Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Session Topic 8.19. Macroscopic Structure of Woody Plant Stems

General Remarks. Deciduous and coniferous tree species grow on the territory of Ukraine, forming corresponding forests. Their productivity is determined by The Development of tree trunks. The trunk is of great biological importance because it provides structural support to the plant, bears numerous metamers, branches, and leaves, and serves as a connecting link between aerial and ROOT Nutrition.

Object: a stem cross-section block of pedunculate oak (Quercus robur L.)

Task:

1. Using oak blocks as an example, study the Structural Features of the trunk in woody deciduous species.

2. Draw a sector of the transverse section of a woody plant stem and label its constituent parts.

Materials and equipment: magnifying glasses, microscopes, blocks of oak, pine, alder, charts, and other accessories.

Methods FOR PREPARING a transverse section block of pedunculate oak. For macroscopic examination of the oak trunk, it is advisable to make 2–3 cm thick cross-sections from different heights. This allows observation of the formation and changes in The Structure of the trunk's constituent elements. The blocks should be cleaned and polished so that annual rings, pith rays, pith, etc., are clearly visible. To better identify and count the annual rings, incisions up to 5 mm deep and up to 1 cm wide are made along two mutually perpendicular lines. Counting is carried out along all four radii, with marks made every five or ten years, starting from the pith and ending at the ring adjacent to the cambium. To determine the growth rate over 5 or 10 years, their annual increment is measured, and based on the data obtained, a graph of stem growth in diameter is constructed.

Macroscopic examination of the pedunculate oak trunk. Visually or with the aid of a magnifying Glass or stereoscopic Microscope, study the macroscopic STRUCTURE OF THE woody plant stem in detail. Having identified the features, draw a diagram of the macrostructure in your notebook. Further detailed study makes it possible to distinguish A number of tissue regions.

Bark (rhytidome). Externally, the block is covered by a thick brown bark, which belongs to the tertiary protective tissue. It reliably protects the tree trunk from Temperature fluctuations, Burns, and the penetration of diaspores. On the inner side, it is continuous and even, whereas on the outside, it is fissured, notched, and features depressions of various sizes. The surface has cracks resulting from stem growth. Under high magnification with a magnifying glass, and on some sections even with the naked eye, denser and darker transverse strips of tissue can be seen. These are layers of cork. Between the layers of cork, a more or less homogeneous amorphous mass is visible, formed by the cortex parenchyma, mechanical, conductive, and other Tissues, which together with the cork form the dead tissue known as the bark or rhytidome (Fig. 62).

Secondary cortex (phloem/inner bark). You can easily recognize it: firstly, it has a lighter color, and secondly, it is located beneath the rhytidome as a continuous band. In the secondary cortex, transverse light, yellowish, thin strips of pith rays stand out in certain places. Between them, areas formed by Hard and Soft bast (phloem) are visible.

The cambium lies beneath the secondary cortex as a narrow zone. It is practically invisible on the block. In practice, the cambium is defined by its Location, specifically between the secondary cortex and the adjacent tissue—the secondary xylem.

Secondary xylem, or wood, occupies the largest part of the block. It is easily recognized by a number of parameters: firstly, it is characterized by high density and hardness; secondly, its color is light brown; and thirdly, annual rings are clearly distinguishable within it. In the wood, the outer part is light and represented by the lighter sapwood, while the remainder is the darker heartwood. As a result of Physical and Chemical changes, not only the color but also the Functions change. Heartwood does not conduct Water and dissolved substances because the vessels become plugged. In this state, it performs a mechanical function. Water and dissolved mineral salts move through the Vessels of the sapwood, which is physiologically active.

Annual rings of wood vary in width depending on the development patterns of the tree species. Each ring is differentiated into latewood (autumn wood) and earlywood (spring wood). The latewood of an annual ring is dense, darker, and thicker, consisting mostly of tracheids. Earlywood is represented by a narrow band, is loose, and consists mostly of vessels.

Pith. On the block, the pith is located in the very center and occupies a negligible area. It is formed by thin-walled parenchyma.

Pith rays. Primary pith rays extend radially from the pith towards the periphery and the cortex. Rays located only within the wood and not reaching the pith are called secondary pith rays. Pith rays appear as light lines of tissue arranged radially. They facilitate gas exchange and the horizontal translocation of nutrients.

Conclusion. The macroscopic structure of a woody plant stem is driven by the accumulation of organic matter produced during Photosynthesis, accompanied by an increase in Cell number and their differentiation into permanent tissues. Individual tissue regions perform their inherent functions.

Class="center">Self-Assessment Questions

1. What are the characteristic structural Features of the pedunculate oak stem?

2. Name the Main Functions of a woody plant trunk.

3. What is sapwood? What physical and chemical changes occur during The formation of sapwood?

4. How does the cortex differ from the rhytidome (bark)? What are their most defining features?

5. What types of pith rays did you find when studying the macroscopic structure of the oak block? What functions do they perform?

7. How does heartwood differ from sapwood?

8. Name the common features shared by heartwood and sapwood.

9. What type of stem structure is characteristic of a pedunculate oak coppice SHOOT?

10. Which tissue is responsible for the Secondary Growth of the trunk in thickness?