Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Session Topic 8.18. Anatomical Structure of the Stem in Herbaceous Dicotyledonous Plants. Bundled and Transitional Types of Structure

General Remarks. In this type of stem Structure, vascular bundles are clearly distinguished, separated by pith rays. The vascular bundles are open, collateral, and arranged in a regular circle. They originate from the procambium, which is formed in the apical meristem as separate procambial strands that give rise to the cambium.

Objects:

1. Sunflower stem (Helianthus annuus L.)

2. Dutchman's pipe stem (Aristolochia sipho Lam.)

Tasks:

1. Prepare temporary cross-sections of the Dutchman's pipe and sunflower stems on your own.

2. Examine the slide of the sunflower stem cross-section as an example of a transitional type of structure.

3. Observe and study the microscopic STRUCTURE OF THE Dutchman's pipe stem.

4. Sketch the Structural Features of the sunflower and Dutchman's pipe stems, labeling their constituent parts.

Equipment and Materials: MBR-1 microscopes, magnifying glasses, razors, scalpel blades, tweezers, handouts, Reagents, prepared slides, charts, etc.

Method for Preparing a Sunflower Stem Cross-Section. Place a drop of Water or an iodine-potassium iodide solution onto a Glass slide. Take a piece of elderberry pith in your left hand and make a 1.5–2 cm deep slit in the middle. Place a piece of the sunflower stem into this slit. Level The surface of the pith along with the stem using a scalpel. Take a razor in your RIGHT HAND AND make a series of cross-sections. The slice must be strictly perpendicular to the axis of the sunflower stem. Using a magnifying glass, select 2–3 of the best sections—thin, transparent, taken across the entire stem or a portion of it including the periphery. Place the sections into the drop of solution, moisten them from above, and cover with a coverslip. Such a slide is ready for examination. After examining the slide, make a schematic drawing of these parts, showing their proportions and spatial distribution. It is best to select a sector of the cross-section (Fig. 60).

Under high magnification, carefully examine the cellular details of individual Tissues and sketch them schematically. The slide shows that the stem is covered by a single-layered epidermis. Its Cells are thin-walled, quadrangular, horizontally elongated, and filled with Cytoplasm. Their outer walls are covered with a continuous thin cuticle film. In some areas, multicellular hairs with pointed tips can be found. Stomata formed by two guard cells are visible here and there in the epidermis.

The primary cortex lies beneath the epidermis and consists of collenchyma, parenchyma, and endodermis. The collenchyma is directly adjacent to the epidermis. It is a living mechanical tissue formed by polygonal cells whose walls are thickened at the corners, with tangential walls also partially thickened, forming angular-lamellar collenchyma. The parenchyma of the primary cortex is clearly demarcated from it. This is a rather thick, multi-layered band formed by living, polygonal, thin-walled parenchymatous cells. Resin canals are interspersed among them; they are formed by a single layer of living epithelial cells surrounding a cavity. The inner layer of the primary cortex is formed by the endodermis, whose living parenchymatous cells are filled with starch grains.

Caps of sclerenchyma of pericyclic origin adjoin the endodermis in places. It is formed by polygonal, tightly packed prosenchymatous cells with uniformly thickened walls. The sclerenchyma shields the peripheral part of the open collateral vascular bundle's phloem. On prepared slides, the phloem is stained blue. It is formed by companion cells, sieve tubes, and phloem parenchyma. The companion cells are small, quadrangular, and filled with dense cytoplasm.

In addition to those mentioned, the phloem contains living Cells of the phloem parenchyma, which sometimes appear as compressed parenchyma.

The phloem and xylem of the bundle are separated by the fascicular cambium, which looks like an openwork, delicate net. Its cells are living, thin-walled, rectangular, and filled with dense cytoplasm. A key feature of cambium cells is their ability to divide.

The xylem occupies the major part of the vascular bundle and is easily recognized by the presence of rather large, lignified vessels arranged radially. In addition to the vessels, fine-pored thick-walled tracheids are visible, distributed individually or diffusely among the vessels. The rest of the xylem is filled with xylem parenchyma. These are living parenchymatous cells with thin lignified walls.

Toward the center, the vascular bundles border the pith. Its cells are living, parenchymatous, generally rounded, thin-walled, with numerous intercellular spaces. The cells increase in size toward the center. Pith rays, formed by radially elongated parenchymatous cells, run from the pith to the periphery between adjacent vascular bundles.

At the level of the fascicular cambium, one can see the interfascicular cambium crossing the parenchyma of the pith rays. By origin, this is a secondary meristematic tissue formed from the cells of the pith rays.

Areas of the interfascicular cambium merge with the fascicular cambium to form a continuous cambium ring, which deposits continuous layers of xylem and phloem, thus establishing a non-bundle (closed) type of stem structure. Consequently, the anatomical structure of the sunflower stem represents a transitional type from the bundle to the non-bundle structure.

Microscopic Examination of a Dutchman's Pipe Stem Cross-Section. Under low magnification, the following structures can be distinguished: epidermis, primary cortex, sclerenchyma, vascular bundles, and pith with pith rays (Fig. 61).

Sketch a sector of the Dutchman's pipe stem cross-section schematically and lightly outline the mentioned structures with a pencil, showing their spatial arrangement and proportions.

Under high magnification, examine the structural details of each tissue and draw them into your diagram. The epidermis is clearly distinct, single-layered, and formed by living, thin-walled parenchymatous cells. The cells are elongated parallel to the stem surface. The outer Cell walls are impregnated with cutin. Stomata formed by two guard cells are visible in certain areas of the epidermis.

The outer layer of the primary cortex consists of 3—5 layers of lamellar collenchyma, which is easily recognizable: it lies directly beneath the epidermis and is composed of parenchymatous cells with thickened tangential walls and dense cytoplasmic contents. It borders a clearly defined cortical parenchyma. The cells of this parenchyma are thin-walled, large, and arranged perpendicularly to the collenchyma. Crystals (druses) can often be seen within them, and METABOLISM/14.html">Chloroplasts occur even more frequently, just as in the collenchyma cells. The cells of the innermost layer of the primary cortex—the endodermis—contain numerous starch grains, which is why it is referred to as the starch sheath.

Adjoining the inner side of the endodermis is a continuous, robust ring of pericyclic sclerenchyma. Its cells are dead, prosenchymatous, with thickened and lignified cell walls. Furthermore, the degree of lignification is greater in the peripheral part, where the cells are more intensely stained than in the inner part, which forms a wavy boundary with slight indentations opposite the vascular bundles. On its inner side, the sclerenchyma borders a distinct band of living, thin-walled parenchyma, the cells of which result from the transformation of the inner zone of the pericycle.

Below lies a ring of collateral vascular bundles. Each bundle is differentiated into three parts: the peripheral phloem, the central fascicular cambium, and the inner xylem. The phloem is divided into two parts: a narrow outer strip of crushed primary phloem cells and a robust secondary phloem region. Thin-walled sieve tubes are clearly visible in the preparation. Adjoining them are small, rectangular companion cells with dense cytoplasmic contents, which sets them apart from the phloem parenchyma cells.

Beneath the phloem lies a layer of living meristematic tissue known as the fascicular cambium. In the preparation, it appears as an openwork lattice formed of rectangular, thin-walled cells overlapping in radial rows.

The activity of the cambium produces secondary xylem, which pushes the primary xylem toward the center. The bulk of the secondary xylem is made up of polygonal vessels with uniformly thickened, lignified walls. Thin-walled, small, living parenchymatous cells with lignified walls are diffusely distributed among the vessels, tracheids, and wood fibers.

The inner part of the xylem is formed by the primary xylem, which consists of smaller vessels with pitted walls. The rest of the xylem is composed of xylem parenchyma.

The central part of the preparation is occupied by the pith, formed by large, thin-walled parenchymatous cells with numerous intercellular spaces. Medullary rays, formed by parenchyma, extend from the pith toward the periphery.

At the level of the two fascicular cambiums, the medullary rays are intersected by the interfascicular cambium, which is similar in nature to the fascicular cambium but differs in having larger cells.

Conclusion. The bundle-type stem structure is characteristic of monocots and dicots. The distinctive Features of the bundle-type stem structure in dicots include: a circular arrangement of vascular bundles; open collateral bundles capable of secondary thickening; and the presence of collenchyma rather than sclerenchyma beneath the epidermis.

Class="center">Review Questions

1. Name the Characteristic Features of herbaceous dicotyledonous plants.

2. Which tissues are characteristic of a collateral vascular bundle?

3. Explain the origin and arrangement of vascular bundles.

4. What accounts for the circular arrangement of vascular bundles?

5. What types of meristematic tissue are present in the stem structure of dicots?

6. Name the mechanical tissues in the stem of dicots. What is their arrangement and how do they function?

7. Into which tissue is the pericycle transformed in a sunflower stem?

8. What living tissues are present in The structure of a sunflower stem?

9. What are the CHARACTERISTICS OF THE transitional type of stem structure?

10. What is the structural peculiarity of the birthwort (Aristolochia) stem?

11. What are the commonalities and differences in the stem structure of the sunflower and birthwort?