Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
Laboratory Class. Topic 5.11. Vascular Tissues. Vascular Bundles

General Remarks. Plants possess two poles of Nutrition: aerial and subterranean. Roots absorb Water containing dissolved mineral nutrients, which are transported upward through the stem to sites of utilization, constituting the ascending stream. Carbon dioxide and water enter the leaves. Through Photosynthesis, utilizing chlorophyll and solar energy, Organic compounds are synthesized and transported downward to consumption sites as well as storage Tissues (such as seeds, fruits, tubers, bulbs, rhizomes, medullary rays, and wood parenchyma). Thus, two opposing streams operate within plants, facilitated by Vascular Tissues. These tissues originate from the procambium and cambium, classifying them as either primary or secondary.

Water and dissolved mineral salts are transported via tracheids and vessels (tracheae). Tracheids are evolutionarily older, unicellular conducting elements characteristic of gymnosperms and primitive angiosperms, and are distinguished by bordered pits. Vessels are more advanced, characteristic of angiosperms and certain highly evolved gymnosperms.

Vessels, much like tracheids, are multicellular, non-living structures. Based on the pattern of their secondary wall thickenings, they can be annular, spiral, annular-spiral, scalariform, or pitted. Scalariform and pitted vessels represent the most advanced and evolutionarily recent types. During individual plant development, they differentiate last, whereas annular and spiral vessels are the first to form.

Vessels and tracheids, together with ground tissue (xylem parenchyma) and mechanical tissue, constitute the xylem, or wood.

Plastic substances produced during photosynthesis are transported downward from the leaves to the roots and other consumption sites. Sieve tubes serve as the pathways for their translocation. Sieve tubes feature transverse sieve plates resembling miniature sieves, which facilitate an even flow of assimilates. They are interconnected by transverse septa pierced by numerous pores on their walls, resembling fine sieves.

Sieve tubes and companion Cells, together with ground tissue (phloem parenchyma) and mechanical tissue (bast fibers), form the phloem.

Xylem and phloem combine to form vascular bundles (in stems with a bundle-type Structure) or occur as continuous solid masses in stems with a non-bundle structure.

In dicotyledonous plants, a cambium layer lies between the phloem and xylem, resulting in The formation of open vascular bundles characteristic of dicots. Conversely, monocots lack a cambium, leading to the formation of closed vascular bundles. When a patch of phloem lies adjacent to a patch of xylem, the bundle is termed collateral. If phloem flanks both sides of the xylem—both externally and internally—it forms a bicollateral vascular bundle (as seen in squash). When phloem encircles a ring of xylem (in iris rhizomes) or xylem encircles a ring of phloem (in bracken fern rhizomes), these vascular bundles are classified as concentric. In the Introduction/19.html">Primary Structure of the ROOT, the vascular bundle type is radial. The absence of either phloem or xylem results in an incomplete vascular bundle.

Objects:

1. Stem of common sunflower (Helianthus annuus L.)

2. Stem of Scots pine (Pinus sylvestris L.)

3. Stem of maize (Zea mays L.)

4. Stem of field pumpkin (Cucurbita pepo L.)

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

6. Rhizome of bracken fern (Pteridium aquilinum (L.) Kuhn.)

Tasks:

1. Examine and draw The structure of conducting tissues using a prepared slide of a Cytology/practical/54.html">Longitudinal section of a sunflower stem.

2. Examine and draw the structure of tracheids using prepared slides of radial and tangential sections of pine wood.

3. Examine and draw the structure of a collateral closed vascular bundle using a prepared slide of a Cross section of a maize stem.

4. Examine and schematically draw the structure of a collateral open vascular bundle using a prepared slide of a cross section of a sunflower stem.

5. Examine and draw the structure of a bicollateral vascular bundle using a prepared slide of a cross section of a pumpkin stem.

6. Examine and draw the structure of a radial vascular bundle using a prepared slide of an iris root cross section.

7. Examine and schematically draw the structure of an amphicribral concentric vascular bundle using a prepared slide of a bracken rhizome cross section.

Equipment and Materials: MBR-1 or Biolam Microscope, magnifiers, prepared slides of longitudinal sections of sunflower and pine stems, cross sections of maize, sunflower, and pumpkin stems, iris root, bracken rhizome, etc.

Microscopic Examination of the Sunflower Stem Longitudinal Section Slide. First, examine and study the slide under low magnification, then switch to high magnification. It is clearly visible that the stem is externally covered by a single-layered epidermis, inward from which, past a layer of mechanical tissue, lie the sieve tubes. They can be identified by the plasmolyzed strands of Cytoplasm that widen toward the sieve plates. Narrow companion cells are situated alongside the sieve tubes. Each segment of a sieve tube corresponds to several companion cells arranged in a single vertical row. Draw 1–2 segments of the sieve tubes along with their companion cells. Indicate the sieve plate and plasmodesmata in your drawing (Fig. 35).

Let us proceed to examine the vascular tissues. Under high magnification, near the sieve tubes and companion cells, we can observe a zone of parenchymatous, living, quadrangular cambium cells, followed by the xylem, which contains five types of vessels. Our examination begins at the center of the organ, where evolutionarily older vessels are located—annular vessels, followed by spiral, scalariform, pitted-reticulate, and finally, near the cambium, the evolutionarily younger and more advanced pitted vessels (Fig. 36). Make an appropriate drawing.

A radial section of pine wood reveals unicellular conducting elements—tracheids with tapered ends. Their walls display distinct bordered pits shaped like two concentric circles. The spring tracheids are wide and thin-walled, gradually transitioning into the thick-walled tracheids of autumn wood with narrow lumina. Sketch 2–3 tracheids showing pits (Fig. 37).

In a tangential section of pine wood, bordered pits are no longer visible on the tracheid walls, nor are they present on the tangential walls. Bordered pits can be seen in a tangential section only where the plane cuts through the radial walls. Make a drawing illustrating the tracheids.

On a cross-section of a corn stem under low magnification, locate a vascular bundle with a clear structure, move it to the center of the field of view, and switch to high magnification. The phloem of the vascular bundle can be recognized by its sieve tubes, and the xylem by four large vessels of varying sizes and a rupture lacuna. There is no cambium between the xylem and phloem; this is a closed vascular bundle typical of monocotyledonous plants (Fig. 38).

Towards the center from the phloem lies a strip of cambium formed by rectangular, thin-walled parenchymatous cells. Beyond it lies the xylem arranged in radial rows of thick-walled vessels; vessels closer to the cambium have a larger diameter (pitted vessels), whereas those oriented toward the pith have a smaller diameter (annular and spiral vessels).

Based on the arrangement of the phloem and xylem, this is an open collateral vascular bundle. Make a schematic drawing showing the xylem, cambium, phloem, and the region of sclerenchyma.

In a cross-section of a pumpkin stem, the bicollateral vascular bundle reveals the xylem in the middle, flanked on both sides by two layers of cambium, which in turn are bordered by two regions of phloem—external and internal (Fig. 40).

Figure 40. Bicollateral vascular bundle of a pumpkin stem:

1 — external phloem; 2, 5 — cambium; 3 — secondary xylem; 4 — primary xylem; 6 — internal phloem

Make a schematic drawing showing the xylem, external phloem, internal phloem, and the two strips of cambium. In the cross-section of an iris root, we can see that the vascular bundle (Fig. 41) is located within the central cylinder: the xylem forms rays radiating from the center to the periphery—the vessels near the center are large, oval, or rounded, while those adjacent to the pericycle are angular.

Phloem regions are located between the xylem rays—that is, xylem rays and phloem regions alternate along radii and are separated by ground parenchyma, which characterizes a radial vascular bundle. Make a schematic drawing showing the rays of primary xylem (tracheids and Trachea) and the regions of primary phloem (sieve tubes, companion cells, phloem parenchyma).

Examine the STRUCTURE OF THE concentric vascular bundle in a cross-section of a bracken fern rhizome. The xylem is clearly visible in the center, surrounded by a ring of phloem. This is a concentric vascular bundle. Make a schematic drawing showing the xylem and phloem (Fig. 42).

Conclusions. 1. Plants possess bidirectional flows maintained by conducting tissues—vessels, tracheids, sieve tubes, and companion cells.

2. Conducting tissues, combined with fundamental and mechanical tissues, form complexes that give rise to vascular bundles.

3. The following types of vascular bundles are known: closed, open, collateral, bicollateral, radial, and concentric.

Self-Control Questions

1. What causes the differentiation of various types of conducting tissues, and how did they evolve?

2. How do vessels (tracheae) differ from tracheids? Name their types.

3. What histological elements are involved in the COMPOSITION OF THE phloem?

4. Name the histological elements of the xylem. Which of them are living?

5. What types of vascular bundles are characteristic of monocotyledonous plants?

6. What types of vascular bundles are typical for the primary and Secondary structure of roots?

7. Name the constituent parts of a bicollateral vascular bundle.

8. In which plants do concentric vascular bundles occur?

9. Explain which vascular bundles are called complete and which are incomplete.