Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Class Topic 7.15. Features of Anatomical Structure

General Remarks. ROOT crops, or the fleshy roots of radish and carrot, exhibit a Secondary Structure resulting from The activity of the cambium, whereas the root crop of beetroot shows a tertiary structure associated with the activity of the cambium and several additional cambial rings. Their characteristic feature is The production of a large amount of ground parenchyma Cells, which serve as a storage site for nutrients. As a result, the roots transform into root crops. At the same time, modifications occur in both the root proper, the hypocotyl, the root collar, and the epicotyl. A defining characteristic of all root crops is the excessive proliferation of the primary storage tissue. Particular attention should be paid to these very aspects—their structure, genesis, and structural changes.

Objects:

1. Root of cultivated radish (Raphanus sativus L.)

2. Root of cultivated carrot (Daucus sativus (Hoffm.) Roehl.)

3. Root of common beet (Beta vulgaris L.)

Tasks:

1. Using the roots of radish and carrot as Examples, study the Features of the Introduction/11.html">Secondary structure of root crops.

2. Study the features of the Tertiary Structure of the root using the beetroot crop as an example.

3. Sketch the Anatomical Features of the secondary and tertiary root structure in your lab notebook.

Equipment and Materials: MBR-1 and Biolam microscopes, root crop slides, hand-out materials, scalpels, razor blades, magnifiers, charts, etc.

Method for preparing a slide of a cultivated radish root. Take a Microscope slide, place it across a pencil case, and apply a drop of Water or an iodine-potassium iodide solution.

Make a longitudinal cut 1–2 cm deep in the middle of an elderberry pith. Insert a 4–5 mm thick radish rootlet into the cleft. Using a scalpel or razor blade, make a cut strictly perpendicular to the axis of the root. Then, make a series of complete sections, making sure to include the peripheral part of the root. Using a magnifier or under low microscope magnification, select a few of the thinnest and most complete sections, place them in a drop of water, and cover with a coverslip. Place the prepared slide on the microscope stage and secure it with stage clips.

Microscopic examination of a cross-section of a radish root crop. On the self-made or permanent slide, even under low magnification, it is clearly noticeable that the root is externally covered not by a single-layered epiblema with root hairs, but by a multi-layered cork tissue. Beneath the cork lies a multi-layered, thin-walled, living cortical parenchyma, in which starch grains can be observed. On two opposite sides, areas of primary phloem adjoin it, standing out on the permanent slide by their smaller cells and intense staining. It is located above a continuous multi-layered ring of secondary phloem, which constitutes the major part of the cross-section. The cells have thin Cell walls, are filled with dense cytoplasmic contents, and often contain starch grains. Against the Background of the phloem parenchyma, sieve tubes and small companion cells are clearly visible. Below the secondary cortex, the cambium is located as a continuous ring. The cambium cells are small with thin walls, forming a delicate mesh of cells. Below the cambium lies a thick layer of secondary xylem, which is a characteristic feature of the radish root crop. The bulk of the secondary xylem consists of xylem parenchyma. It is formed by polygonal cells with thin walls and cytoplasmic contents, occasionally containing numerous starch grains. In the xylem parenchyma, lumina of vessels of considerable size are usually arranged radially. In the center is the primary xylem, which represents the remains of a diarch vascular bundle (Fig. 49).

The continuation of the primary xylem within the secondary xylem is represented by two radial rays.

Microscopic examination of a cross-section of a carrot root crop. The structure of radish and carrot root crops is similar, and both exhibit a secondary root structure. Therefore, the slide reveals the same Tissues as in the radish root crop. A distinctive feature of the carrot root crop structure is the prominent Development of the secondary phloem and, conversely, a weaker development of the secondary xylem. In contrast to carrot, radish has a highly developed secondary xylem, whereas the secondary phloem forms only a narrow band, as can be seen from the diagram (Fig. 50).

Microscopic examination of a cross-section of a beetroot crop. Examine the anatomical features of the beetroot crop under low and high microscope magnification. On this slide, familiarize yourself with the tertiary structure of the root. We recommend making a close-up pencil sketch of the layout of individual tissue groups, and then sketching the features of the tertiary root structure under high magnification. The slide shows that the root is externally covered by cork. As in the previous slides, it is represented by multiple layers of dead cells.

Below lies the cortical parenchyma, which is genetically bordered by a thick layer of phloem-pericyclic parenchyma. It has the appearance of an openwork mesh of cells filled with living content. Within the ring of this tissue, 3–7 additional cambial rings stand out, consisting of interfascicular and fascicular cambia in the form of a thin-walled network. The cells are living, filled with dense cytoplasmic contents.

Open vascular bundles are formed on the additional cambial rings, outwardly bounded by the tertiary phloem and tertiary xylem. Its composition typically features 5–7 polygonal vessels, making it easy to identify. The vessels are embedded within a thin-walled, living xylem parenchyma. Further outward lies the thin-walled primary phloem, beneath which is the secondary phloem. Below, a continuous ring of cambium is visible as a strip of neatly stacked rectangular thin-walled cells. In the centripetal direction, the cambium cells deposit secondary xylem. On the slide, it is represented by relatively large vessels arranged in radial rows. Between them lies small-celled xylem parenchyma with thin cell walls and dense cytoplasmic contents. In the center of the slide, a diarch primary xylem with 5–9 cells arranged in a single row is discernible. Two radial rays, formed by thin-walled living parenchyma, extend from them in both directions.

Conclusion. As a result of the activity of the cambium and additional cambial rings, the active proliferation of ground parenchyma cells, and its excessive growth associated with the accumulation of reserve nutrients, the root undergoes modification and transforms into a root crop. In connection with the changing Functions of the root crop, the pattern of its internal structure changes accordingly.

Self-Control Questions

1. What type of structure is characteristic of the radish root?

2. Which tissue predominates by mass in the radish root crop?

3. Due to which tissue are the secondary xylem tissues formed?

4. What histological elements are present in the secondary phloem of beet?

5. What type of vascular bundle is characteristic of radish and carrot root crops?

6. What is the Internal Structure of radish and carrot root crops?

7. What tissue predominates in the carrot root crop and what is its function?

8. Which tissues are laid down outward by the cambium?

9. What is the difference between Primary and secondary phloem?

10. The cells of which tissues undergo changes during the transition from the primary to the secondary structure of the root crop?

11. Explain the Mechanism of formation of additional cambial rings.