Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
Laboratory Class Topic 4.8. Secondary and Tertiary Protective Tissues

General Remarks. When studying the secondary and tertiary Dermal Tissues, pay attention to the secondary meristematic tissue — the phellogen, or cork cambium. Its activity leads to The formation of the secondary dermal tissue: the cork (phellem), which is deposited toward the periphery of the organ, and the phelloderm, which is cut off inwardly. Together, the cork, phellogen, and phelloderm make up a complex tissue known as the periderm.

It is The Development of the periderm in the deeper layers of the cortex that forms the tertiary dermal tissue — the bark (rhytidome). Note that it is thicker and layered, incorporating not only the periderms but also the ground parenchyma, vascular, and mechanical tissues. By nature, it is a complex of dead tissues that reliably protects woody plants from Temperature fluctuations, pests, and disease.

Objects:

1. A branch of European elder (Sambucus nigra L.)

2. Bark of common oak (Quercus robur L.)

Tasks:

1. Examine the arrangement and appearance of lenticels on a small piece of elder branch. Make a drawing of the branch with lenticels.

2. Using a prepared cross-section slide of an elder branch under low and high Microscope magnification, study The Structure of the periderm and lenticels. Make a detailed drawing of the periderm and lenticel.

3. Using a prepared cross-section slide of oak bark under low and high magnification, study its Anatomical Structure. Under high magnification, draw a section of the bark.

4. Equipment and Materials: microscopes, hand lenses, pieces of elder branch, prepared cross-section slides of elder and oak bark, charts.

Macroscopic Study of the elder branch. The elder branch is covered with elongated lenticels aligned along the main axis of the organ, rising above the bark as small bumps of loose Cells protruding through a crack in the periderm. The lenticels are up to 2–3 mm long and up to 1 mm wide. There are 3–4 lenticels per 1 cm². Draw the lenticels on the elder branch (Fig. 29).

Microscopic study of the cross-section of an elder branch. Under low microscope magnification, scan the entire perimeter of the stem cross-section to find the best lenticel. The section must be thin enough to reveal all tissue cells (some cells may fall out during slide preparation); orient it with the rupture pointing upward. Examining the cross-section under high magnification, we can observe the partially degraded epidermis on the outer surface of the organ, followed by regular rows of the secondary dermal tissue — the cork (phellem) — featuring thick walls and lacking protoplasts. Beneath the cork lies a layer of thin-walled, living cells belonging to the secondary meristem, the phellogen. Further inward are the living Cells of the ground tissue, the phelloderm.

On the slide, the lenticel appears as a fissure with slightly elevated edges; the opening itself is filled with loose complementary tissue cells, through whose intercellular spaces gas exchange occurs between the stem tissues and the external environment. Lenticels are typically located opposite the medullary rays. In your drawing, indicate the cork, phellogen, phelloderm, cortical parenchyma, and complementary cells of the lenticels (Fig. 29).

Microscopic study of oak bark. The structure of oak bark is best studied on a thin section preserved in an alcohol-glycerin mixture. The periderm layers are easily recognized in the section by the regular radial rows of cork cells. Interspersed between several periderm layers are dead tissues: a thin-walled ground parenchyma, thick-walled sclerenchyma fibers (5–6-sided) in small groups, and clusters of oval, thick-walled stone cells with large lumens and prominent pits. Make a drawing showing: the periderm, cortical parenchyma, stone cells, and bast fibers (Fig. 30).

Conclusions. 1. The secondary dermal tissue (cork) is dead, characteristic of dicotyledonous plants, and particularly well-developed in woody species.

2. The tertiary dermal tissue (bark) is a composite of several types of dead tissues, occurring exclusively in woody plants.

Self-Control Questions

1. Why is the epidermis replaced by cork in dicots, particularly in perennial plants?

2. How do cork cells differ from epidermal cells?

3. Why is cork referred to as a secondary dermal tissue?

4. Which tissue complex includes the cork?

5. How do gas exchange and Transpiration take place through the cork?

6. In which plants is the bark (rhytidome) most pronounced?

7. Why does the periderm transform into bark in these plants?

8. What tissues does bark contain?