Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Organs of Plants
Laboratory Class Topic 6.14. Secondary Anatomical Structure of the Root

General Remarks. When studying the ROOT Structure, pay attention to the appearance of secondary meristematic tissue. Significant structural Changes in the roots of dicotyledonous plants are associated precisely with The Emergence of this tissue.

The Primary Structure persists throughout the entire life in monocots, whereas in dicots and gymnosperms it is present only in young rootlets. In roots older than 7–10 days, changes occur that lead to The formation of the Secondary structure, which is produced by The activity of Three types of Meristems: fascicular cambium, interfascicular cambium, and phellogen.

Fascicular cambium, which forms secondary phloem and xylem elements, originates from the thin-walled phloem parenchyma Cells of the vascular cylinder.

The interfascicular cambium, initiated by pericycle regions located opposite the protoxylem rays, forms the parenchyma of the radial rays. These changes lead to an increase in root volume and the shedding of the primary cortex. This process is further accelerated by the Formation of the phellogen from the pericycle, which produces the secondary dermal tissue—cork (phellem)—and the ground tissue—phelloderm. Together, these constitute a complex of three tissue types known as the periderm.

In the final phase of transition, the secondary structure comprises the following components (Fig. 48): periderm, secondary cortex parenchyma, open collateral bundles, radial rays extending from the primary xylem rays located in the center of the root.

The second characteristic feature of the secondary root structure compared to the primary one is the rearrangement of Tissues in accordance with their Functions: instead of a radial vascular bundle, several open collateral ones emerge; the xylem acquires an axial arrangement, which provides the plant with high tensile strength; the root plays a role not so much in absorbing nutrients and Water, but rather in strengthening and anchoring the plant in the soil.

Object: pumpkin root (Cucurbita pepo L.)

Task:

1. Using a self-prepared or ready-made pumpkin slide, examine the Features of the Introduction/11.html">Secondary structure of a herbaceous plant root.

2. Sketch the Structural Features of the pumpkin root and label its component parts.

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

Studying the cross-section slide of a pumpkin root. Under low magnification, examine and analyze The structure of the slide. Sketch a diagram showing the arrangement of individual tissue blocks of the secondary root structure. Draw the diagram using a pencil. It is advisable to carry out the observation and practical work under high magnification, examining each tissue block sequentially.

The slide shows that the root is covered externally by the periderm, whose cells are neatly stacked one above another. This tissue consists of dead cork cells. Beneath the cork, 1–2 layers of meristematic tissue—the phellogen—are quite clearly visible in certain areas. Its cells are filled with cytoplasmic contents and appear lighter. Below lies several layers of phelloderm, whose cells, together with the medullary ray cells, form the cortex parenchyma. These cells are large, parenchymatous, thin-walled, and have intercellular spaces (see Fig. 48).

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Four collateral-type vascular bundles, separated by the parenchyma of radial rays, stand out clearly on the slide. With their narrowed ends, they abut the small rays of the primary xylem. The latter is represented by a large vessel located right in the center, from which rays of small tracheids and xylem parenchyma extend in four directions. In each bundle, the primary phloem is distinguishable, adjoining the cortex parenchyma. The secondary phloem is deposited centrifugally, consisting of phloem parenchyma, companion cells, and sieve tubes. Special attention should be paid to the sieve tubes, which feature large sieve plates with sieve pores. They are living and exhibit a blue coloration. Alongside them are small companion cells filled with dense cytoplasmic contents. Toward the center, behind the phloem, lies the fascicular cambium, represented by small, living parenchymatous cells that form a delicate net-like mesh. At the level of the fascicular cambium within the radial rays, the interfascicular cambium is formed. A significant portion of the slide, and of the vascular bundle in particular, is occupied by the secondary xylem. It is easily recognized by its red coloration, as well as by the presence of large vessels separated from one another by living, thin-walled xylem parenchyma cells.

Conclusions. Changes in the secondary root structure are associated with the functioning of the cambium. Most tissues are of secondary cambial origin. These tissues occupy the main part of the root.

Class="center">Review Questions

1. Due to the division of which tissues does the phellogen arise during the transition from the primary to the secondary root structure?

2. What changes occur in the central cylinder during the transition from the primary to the secondary root structure?

3. Name the Tissues of the periderm and the tissue from which it originates.

4. Which tissues make up the secondary phloem?

5. List the tissues of the secondary xylem of the root and their functions.

6. From which tissue is the interfascicular cambium formed, and what tissues does it regenerate?

7. Which of the primary tissues are preserved in the secondary root structure and where are they located?

8. Which complex of tissues occupies the largest volume of the secondary root structure?