Botany - B.Ye. Yakubenko 2017
Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Class Topic 8.20. Anatomical Structure of the Stem of Hardwood Species
General Remarks. Natural stands and forest plantations in Ukraine feature a great diversity of deciduous tree species. Compared to conifers, they appeared later in evolutionary terms and possess a more advanced anatomical stem Structure. Internally, this is manifested in The Development of highly specialized tissue types, including dermal, vascular, mechanical, and others—most notably vessels, companion Cells, collenchyma, libriform fibers, etc. The wood of deciduous species is characterized by the absence of resin ducts.
Object: Stem of the small-leaved lime (Tilia cordata Mill.)
Assignment:
1. Prepare a cross-section of the lime stem on your own.
2. Study the stem structure of deciduous species using both your freshly prepared and permanent slides.
3. Draw a sector of the lime stem cross-section in your lab notebook and label its component parts.
Equipment and Materials: MBR-1 microscopes, hand lenses, razors, scalpel blades, other tools, handout materials, Microscope slides, charts, Reagents.
Procedure for Preparing a Cross-Section of the Lime Stem. Place a drop of Water or an iodine-potassium iodide solution onto a Glass microscope slide. Take a piece of elderberry pith in your left hand and make a longitudinal slit in the middle to a depth of 1–1.5 cm. Insert a segment of the lime stem, matching the length of the slit, into the elderberry pith. Using a scalpel or razor blade, level The surface of the pith along with the embedded lime stem. Sections must be cut strictly perpendicular to the stem axis. Cut a series of thin cross-sections. Using a hand lens, select a few of the thinnest sections and transfer them into the drop of water or iodine-potassium iodide solution. The section does not need to span the entire stem, but it must reach at least the midpoint and necessarily include the peripheral region. Moisten the section additionally with water or phloroglucinol reagent. Cover the prepared section with a coverslip.
Microscopic Examination of the Slide. Under low magnification, examine the slide and sketch the main tissue regions schematically: periderm, Primary and secondary cortex, cambium, secondary xylem, and pith. Under high magnification, examine each of these regions individually: note the specifics of their arrangement, proportions, and cellular structures.
Periderm. The outer part of the periderm is visible as a multi-layered cork (Fig. 63), which serves as reliable protection for the stem against Temperature fluctuations and spore penetration. Its distinctive feature is that it is formed by parenchymal cells neatly stacked in radial rows.
The cork cells are dead and hollow, with Cell walls impregnated with suberin, making them impermeable to water and gases.
Beneath the cork lies the cork cambium, or phellogen, arranged in a precise pattern. It is easily identified: it consists of a single layer of living cells containing a Nucleus and dense cytoplasmic content. In some areas, mother cells can be observed dividing into two daughter cells.
Located just below is the phelloderm. Like the phellogen, it is formed by living parenchymal cells with nuclei and dense Cytoplasm. It differs in having thicker cell walls, larger cell sizes, and a less regular arrangement, lacking the strict radial ordering seen in the cork.
Primary cortex. In the preparation, the primary cortex is separated from the periderm by lamellar collenchyma. This is a living mechanical tissue formed by relatively large, rectangular parenchymal cells with living contents. A key feature is the thickening of their tangential walls, which imparts significant strength to the stem. Beneath the collenchyma lies the cortical parenchyma, easily recognized not only by its Location but primarily by its structure. It is formed by large, hollow, polygonal or oval parenchymal cells with thin walls. The parenchyma constitutes a thick layer of the cortex and serves as a nutrient storage site: starch grains can be observed within its cells, and calcium oxalate crystals are visible in some.
The innermost layer of the primary cortex is the endodermis, or starch sheath. This is a single-layered tissue composed of living parenchymal cells with thin walls. A characteristic feature of these cells is the presence of starch grains; this starch is not depleted even during periods of nutrient deficiency in the plant.
Secondary cortex. It comprises trapezoidal areas of secondary phloem and medullary ray segments. The prepared slide clearly shows hard bast, which appears as layers formed by thick-walled sclerenchyma. These cells are elongated, tightly packed, polygonal, with uniformly thickened and lignified walls. This ensures high elasticity and strength of the tissue. Phloem layers are situated between adjacent bands of hard bast (sclerenchyma). The phloem consists of large, hollow, living parenchymal cells, sieve tubes, and small companion cells filled with dense cytoplasm. The areas of secondary phloem are separated by sections of primary medullary rays, which are recognized by the following features: on the slide, they appear blue and are triangular in shape, with their apex pointing toward the cambium. The cells are large, typically quadrilateral, with a protoplast and a thin wall. Starch grains are sometimes visible within them.
Cambium is easily spotted by two characteristics: its Location and Structure. It is situated between the secondary phloem and secondary xylem (between the bark and the wood). Structurally, its cells are rectangular and stacked in rows, forming a delicate network. The cells are living and possess the capacity to divide and generate permanent tissue elements.
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Class="center"> Figure 63. Anatomical Structure of the lime stem: I — periderm; II — primary cortex; III — secondary cortex; IV — cambium; V — secondary xylem (wood); VI — pith; 1 — cork; 2 — phellogen; 3 — phelloderm; 4 — collenchyma; 5 — cortical parenchyma; 6 — starch sheath (endodermis); 7 — sclerenchyma; 8 — primary phloem (pericyclic parenchyma); 9 — hard bast (bast fibers); 10 — sieve tubes; 11 — companion cells (10, 11 — soft bast); 12 — cambium; 13 — annual rings; 14 — spring wood; 15 — summer/autumn wood; 16 — vessels; 17 — primary xylem; 18 — perimedullary zone; 19 — pith; 20 — primary medullary ray; 21 — secondary medullary ray |
Secondary xylem, or secondary wood, occupies the largest portion of the cross-section. A defining feature of this tissue is the presence of annual rings. Each annual ring consists of spring wood and autumn wood. Spring wood is formed by large-pored, thin-walled vessels and small-celled parenchyma. The wood is porous and makes up the larger part of the annual ring. Autumn wood is composed predominantly of small-pored, thick-walled tracheids. Due to the sharp contrast between the denser autumn wood formed In the second half of the previous year (which appears darker) and the spring wood of the current year (which is lighter than the autumn wood), the boundary between yearly growth increments is clearly demarcated. The age of the tree can be determined by counting these annual rings. In addition to parenchyma and Vascular Tissues, the secondary xylem contains libriform fibers.
The inner part of the wood is represented by the primary xylem. It is visible as distinct sectors separated by medullary rays. The xylem is formed by small-pored annular and spiral vessels with heavily thickened, lignified walls. Interspersed among them are small living cells of xylem parenchyma.
At the center of the wood lies the pith, which is easily identified by its position and tissue structure. It is formed in the center by large, parenchymal, thin-walled, polygonal cells. Among them, smaller cells or groups of cells with thickened walls and dense brown contents occasionally occur.
The peripheral part of the pith bordering the primary xylem is formed by a narrow band of small-pored cells with thickened cell walls. This region of the pith is referred to as the perimedullary zone.
Primary medullary rays extend outward from the pith toward the periphery, widening within the secondary phloem into triangles with their apexes pointing toward the cambium. Here, they are composed of large, quadrilateral cells with granular contents. In the wood, the primary medullary rays are multiseriate (two- or multi-rowed), whereas the secondary rays are uniseriate and do not reach the pith. They are located within the secondary xylem.
Conclusion. The anatomical stem structure of woody deciduous plants is characterized by a high degree of tissue specialization, complex structural and spatial Organization, and temporal changes. During ontogeny, primary tissues give way to secondary and tertiary types (bark), undergoing displacement and rearrangement in connection with their conducting, protective, and mechanical Functions.
1. Name the tissue blocks in the anatomical STRUCTURE OF THE stem of deciduous woody plants.
2. What tissues make up the primary cortex?
3. Characterize the types of mechanical tissues typical for the stems of deciduous woody plants.
4. Due to The activity of which tissue and through what mechanism is the cork formed?
5. Name the constituent parts and Characteristic Features of the elements of Hard and Soft bast (phloem).
6. Features of cambial activity.
7. List the tissues that comprise the secondary xylem.
8. How are annual rings formed in deciduous woody plants?
9. What varieties of mechanical tissue are present in the secondary
xylem and secondary phloem?
10. What are the differences between the primary and secondary cortex?
11. What is The Role of pith rays?
Last update: 07/08/2026
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