Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Session Topic 6.13. Root Zones. Primary Anatomical Structure of the Root

General Remarks. A distinctive feature of the primary ROOT Structure is the Formation of primary Tissues from the apical meristem (growing cone). Through the division of the outer layer of Cells in the division zone (dermatogen), a permanent tissue is formed — the epiblema (rhizodermis) bearing root hairs. The Cells of the periblem give rise to the permanent elements of the primary cortex, while the plerome cells form all the Components of the central cylinder.

Objects:

1. Rootlet of common wheat seedlings (Triticum aestivum L.)

2. Root of German iris (Iris germanica L.)

3. Root of common reed (Phragmites australis (Cav.) Trin. ex Steud.)

Tasks:

1. Study the morphological and genetic Zones of the root using a self-prepared slide of a germinated wheat grain.

2. Study the primary root structure using ready-made slides or self-prepared cross-sections of the root of German iris or common reed.

3. Clearly illustrate the Features of the morphological and genetic zones of a young wheat rootlet and the primary root structure of iris or reed in your laboratory notebook.

Equipment and Materials: MBR-1 or Biolam microscopes, scalpels, blades, razors, magnifying glasses, wheat seedlings, ready-made slides of iris root, reed roots, charts, etc.

Procedure for preparing a slide of the wheat root tip. Place a drop of Water or iodine-potassium iodide solution on a Glass slide. Take a germinated wheat grain and, using a scalpel or blade, cut off a young wheat rootlet 1—2 cm long, then place it into the drop of water on the slide. Using a magnifying glass or a low-power Microscope, carefully examine the root tip and make sure it is covered by the root cap, and at a certain distance from it, possesses root hairs. If these parts are not visible, take another rootlet that clearly shows both the root cap and root hairs. Place it in a drop of water, cover with a coverslip, and place it on the microscope stage.

Visual and microscopic examination of the young wheat seedling rootlet slide. Three rootlets are clearly distinguishable in a wheat grain: one is the main root and two are adventitious roots developing from the hypocotyl. Each rootlet has a uniform structural type. Therefore, all zones characteristic of monocotyledonous plant roots can be distinguished on the slide: the root cap, division zone, elongation zone, root Hair zone, maturation (differentiation) zone, and lateral root zone in sufficiently developed seedlings.

Using low magnification, locate the root cap. The slide clearly shows individual cells shedding from it. Just beneath the root cap lies the Cell Division zone: within it, one can distinguish the dark central part (plerome) from which the central cylinder develops, a light band of periblem cells on both sides of the plerome, and the outermost cell layer — the dermatogen (Fig. 46).

Above the division zone lies the growth zone, or elongation zone, which drives root elongation. This zone ends with The formation of the first protuberances of future root hairs. Next is the root hair zone, characterized by a well-developed system of hairs through which the plant absorbs the necessary amounts of water and nutrients.

The region of the root extending from where the root hairs begin to die off down to the first primordia of lateral roots is called the maturation (conduction) zone. The zone containing developed and rudimentary lateral roots is referred to as the lateral root zone.

Microscopic examination of the iris root slide. Using low and high magnification of the microscope, locate the three main tissue blocks: the epiblema, primary cortex, and central cylinder (Fig. 47).

The epiblema represents the outer layer of parenchymatous cells elongated horizontally. Young cells are living and appear blue-stained on the slide. In some of them, the outer walls bulge and elongate into root hairs. Beneath the epiblema lie 3—5 layers of polygonal, tightly packed exodermal cells. Their cell walls are impregnated with suberin, making them impermeable to water and pests, thus providing a protective function after the epiblema sloughs off. The major part of the slide is occupied by the mesodermis; its cells are large, parenchymatous in shape, rounded, thin-walled, and possess intercellular spaces. The innermost layer of the primary cortex is formed by the endodermis. Endodermal cells are easily recognized: they are stained red and feature wall thickenings on three sides — the inner tangential and two radial walls. These cells are lignified to varying degrees. This ring of cells is interrupted in certain places by passage cells, which are living, thin-walled, and filled with dense contents. Water and nutrients seep through them toward the conductive elements of the central cylinder.

The latter begins with the pericycle, whose living cells are arranged parallel to the endodermis.

Pericycle cells are capable of division: they form the cells of the interfascicular and cork cambium (phellogen) and give rise to lateral roots; therefore, this tissue is referred to as root-forming or meristematic. Deeper lies the parenchyma of the central cylinder, which often becomes lignified in the central region and acquires a reddish hue upon staining. Embedded within this tissue is a radial vascular bundle. This exact type of bundle is characteristic of the primary root structure. It is formed by radial rays of xylem, which near the endodermis consist of smaller tracheids and Vessels of the protoxylem, and closer to the center — of larger vessels of the metaxylem. Between the xylem rays lie regions of phloem, where the proto-phloem oriented toward the endodermis and the meta-phloem directed toward the center can be distinguished.

Conclusion. The primary root structure is characteristic of monocotyledonous plants throughout their entire life, and of dicotyledonous plants only at a young age. The tissue arrangement ensures the rapid absorption of water and mineral salts from the soil and their transport to the conductive tissue. The functioning of the pericycle is associated with the formation of lateral roots and the plant's root systems, while in dicots it ensures the transition to Secondary structure.

Self-Assessment Questions

1. In which part or zone of the young rootlet does tissue specialization take place?

2. Which zone is characterized by growth?

3. What is the pericycle and what is its role?

4. Name and characterize all the tissues that make up the primary cortex.

5. Which tissues result from The activity of plerome cells?

6. Name the Structural components of the primary xylem. What is their significance?

7. What type of vascular bundle is characteristic of the primary root structure?

8. Which tissue block does the endodermis belong to? What are the Structural Features of its cells?

9. Which tissue is responsible for the formation of lateral roots?