Plant Anatomy: A Practical Guide - Paniuta O.O. 2019

Topic 2. Tissues
Laboratory Work No. 7. Mechanical Tissues

Theoretical Background. Mechanical (or supporting) Tissues provide structural strength to individual plant Organs and protect the plant against various mechanical stresses, such as compression, bending, tension, tearing, and breaking. There are two primary types of mechanical tissues: collenchyma and sclerenchyma. Various Cell types can differentiate into mechanical tissues.

A common feature of all mechanical tissues is the significant and diverse thickening of their cell walls, which undergo complex chemical modifications.

When all cell walls thicken uniformly and become lignified, The Cell protoplast dies, forming a dead mechanical tissue known as sclerenchyma. Sclerenchyma Cells are prosenchymatous with tapered ends, fitting tightly together. Their Cell walls contain a small number of slit-like pits. Sclerenchyma elements are typically found in plant organs that have completed their longitudinal growth. Examples of such sclerenchymatous elements include bast fibers and wood fibers, or libriform fibers.

In actively growing organs, a second type of mechanical tissue is prevalent: collenchyma. Collenchyma cells are living and retain the capacity for growth. Their cell walls are unevenly thickened but remain cellulosic throughout their lifespan. The pattern of wall thickening in collenchyma cells varies. Thickenings may develop at the corners where adjacent cells meet, forming angular collenchyma; alternatively, two opposite cell walls may thicken to form lamellar (plate) collenchyma. A third, intermediate form is angular-lamellar collenchyma, and the fourth is lacunar collenchyma, characterized by thickenings adjacent to intercellular spaces.

It should be noted, however, that transitional forms frequently occur among these collenchyma types, making exact Classification into a single category difficult at times.

All of these mechanical tissue cell types, in specific combinations with other living cells, form the plant's structural Skeleton, providing mechanical strength and resilience against environmental factors.

Not all of these mechanical elements develop simultaneously within the plant. The Nature of mechanical cells changes as the plant ages, correlating with specific developmental stages. For instance, in young plants prior to the onset of meristem differentiation, mechanical cells are entirely absent. Structural support and rigidity in young plants are maintained by turgor pressure. As the plant develops and turgor alone becomes insufficient to provide adequate support and strength, mechanical tissues begin to form in young, growing organs—initially collenchyma, followed later by sclerenchyma.

Sclerenchyma is characterized predominantly by dead cells with heavily thickened, lignified cell walls. The cell contents disappear, leaving the cells typically empty. Sclerenchyma cell walls exhibit distinct stratification and are permeated by numerous pits.

To identify the lignification of sclerenchymatous cell walls, one must utilize the property of lignified walls to change color when exposed to specific Reagents. For example, Treatment with phloroglucinol and Hydrochloric acid stains lignified cell walls a bright cherry-red, while aniline sulfate imparts a bright yellow color.

Objective: to study the Structural Features of mechanical tissues.

Materials and equipment: light microscopes, Glass slides and coverslips, dissecting needles, forceps, glass stirring rods, filter paper, distilled Water, phloroglucinol with hydrochloric acid, aniline sulfate, chlor-zinc-iodide, chromic acid, 20% potassium hydroxide solution, phloroglucinol, hydrochloric acid, plant material.

Slide preparation. Angular collenchyma in the stem of pumpkin (Cucurbita pepo L.)

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Fig. 34. Angular collenchyma in a pumpkin stem

Take a preserved pumpkin stem and use a razor blade to make a thin cross-section, ensuring the cut passes through a stem rib. Place the section in a drop of water on a glass slide, cover with a coverslip, and examine under a Microscope at low magnification to locate the peripheral region and observe the groups of collenchyma cells. They are very easy to distinguish. Because collenchyma cells have corner thickenings that strongly refract light, they appear under the microscope as gleaming spots. Typically, collenchyma cells are arranged in small patches beneath the stem rib. Center the desired area in the field of view and examine it under high magnification (Fig. 34).

The preparation clearly reveals the epidermal cells, beneath which lie the collenchyma cells. Granular Cytoplasm and occasionally METABOLISM/14.html">Chloroplasts are visible inside the cells along the inner wall. The collenchyma cells fit closely together.

After examining the slide, make a drawing of it. Then, add a drop of chlor-zinc-iodide to the preparation, which will stain the collenchyma cell walls blue-violet. This indicates that their walls are composed of Cellulose.

Slide preparation. Angular collenchyma in the begonia leaf petiole (Begonia sp.)

Take a small piece of begonia petiole, flatten the surface neatly, and make several thin cross-sections, making sure to include the epidermis, as the collenchyma lies directly beneath it.

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Fig. 35. Angular collenchyma in a begonia leaf petiole:

1 - epidermis, 2 - collenchyma, 3 - cytoplasm, 4 - chloroplasts, 5 - Nucleus

Sections should not be cut across the entire stem surface, but rather through a portion of it, as making a thin section across the entire organ is quite difficult. Using a small brush, transfer a section from the razor blade into a drop of water on a microscope slide. Select the thinnest of the prepared sections, cover them with a coverslip, place the slide on the microscope stage, and examine under low magnification. By moving the slide, locate the peripheral region of the section, center it in the field of view, and then examine the preparation at high magnification.

The peripheral part of the section (Fig. 35) shows the epidermis, which consists of flattened cells fitting tightly together.

Beneath the epidermis, there are 5–7 rows of small-celled tissue, in which triangular thickenings are clearly visible at the cell corners where the cells Touch one another. This is the collenchyma. It forms a cylinder in the petiole that surrounds all the deeper-lying tissues. The thickenings in the collenchyma cells appear as shiny spots due to their high light refraction. Collenchyma cells are living; their cytoplasm, which fits snugly against The Cell wall, chloroplasts, and—in some—The Nucleus, are clearly visible. When examining the slide, one should adjust the micrometer screw of the microscope to clearly view all the cell components located in different planes.

After examining and drawing the slide, use a small piece of filter paper to remove the water from under the coverslip and place a drop of chlor-zinc-iodine onto the microscope slide. The walls of the collenchyma cells stain blue-violet. This indicates that both the thin cell walls and the formed thickenings consist of cellulose.

Slide. Angular-lamellar collenchyma in the stem of goutweed (Aegopodium podagraria L.)

This intermediate form of collenchyma is characterized by the fact that cell thickenings develop not only at the corners but also along most of the cell walls. Only small portions of the cell walls remain unthickened. The stem of goutweed serves as a convenient subject for studying angular-lamellar collenchyma.

In the goutweed stem, the angular-lamellar collenchyma is located in the ridged area; therefore, when preparing a section, it is essential to capture several stem Ribs, though a full cross-section of the entire stem is not strictly necessary. After making several thin cross-sections, use a brush to transfer the best ones from the blade into a drop of water on a microscope slide, and cover with a coverslip. First, examine the prepared slides under low magnification. Position the slide so that one of the stem ribs is in the center of the microscope's field of view. Even at low magnification, collenchyma is easily distinguished from other tissues because its thickened walls strongly refract light and appear very glossy. For a more detailed study, examine the slides carefully under high magnification. By turning the micrometer screw, one can see that the collenchyma cells fit tightly together and have thickened walls not just at the corners, but along almost their entire length (Fig. 36, A).

Fig. 36. Angular-lamellar collenchyma in the stem of goutweed:

A - in cross-section: 1 - epidermis, 2 - collenchyma, 3 - oil duct, 4 - trichome; B - in longitudinal-radial section: 1 - epidermis, 2 - parenchyma, 3 - collenchyma, 4 - cytoplasm, 5 - nucleus

Collenchyma cells are living, and fine-grained cytoplasm is visible within them. Nuclei and Organelles are not always distinct.

By carefully scanning the entire slide while moving it under the objective, one can spot a small cavity lined with thin-walled epithelial cells among the collenchyma cells. This is secretory tissue. The epithelial cells secrete Essential Oils that accumulate in the cavity, which forms a duct.

Make a detailed drawing of the slide, starting with the epidermis to clearly show that the collenchyma is located directly beneath it. To examine the Nature of the thickenings in the collenchyma cell walls, remove the water from under the coverslip and replace it with a drop of chlor-zinc-iodine, which stains the cell walls blue-violet. This proves that the cell walls are composed of cellulose.

Slide. Collenchyma cells in the stem of goutweed (Aegopodium podagraria L.) in longitudinal radial section

Cut the goutweed stem lengthwise in half, and then make longitudinal sections from the exposed surface, making sure to capture the edge of the stem with its rib. For convenience when cutting longitudinal sections, slice off small pieces at the ends of the stem to create stepped levels. Place the thinnest sections in a drop of water on a microscope slide, cover with a coverslip, and examine first under low and then under high magnification. Under high magnification, it is evident that the collenchyma cells (Fig. 36, B) in the longitudinal section are so elongated that they do not fit entirely within the field of view. Therefore, the slide must be moved gradually to observe the cells in their entirety. The longitudinal cell walls are heavily thickened, while the transverse walls are thin and positioned obliquely. This arrangement of individual cells provides greater tissue strength. By adjusting the micrometer screw, one can observe the parietal layer of the cytoplasm and the nucleus elongated along the wall. To better differentiate the individual PARTS OF THE collenchyma cells, remove the water from beneath the coverslip and replace it with a drop of chlor-zinc-iodine; under its influence, the cell walls stain blue-violet, the cytoplasm yellow, and the nucleus yellow-brown.

Be sure to draw a small area of the examined slide, labeling its individual components.

Slide. Structure of sclerenchyma in the stem of pumpkin (Cucurbita pepo L.)

The pumpkin stem is a convenient subject for studying sclerenchyma tissue.

Cut several thin cross-sections from a small piece of fresh or preserved pumpkin stem, ensuring that the peripheral part of the stem is captured. Gently remove the thinnest sections from the blade with a brush, place them in a drop of water on a microscope slide, cover with a coverslip, and examine the slide under low magnification on the microscope stage. On the periphery of the section, one can see the epidermis with trichomes, beneath which lie distinct areas of collenchyma cells. Several layers of parenchyma cells lie under the collenchyma, followed by several layers of thick-walled, straw-yellow cells. This is the sclerenchyma. By moving the slide so that the sclerenchyma is in the center of the field of view and switching to high magnification, one can determine that the sclerenchyma cells are slightly transversely elongated, thick-walled, and tightly packed; their walls clearly show stratification and pore canals (Fig. 37).

Fig. 37. Sclerenchyma in the pumpkin stem.

To verify that the walls of the sclerenchyma cells are lignified, remove the slide from the microscope stage, take off the coverslip, and apply a drop of aniline sulfate or phloroglucinol with hydrochloric acid to the section.

After staining, transfer the section into a drop of water on a clean microscope slide, cover with a coverslip, and examine under the microscope again. If the cell walls of the sclerenchyma in the pumpkin stem are lignified, they will stain yellow with aniline sulfate, and red with phloroglucinol and hydrochloric acid. Following treatment with these reagents, the unthickened areas of the cell wall—the pits—will be particularly distinct. Draw the area containing the sclerenchymatous cells. Upon completion of the work, carefully wipe the objectives and the microscope stage with a dry, soft, clean cloth to remove any residue of the reagents used.

Slide. Sclerenchymatous fibers of the begonia leaf petiole (Begonia sp.)

Take a begonia leaf petiole, trim the surface evenly, and make several thin cross-sections. When preparing the section, ensure that internal tissues are captured. However, it is not necessary to cut across the entire surface of the stem.

Note. Since the begonia leaf petiole is quite succulent, use a sharp blade and do not press down on the tissues while cutting, otherwise the section will be crushed and difficult to examine.

Make several sections so that the best ones can be selected. Carefully remove the resulting sections from the blade using a brush, place them in a drop of water on a microscope slide, and cover with a coverslip. Then, locate the areas of sclerenchymatous fibers under low magnification and examine them under high magnification.

Fig. 38. Sclerenchyma fibers in a begonia leaf petiole:

A - transverse section: 1 - parenchyma cells, 2 - sclerenchyma fibers; B - longitudinal section: 1 - sclerenchyma fibers, 2 - parenchyma cells

The begonia leaf petiole consists of parenchymatous cells of various sizes (Fig. 38, A). The walls of the parenchyma cells are thin and colorless. In some cells, cytoplasm, a nucleus, chloroplasts, and isolated calcium oxalate crystals and druses can be observed. Scattered throughout the parenchyma are polyhedral, thick-walled cells known as sclerenchyma fibers. Sclerenchyma cells lack internal contents, and their cell walls clearly exhibit layering resulting from progressive lignification.

To verify that the walls of the sclerenchyma fibers have lignified, remove the slide from the microscope stage, lift the coverslip, and apply a drop of aniline sulfate or phloroglucinol with hydrochloric acid to the section. Using a brush, transfer the treated sections to a second microscope slide in a drop of water, cover with a coverslip, and examine again under the microscope. Lignified cell walls will turn yellow with aniline sulfate and red with phloroglucinol and hydrochloric acid. To observe starch grains more clearly, place one of the sections in a drop of chlor-zinc-iodine and examine it under the microscope. Treatment with chlor-zinc-iodine turns starch grains blue-violet.

Slide. Sclerenchyma fibers in a begonia leaf petiole (Begonia sp.) in longitudinal section

Cut a small piece of the begonia leaf petiole in half with a scalpel and trim small pieces of tissue transversely at the ends so that the area to be sectioned is slightly raised. Alternatively, instead of cutting the petiole in half, one can simply remove the surface tissue layer and take sections from deeper layers.

Prepare several thin sections. Carefully lift some of them from the blade with a brush, place them in a drop of water on a microscope slide, cover with a coverslip, place the slide on the microscope stage, and examine under low magnification. Place the remaining sections first in a drop of aniline sulfate, and then transfer them to a clean microscope slide in a drop of water.

In the unstained preparation, sclerenchyma fibers have a grayish-greenish hue. They are elongated and very often extend beyond the field of view (Fig. 38, B); therefore, to determine the fiber length, one must carefully move the slide. Individual fibers are scattered among the parenchyma cells and thus do not hinder the growth of the leaf petiole. Their walls are thickened, with clearly visible layering and pit canals. The ends of sclerenchyma fibers may be pointed, blunt, or sometimes forked.

Examine the preparation treated with aniline sulfate or phloroglucinol with hydrochloric acid in the same manner as described above. In stained preparations, pit canals are clearly visible, and the fibers themselves stand out distinctly from other cells.

Draw The structure of the sclerenchyma fibers and their arrangement within the leaf petiole.

Slide. Sclerenchyma in the stem of the meadow geranium (Geranium pratense L.)

Fig. 39. Sclerenchyma in the geranium stem:

A - transverse section: 1 - primary cortex cells, 2 - sclerenchyma, 3 - ground tissue; B - longitudinal section: 1 - primary cortex cells, 2 - sclerenchyma

Prepare several transverse sections of the geranium stem, ensuring the peripheral tissues are captured. Place some of the resulting sections on one microscope slide in a drop of water, and others on a second microscope slide in aniline sulfate or phloroglucinol with hydrochloric acid. Cover the sections lying in the drop of water with a coverslip and examine them first under low and then under high magnification. Once the desired area is located on the slide, study it under high magnification. If the outer tissues were captured during sectioning, the preparation will reveal a single-layered epidermis, beneath which lie the thin-walled parenchyma Cells of the primary cortex, followed by a layer of mechanical tissue—the sclerenchyma (Fig. 39, A). The sclerenchyma encircles the stem in a continuous ring and consists of thick-walled, polyhedral, shiny cells that fit tightly together. The Cell size and wall thickness are not uniform. Cells closer to the periphery are larger and have thinner walls compared to those located nearer the center of the sclerenchyma tissue. The cell walls of the sclerenchyma strongly refract light and are easily distinguished from other cells by their glossy appearance.

Pit canals run through the cell walls, connecting with the pit canals of adjacent cells. The cells under examination lack internal contents and are empty; thus, this is dead tissue.

The ground parenchyma lies beneath the sclerenchyma.

After carefully examining the preparation, remove it from the microscope stage, prepare a new slide using the sections treated with aniline sulfate or phloroglucinol, and examine it under the microscope. The sclerenchyma cell walls stain yellow with aniline sulfate or red with phloroglucinol and hydrochloric acid, indicating that they are fully lignified. By carefully moving the slide, one can observe that the cells of the thin-walled cortical parenchyma have also undergone lignification.

The slide can also be treated with chlor-zinc-iodine, which turns lignified walls dark brown, non-lignified walls blue-violet, and the cytoplasm yellowish-brown. If starch is present in the cells, it stains blue.

Slide. Sclerenchyma cells in the stem of the meadow geranium (Geranium pratense L.) in longitudinal section

Prepare several thin radial or tangential sections. Carefully remove some of them from the blade with a brush, place them in a drop of water, cover with a coverslip, and examine first under low and then under high magnification. Place a portion of the sections on a second microscope slide in a drop of aniline sulfate or phloroglucinol with hydrochloric acid and leave them for a few minutes while examining the sections in the water drop.

The sclerenchyma cells in the geranium stem are elongated with pointed ends that overlap one another (Fig. 39, B). The cell walls are heavily thickened and pierced by pit canals, which appear as thin transverse lines in longitudinal section. After examining the section in the drop of water, remove it from the microscope stage and prepare a slide from the sections treated with phloroglucinol and hydrochloric acid or aniline sulfate. Transfer the sections to a new slide in a drop of water, cover with a coverslip, and examine them under the microscope as well. The sclerenchyma cell walls stain yellow (with aniline sulfate) or red (with phloroglucinol and hydrochloric acid). Consequently, the cell walls are lignified and the cells are dead.

Make a drawing of the STRUCTURE OF THE sclerenchyma cells.

Upon completion of the work, thoroughly wash the microscope slides and coverslips and wipe the microscope stage clean with a cloth. Always remember that reagents such as aniline sulfate and phloroglucinol with hydrochloric acid have a damaging effect on optical components.

Slide. Bast fibers of common flax (Linum usitatissimum L.)

Bast fibers provide mechanical strength to the plant and are of great practical importance for manufacturing various industrial fabrics. The typical structure of sclerenchymatous bast fiber is best observed in common flax. Each fiber is a highly elongated cell with tapered ends and uniformly thickened walls. The cell lumen almost disappears.

Typically, bast fibers are arranged in groups within the plant, with individual fibers glued together by intercellular substance.

Therefore, to examine the structure of an individual fiber, it is first necessary to dissolve the intercellular substance, i.e., to macerate the tissue. To do this, place the test object in a test tube with a 10% chromic acid solution and boil for 3...5 min. Alternatively, a 20% potassium hydroxide solution can be used, boiling the plant material in it for 3...6 min. After boiling, the material must be thoroughly rinsed first with water and then with alcohol. For maceration, take the cortical part of the stem, stripping it off with tweezers. The rinsed material is stored in alcohol.

To prepare the slide, take a piece of macerated tissue, mash it between your fingers, and place it in a drop of water on a microscope slide. To better separate individual fibers, the material should be carefully teased apart using dissecting needles. Covering the preparation with a coverslip, examine it first under low and then under high magnification.

The bast fiber is so elongated that it does not fit within the microscope's field of view. The ends of the fiber are thin and pointed. The lumen is preserved only in the central part of the fiber and disappears completely at the tapered ends.

Remnants of cytoplasmic content can be seen within the lumen of the bast fiber. The cell walls are cellulosic, pierced by slit-like pits.

Note. A slide to study bast fibers in hemp can be prepared in the same way. It differs from flax fiber by its shorter length, thicker walls, and blunter ends.

Periwinkle or nettle can also be used to study the structure of bast fibers. Periwinkle is convenient because fibers can be isolated from it without prior maceration. The walls of bast fibers in these plants are cellulosic and stain blue-violet with chlor-zinc-iodine.

Slide. Stone cells (sclereids) of the common pear fruit (Pyrus communis L.)

Changes in the cell wall associated with the mechanical Functions of the cell are most pronounced in stone cells. These changes are particularly noticeable in the stone cells of pear flesh. When a pear is cut, yellowish, hard, stone-like granules are visible in its flesh even to the naked eye. These are clusters of stone cells, which are especially abundant in wild pear fruits. To examine THE POSITION OF stone cells among the flesh, make a section of it and prepare a slide. Stone cells are gathered in dense groups, surrounded by large, sac-like, thin-walled living flesh cells filled with cell sap (Fig. 40, A). Thus, stone cells act as support points for the thin-walled cells.

The slide should be carefully examined and sketched.

Fig. 40. Stone cells:

A - stone cells of pear; B - stone cells of horseradish; C - stone cells of plum pits

Next, isolate a single granule from the pear flesh, place it in a drop of water on a microscope slide, and crush it with a scalpel so that it breaks down into individual stone cells. Having dispersed the crushed granule in the drop of water, cover the preparation with a coverslip and examine it first under low and then under high magnification.

Selecting an area with individual cells on the slide, one can observe that their shape is rounded or elongated, the walls are heavily thickened, and the cell lumen is very small. Canals extend from the cell lumen through the entire wall, branching at their ends. Through these canals, which represent unthickened areas in the wall, neighboring cells communicate with one another.

After examining individual cells, locate an area on the slide where several unseparated stone cells are present. A careful study of these cells reveals that the pit canals of one stone cell align with the canals of the adjacent cell (Fig. 40, A).

To determine what chemical changes have occurred in the walls of the pear flesh stone cells, remove the slide from the microscope stage, lift the coverslip, and add a drop of phloroglucinol to the microscope slide, followed by hydrochloric acid. After a few minutes, transfer the granules to a clean microscope slide in a drop of water, cover with a coverslip, and examine under the microscope. The cell walls stain a bright cherry-red color, indicating that they are lignified.

Note. Stone cells can also be observed in quince fruits.

Slide. Stone cells from the horseradish ROOT (Cochlearia armoracia L.)

Take a horseradish root and make several cross-sections so as to capture the outer part of its cortex.

Some of the sections are placed in a drop of water, some are treated with chlor-zinc-iodine, and the rest with phloroglucinol and hydrochloric acid or aniline sulfate.

The preparations are first examined under low magnification. In the cortex, A large number of loosely arranged thin-walled rounded cells can be seen, among which thick-walled stone cells are scattered individually or joined in small groups (Fig. 40, B).

Having examined the slide under low magnification, switch the microscope to high magnification, positioning the slide so that the stone cells are in the center of the field of view. The walls of the stone cells are greatly thickened, and their transverse layering is clearly visible (Fig. 40, B). Due to such wall thickening, the cell lumen becomes very small, and the cell contents disappear. The cell walls are pierced throughout their thickness by simple or branched canals. As a rule, the pits of one cell align with the pit canals of another, through which the lumens of adjacent cells communicate. The pit canals of neighboring cells are separated only by a thin primary wall.

After sketching the studied slide, remove it from the microscope stage and examine the sections treated with chlorzinc iodine. Even under low magnification, it is evident that the walls of the loosely connected parenchymal cells have turned blue-violet, indicating their cellulosic composition.

Take a clean glass slide, place a drop of water on it, and transfer the sections previously treated with phloroglucinol and hydrochloric acid or aniline sulfate. Cover the sections with a coverslip and examine them under the microscope. Thin-walled parenchymal cells remain unchanged, whereas stone cells turn red with phloroglucinol and hydrochloric acid, and yellow with aniline sulfate, which indicates their lignification.

Slide. Stone cells of the plum pit (Prunus domestica L.)

Take the hard shells of plum pits, grind them thoroughly in a mortar, treat the resulting powder first with sulfuric acid, and then wash it thoroughly with water. Store the material prepared in this way in glycerin. During the practical session, use a dissecting needle to take a small amount of the prepared mass and disperse it on a glass slide in a drop of water. Cover the preparation with a coverslip and examine it first under low and then under high magnification.

The stone cells of the plum pit have heavily thickened walls; they are slightly elongated and appear somewhat flattened. The cell lumen is also elongated in the same direction as the walls and is greatly reduced. The cell contents disappear, and the lumina become filled with air, thus appearing dark (Fig. 40, B). The cell walls are pierced by pits throughout their entire thickness. Treating the slide with phloroglucinol and hydrochloric acid or aniline sulfate confirms that they are lignified.