pedestal.

Hair cell of stinging nettle

is living and filled with protoplasm. The tip of the cell is rounded. The cell walls are impregnated with silica, making them extremely fragile. Upon contact, the hair breaks off obliquely just above its expanded base. The sharply pointed tip of the hair punctures the skin, injecting formic acid, which causes a burn.

Slide. Wax on aloe leaves (Aloe vera L.)

Take a young leaf and gently peel off the epidermis, taking care not to remove the bluish waxy bloom. Place a portion of the peeled epidermis under the microscope and examine it under low and high magnification. Here, one can observe that the surface of epidermal cells is covered with tiny scales and wax grains. These are responsible for the bluish hue, which is also characteristic of plum fruits. The formation of wax on the epidermal surface has an adaptive significance: it protects the epidermis from getting wet. This is particularly important for stomata, which in many plants are protected by a layer of wax. In some plants, the wax layer can reach a thickness of 5 mm, as observed in certain palms.

Slide. Periderm of black elder stem (Sambucus nigra L.)

Using a blade, cut several transverse sections from an elder branch, place them in a drop of water, cover with a coverslip, and examine under the microscope. Inspect the slide first under low and then under high magnification.

Located at the periphery is the epidermis, which is starting to peel off in places. Beneath the epidermis lie several layers of cork, whose cells are arranged in radial rows one above the other. Their walls are thin and suberized. These cells contain no living protoplast. The older the branch, the thicker the layer of cork cells will be.

Underneath the cork lies a strip of narrow cells with thin cellulose walls and living contents. This is the phellogen, or cork cambium. The lumen of the phellogen cells is filled with dense granular cytoplasm, making them appear dark and stand out distinctly in the section. A nucleus can often be seen within these cells.

Located beneath the phellogen is the phelloderm, which consists of typical living, rounded parenchymatous cells. Their cell walls are thin and cellulosic. Chloroplasts can often be observed in the phelloderm cells alongside cytoplasm.

After examining the slide, you should make a drawing of it.

Slide. Periderm of black elder stem (Sambucus nigra L.) in longitudinal section

Cut a small piece from an older elder branch, split it in half, and then use a sharp blade to make ultra-thin radial sections that include the epidermis. Place the prepared sections in a drop of water, cover with a coverslip, and examine under the microscope using low and high magnification. In longitudinal section, the cork cells are arranged in regular rows, but here they appear slightly elongated. The phellogen and phelloderm cells are also elongated. Thus, the periderm cells look like broad, slightly stretched parallelograms. They are thin in the cork, extremely thin (almost plate-like) in the phellogen, and thick with a convex outer wall in the phelloderm.

Slide. Lenticel of black elder stem (Sambucus nigra L.)

To study the structure of lenticels, take fixed two-year-old elder branches, locate the lenticels on them, and make a cut with a sharp knife directly above them. Then, prepare thin transverse sections passing through the middle of the lenticel. Transfer the prepared sections into a drop of water and examine them under the microscope (Fig. 29).

Fig. 29. Lenticel of elder in transverse section:

1 - filling tissue (complementary tissue), 2 - epidermal remnants, 3 - phellem (cork), 4 - phellogen, 5 - phelloderm, 6 - bast fibers, 7 - primary cortex parenchyma,

8 - ruptured closing layer of phellem

Examine the epidermis, multilayered cork, phellogen, phelloderm, and thick-walled collenchyma. The epidermis is ruptured above the lenticels. The inner part of the lenticel is filled with complementary cells that do not fit tightly against one another, leaving large intercellular spaces. Lenticels contain a cork cambium that originates from parenchymatous cells surrounding the air cavities beneath the stomata. The phellogen cells divide quite vigorously, forming the filling tissue of the lenticels. The complementary cells of lenticels are living.

After examining the slide, you should make a drawing of it.

Slide. Periderm of black currant stem (Ribes nigrum L.)

Take a fixed two- or three-year-old branch of black currant, cut off a small piece, and split it in half.

Prepare thin transverse sections from this piece, transfer them with a brush onto a glass slide in a drop of water, and examine under the microscope. Pay attention to the characteristic feature of the black currant periderm, which is that it forms in the deep layers of the primary cortex (Fig. 30).

Fig. 30. Periderm of black currant: 1 - cork (phellem), 2 - phellogen, 3 - phelloderm, 4 - collenchyma, 5 - epidermis, 6 - dead primary cortex

The periderm consists of a multilayered cork, cork cambium, and phelloderm, followed by the cortical collenchyma cells. The cells located above the cork are compressed, die off, and exfoliate.

A similar structure is characteristic of red and white currants. The prepared and examined slides must be drawn, and individual tissues labeled on the diagrams.

Slide. Periderm of birch stem (Betula sp.)

Take a birch branch aged 3 to 5 years, cut it into pieces, and place them in an alcohol-glycerin mixture to keep the material soft and elastic. Make a cross-section from such a piece, place it in a drop of phloroglucinol with hydrochloric acid to better differentiate the wood and phloem elements. After treating the slide, transfer it to a second glass slide in a drop of water or glycerin, cover with a coverslip, and examine under a microscope.

At the periphery of the slide, the protective tissue formed by The activity of the cork cambium, or phellogen, is clearly visible. This is the cork. The slide clearly shows annual rings in which wide, thin-walled cells formed by the cork cambium in spring alternate with flattened, thick-walled cells produced in autumn.

Examining the slide under high magnification reveals that the birch cork cells are filled with a granular content called the glucoside betulin. It has a white color and determines the white coloration of the birch trunk. Betulin is insoluble in water but soluble in alcohol. Remove the water from the slide using filter paper and add pure alcohol. After some time, the slide will begin to lighten as the betulin dissolves. In such a clarified slide, the alternation of thick-walled and thin-walled cork layers is noticeable. This explains why birch cork easily peels off in thin films, as the thin-walled cells rupture easily. Such cork is referred to as heterogeneous.

Slide. Rhytidome (bark) of black elder stem (Sambucus nigra L.)

Carefully cut a small piece (3-4 cm long) from a thick elder stem (approximately 5 cm in diameter) with a cracked, scaly surface, and gently split it into 8...10 parts, taking care not to damage the outer cork tissue. The pieces of wood should be placed for several days in a mixture of alcohol and glycerin, which impregnates them and makes them softer and less brittle.

The prepared material can be stored for several years.

Prepare several cross-sections of the elder piece so as to capture the outermost layer of the protective tissue, and place them on a watch glass in alcohol to remove air bubbles. Then, water is added drop by drop to the alcohol. This operation must be repeated several times until all the air is displaced. To avoid cutting through the entire width of the object, trim its end obliquely so that all the wood is removed, leaving only the cortex. Transfer the sections from the alcohol into a drop of water on a glass slide, cover with a coverslip, and examine under low magnification. Having selected the thinnest area under low magnification (Fig. 31), one can observe that the rhytidome consists of dead parenchymatous cells that fit quite tightly against each other.

Fig. 31. Rhytidome of elder in cross-section:

1 - rhytidome, 2 - periderm, 3 - dead cells of the primary cortex, 4 - living thin-walled cortical parenchyma

This dead parenchymatous tissue is intersected by the periderm, which is very easily distinguished by the arrangement pattern of the cork cells lying one above the other. When there are several periderms on the slide, the oldest one is located externally, while the youngest adjoins the living cortical parenchyma. Typically, the phellogen and phelloderm can also be distinguished in the youngest periderm.

Between the periderms lie dead thin-walled parenchyma and small areas of sclerenchyma—fibrous cells that are easily distinguished on the slide by their greatly thickened walls.

Having examined the slide, one should draw its general diagram, show the arrangement of individual elements, and switch the microscope to high magnification.

Under high magnification, the entire rhytidome does not fit into the microscope's field of view, and therefore the slide must be moved. By rotating the micrometric screw, examine the slide carefully and establish that its upper end contains the cork layers of the first periderm. The cork cells are dead, brownish, and arranged in regular columns one above the other. The cork cells are slightly compressed in the tangential direction, resulting in slightly wavy walls. The deeper-lying periderm cork cells have a similar structure, but the deeper the periderm lies, the younger it is, and the thinner and lighter its cell walls are. The cortical cells lying between the periderms are dead, flattened, and sometimes even ruptured; their walls are brown, and their contents either dry up completely (rendering them empty) or retain only remnants of dry brownish matter inside the cell.

After examining the slide carefully, make a drawing of it, labeling its individual constituent parts.

A color reaction can be applied to identify individual elements of the cork. For instance, when the slide is treated with chlor-zinc-iodine, the cell walls of the epidermis, inner cork layers, phellogen, phelloderm, and collenchyma stain violet-blue. Elements such as the cuticle, outer cork layer walls, and inner thickenings of the epidermal walls stain yellowish-brown. Cells containing starch turn blue, while the internal cell contents stain yellow.

Slide. Rhytidome of small-leaved linden stem (Tilia cordata Mill.)

Both fresh and fixed material can be used for research. Cut small blocks of bark from an old linden tree, split them into thin pieces, and keep them for 2-3 weeks

in a mixture of glycerin and alcohol. This is necessary so that the bark softens and does not crumble during sectioning. Make several thin cross-sections from the bark pieces, place them back in glycerin on a glass slide, cover with a coverslip, and examine under low magnification. Having selected the thinnest spot under low magnification, one can study the structure of the rhytidome under the microscope (Fig. 32).

Fig. 32. Cross-section of linden (lime tree) bark:

1 - primary medullary rays, 2 - secondary hard bast (phloem fibers), 3 - secondary soft bast, 4 - secondary medullary rays, 5 - periderm

Here, one can observe that the main layers of the bark consist of brownish dead tissue, through which primary medullary rays are visible. Under the microscope, it is clear that they are shaped like triangles, with their sharp ends pointing toward the center and expanding significantly toward the periphery. Between the medullary rays, strips of secondary phloem are clearly distinguishable, subdivided into Hard and Soft phloem, which also form triangles, but inverted—with their wide bases facing the center and sharp ends pointing outward. The secondary phloem is radially intersected by densely arranged secondary medullary rays. Areas of thin-walled and thick-walled cells alternate with one another.

Running almost tangentially through the bark are dark brown strips with an even outer edge and an inner edge forming projections that cross the medullary rays and phloem bundles. These are the periderms. Together, the periderms, medullary rays, and phloem bundles make up the bark. Thus, in the linden tree, the bark incorporates deep-seated tissues such as the secondary phloem. After examining the slide under low magnification, one should draw a general diagram of its structure. Then, positioning the thinnest section of the preparation in the center of the field of view, switch to high magnification to study in detail the structure of the tissues that form the linden bark. Under high magnification, one can see that the medullary rays consist of square or slightly radially elongated cells. Some of these cells are empty, while others are filled with a fine-grained content. Calcium oxalate crystals can frequently be observed within the cells. The cell walls here are thin and brownish in color.

A characteristic feature of the secondary phloem is that it is composed of heterogeneous cells, some having thickened walls and others very thin ones. This thickening can be so pronounced that, under the microscope, the cell lumen appears merely as a dark speck.

The cells of the soft phloem are thin-walled. They may be in The process of dying off: some still retain living contents, while others are already dead. Dead cells are either empty or contain a brownish mass.

Within the periderm strips, the cork is clearly distinguishable, characterized by its typical cellular arrangement. The cork cells are filled with a dark brown content.

In the linden tree, the periderm develops from living cells of the soft phloem. Consequently, in certain areas of the slide, it can be seen surrounding the hard phloem and, turning to the right or left, passing through the medullary ray. In younger periderm tissue adjacent to the center, the phellogen and phelloderm can also be distinguished.

To better differentiate the individual elements of the preparation, a new section should be prepared and treated with a phloroglucinol-hydrochloric acid solution. To do this, place a drop of phloroglucinol on a clean glass slide, transfer the test section into it, and add a drop of hydrochloric acid. Even to the naked eye, the preparation turns pink. The stained section should then be transferred to a drop of water, covered with a coverslip, and examined under a microscope. The slide must not be examined directly where the staining took place, as hydrochloric acid vapors have a corrosive effect on the microscope's optical system.

All elements of the bark, with the exception of the cork and soft phloem, turn red upon Treatment with phloroglucinol because they are lignified.

After careful examination of the preparation, make a drawing highlighting the characteristic structure of all tissue elements that make up the linden bark.

Slide. Bark of the sour cherry stem (Cerasus vulgaris Mill.)

Using a sharp knife, slice the bark from the lower part of a multi-year-old trunk, cut it into small blocks, and soak them in an alcohol-glycerin mixture for two to three weeks to completely remove air. Alternatively, slides can be prepared from fresh, freshly cut bark.

Prepare several thin cross-sections from the collected material, encompassing the outer part of the bark, place them on a glass slide in a drop of glycerin or water, and cover with a coverslip. Such preparations are first examined under low magnification and then under high magnification. The bark of the cherry tree (Fig. 33) consists of dead parenchymatous tissue, in which some cells are empty while others are filled with a yellowish-brown content. The parenchyma is intersected at several points by strips of cork, which consist of numerous rows of cork cells.

Fig. 33. Cross-section of cherry tree bark:

1 - dead parenchymatous cells, 2 - cork cells