Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
Laboratory Class Topic 5.10. Mechanical Tissues

General Remarks. Mechanical Tissues play a supportive role in the plant Organism and are therefore frequently referred to as strengthening or Skeletal Tissues. Their defining feature is the thickening of The Cell walls. Depending on The Nature of this thickening, Three types of mechanical tissues are distinguished: collenchyma, with partial thickening of The Cell wall; sclerenchyma; and sclereids, or stone Cells, with uniformly thickened walls. Furthermore, the former are formed by prosenchymatous cells, whereas the latter are formed by parenchymatous cells. Both of the last Two Types of mechanical tissue are dead at maturity, possessing thickened cell walls around their entire perimeter that are impregnated with Lignin. This grants these tissues high strength and elasticity.

Objects:

1. Beet leaf petiole (Beta vulgaris L.)

2. Flax bast fibers (Linum usitatissimum L.)

3. Wild or European pear fruit (Pyrus communis L.)

Tasks:

1. Examine the Structural Features of angular collenchyma using a self-prepared slide of a beet petiole.

2. Examine the structural features of sclerenchyma using a prepared slide and handout material of bast fibers.

3. Examine the structural features of sclereids using a self-prepared slide of a wild pear fruit.

4. In your laboratory notebook, draw the structural features of various types of mechanical tissues from cultivated and wild plants, and label their component parts.

Equipment and Materials: MBR-1 microscopes, Biolam microscopes, magnifying glasses, razors, scalpels, blades, prepared slides.

Procedure for preparing the collenchyma slide from a beet leaf petiole. Place a Glass slide transversely across a pencil case and apply a drop of Water or an iodine-potassium iodide solution onto it. Take a piece of the sliced beet petiole in your left hand and a razor in your right, and trim The surface of the petiole. Make a series of cross-sections of the leaf petiole. When making a section, it is crucial that it is cut strictly perpendicular to the axis of the leaf petiole and that it necessarily includes the peripheral region where the epidermis and collenchyma are located. The section must be as thin as possible. The mechanical tissue is concentrated in the ribbed protrusions; therefore, this exact area must be captured when making the sections. Using a magnifying glass, select several sections that meet these criteria and place them into the drop of water or solution. Cover the specimen with a coverslip and remove any excess moisture using filter paper. Secure the slide with clips on the Microscope stage.

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Microscopic examination of the beet petiole collenchyma slide. Under low magnification of the microscope, locate the mechanical tissue—collenchyma—along the ridges beneath the epidermis. It is easily recognized, appearing checkerboard-like on the slide. Continue a more detailed Study of the collenchyma under high magnification. You will observe that the collenchyma is formed by parenchymatous cells characterized by partial cell wall thickening. The thickenings are visible at the corners, which is why this type of collenchyma is named angular collenchyma. Collenchyma cells are living: they contain Cytoplasm in the form of a granular peripheral layer, a Nucleus located within the cytoplasm, green METABOLISM/14.html">Chloroplasts, and a large vacuole occupying the major part of the cell. Pay attention to the fact that the cell wall thickens only at the corners. These secondary thickenings are deposited on the primary wall, which appears as a thin silvery plate reflecting the polygonal shape of the cells before The formation of the collenchyma is fully complete (Fig. 32).

Make a close-up drawing of the collenchyma and label the indicated elements.

Microscopic examination of the flax bast fibers slide. Under low and high microscope magnification, examine the slide of the flax stem cross-section and study the structural Features of the sclerenchyma. It adjoins the starch sheath, or endodermis. Bundles of bast fibers consist of 12–28 cells, or elementary fibers, united in a bundle by middle lamellae that lignify in places and form lignification nodes. In cross-section, the bast fiber cells have regular outlines—round or polygonal—and are tightly packed into bundles. The cells have heavily thickened cell walls around the entire perimeter. By adjusting the fine adjustment knob, one can observe the stratification of the cell wall and the narrow lumen of the cell cavity. Remnants of the protoplast are often present within the cell cavity.

In longitudinal section slides, bast fibers appear as long prosenchymatous cells with tapered ends. Due to this, the cells wedge between one another, achieving a high contact area that imparts strength to the tissues. The cells, or elementary fibers, possess heavily thickened cell walls in which transverse pore canals are visible. A longitudinal lumen is noticeable in the middle, and remnants of the protoplast are sometimes visible (Fig. 33).

Procedure for preparing the pear fruit sclereid slide. Place a drop of water on a glass slide. Extract nut-like stony formations from the flesh of the pear fruit, place several of them between two glass slides, and grind them into a powder by pressing the slides together and rubbing them against each other. Take a portion of this mass and place it into the drop of water on the glass slide. Cover the specimen with a coverslip. Place the slide prepared in this manner onto the microscope stage and secure it with clips.

Microscopic examination of the slide. First under low and then under high microscope magnification, examine and study the structural features of the sclereids (stone cells) of the pear fruit; they are easily spotted in the field of view: in appearance they are silvery in color, gathered in groups of 8–20 cells of polygonal, often elongated parenchymatous shape. Their characteristic feature is the excessive thickening of the cell wall around the entire perimeter. When manipulating the fine adjustment knob, stratification becomes quite clearly visible within the cell walls. The cell wall is not continuous; in certain places, it is interrupted by numerous pore canals that connect with those of adjacent cells. The central part of the cell is occupied by a dead protoplast in young cells or a lumen at maturity (Fig. 34).

A second characteristic feature of sclereids is the impregnation of their walls with calcium oxalate salts, As a result of which the cells become stony—hence their name, stone cells.

In your laboratory notebook, make a close-up drawing of 3–4 cells and label the mentioned component parts on the drawing.

Conclusion. Mechanical tissues evolved as a biological necessity to provide structural stability to plant organisms under terrestrial conditions. These mechanical tissues themselves evolved from angular collenchyma to sclereids, ensuring plant development at various stages—both across different taxonomic groups of plants and during the Ontogeny of the organism and individual Organs.

Self-Control Questions

1. Name the types of mechanical tissues and their localization in plant organs.

2. What types of collenchyma do you know and in which plants do they occur?

3. What are the Similarities and differences between collenchyma and sclerenchyma?

4. Name the features of similarity and difference between collenchyma and sclereids.

5. What features allow collenchyma to remain a living tissue?

6. Name the mechanical tissue characteristic of the secondary cortex (periderm/bark).

7. Which mechanical tissue is part of the wood of hornbeam and oak?

9. Which tissue provides the plant's resistance to bending?