Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Class Topic 9.23. Anatomical Structure of the Leaf

General Remarks. The Water/140.html">Anatomical Structure of the leaf is determined, on the one hand, by the Evolution of the organ resulting from the natural complexification of its structure across various systematic groups of plants, and on the other hand, internal differentiation is the result of plant adaptation to diverse terrestrial habitats and, finally, is associated with the performance of aerial Nutrition, Transpiration, and gas exchange. It is the complex of these factors that drove the genesis and refinement of leaf anatomical structure in various plant groups.

Objects:

1. Lemon leaf (Citrus limon Burm.)

2. Camellia leaf (Camellia japonica L.)

3. Corn leaf (Zea mays L.)

4. Pine needle (Pinus sylvestris L.)

Tasks:

1. Prepare temporary mounts of the designated objects on your own.

2. Study the Structural Features of monocot and dicot leaves.

3. Examine and study the structural features of a pine needle.

4. Draw the anatomical STRUCTURE OF THE leaves of lemon, camellia, corn, and the pine needle, and label their constituent parts.

Equipment and Materials. MBR-1 or Biolam microscopes, magnifying glasses, razors, scalpels, forceps, permanent slides, charts, living and preserved material for distribution.

Methods FOR PREPARING a Cross-Section of a Lemon Leaf. Place a drop of water or iodine-potassium iodide solution onto a Glass slide. Take a piece of elderberry pith and make a longitudinal slit in it to a depth of 1.5 cm. Insert a piece of the leaf into the slit, cut lengthwise on both sides of the midrib, up to 1.5 cm wide. Using a scalpel or a razor blade, level The surface of the pith along with the leaf piece. It should be perpendicular to the axis of the leaf midrib. Using a sharp razor, make a series of sections. With the help of a magnifying glass, select 2–3 of the best ones: thin and transparent, capturing the midrib. Place the sections into a drop of iodine solution, or better yet, treat them with aniline sulfate. Cover the sections with a coverslip.

Microscopic Examination of the Slide. Under low magnification, carefully examine the general structure of the leaf and make a schematic drawing with a pencil showing the structural distribution of individual tissue groups and their proportions. Once this diagram is complete, further study is best conducted under high magnification, successively examining each block regarding its respective Tissues and structural features. In an uncomplicated leaf, the upper and lower epidermis are easily distinguishable. The upper epidermis is formed by a single layer of horizontally elongated Cells. The cells are parenchymatous with thin walls. The outer wall of the epidermal cells is impregnated with cutin, which forms a continuous cuticular film over the upper surface of the epidermis (Fig. 60).

Beneath the epidermis on both sides of the midrib lie 2–4 layers of closely packed palisade parenchyma cells oriented perpendicular to the leaf surface. The cells are elongated, living, parenchymatous, with thin Cell walls, and filled with METABOLISM/14.html">Chloroplasts. Due to the high Abundance of chloroplasts, the palisade parenchyma performs the function of Photosynthesis. In certain areas within the epidermal and palisade cells, sac-like cells filled with prismatic calcium oxalate druses are visible.

Located beneath the palisade parenchyma is the spongy parenchyma, characterized by loosely arranged angular or oval cells with thin walls and fewer chloroplasts than those in the palisade layer. Large intercellular spaces form between the cells, which coalesce to create air passages. Due to this structure, the spongy parenchyma Functions in gas exchange and transpiration.

Together, the palisade and spongy parenchyma form the mesophyll, or leaf pulp. Large cavities filled with essential oil are visible within some of its cells. The inner walls of these cavities are lined with secretory tissue cells. On the lower side, the spongy parenchyma borders the lower epidermis. Like the upper one, it consists of a single layer of living parenchymatous cells with thin walls. The lower epidermis features a weakly developed cuticle and numerous stomata. In the prepared slide, guard cells are clearly visible, separated by a stomatal pore that opens into a substomatal cavity connected to the air passages of the spongy parenchyma.

In the central part of the slide, a large, prominent midrib is visible, flanked by less developed lateral vascular bundles. The central vascular bundle is surrounded by a continuous ring of sclerenchyma, formed by polygonal, tightly packed prosenchymatous cells with heavily thickened walls all around the perimeter. It is easy to notice that within the bundle, the xylem is oriented toward the upper epidermis and divided into distinct sectors by radial rays of living cells; the xylem vessels and tracheids are lignified with thickened cell walls.

A layer of fascicular cambium stands out beneath the xylem. Its cells are rectangular with thin walls, filled with cytoplasmic content, and nuclei are visible in places.

The phloem is adjacent to the cambium. It contains hollow sieve tubes, occasionally with sieve plates, and small companion cells filled with dense Cytoplasm.

Above the midrib, and even more so beneath it, lies angular collenchyma. It is formed by living parenchymatous cells possessing thickened corner and tangential walls.

In your workbook, draw a cross-section of the leaf showing the midrib. Indicate all the structural elements mentioned above on your drawing and provide the necessary labels.

Methods for Preparing a Cross-Section of a Corn Leaf. The corn leaf slide is prepared in the same way as the cross-section of the lemon leaf, but it does not require Treatment with aniline sulfate.

Microscopic Examination of the Slide. Under low magnification, draw a diagram showing the structural distribution of individual tissue groups and their proportions. Further study should be carried out under high magnification. The upper and lower epidermis (Fig. 71), containing numerous stomata, are easily located on the slide.

The lower epidermis is formed by a single layer of living, more or less uniform cells elongated in the horizontal direction. The outer cell walls are covered by a thick layer of cuticle. The upper epidermis, featuring single-celled simple trichomes in its depressions, consists of two cell types: ordinary small living epidermal parenchymal cells and large hollow cells, the so-called motor cells with reduced turgor, which facilitate the rolling of leaves into a tube during drought conditions. The parenchyma located between the two epidermal layers is called the leaf mesophyll. It shows no clear differentiation into palisade and spongy parenchyma; its cells have a more or less uniform oval or rounded, elongated shape and are filled with chlorophyll grains. This is because the leaf has an isobilateral structure, is positioned at an angle, and is well illuminated from both sides. At the same time, it can be observed that the mesophyll cells adjoining the upper epidermis are medium-sized and more tightly packed compared to the loose, intercellular-spaced cells adjacent to the lower epidermis.

The central vascular bundle consists of xylem oriented toward the upper epidermis and phloem facing the lower epidermis. The xylem is composed of a single smaller vessel (protoxylem) surrounded by water-storing parenchyma and two large pitted vessels (metaxylem) interconnected by thick-walled parenchyma. A small amount of xylem parenchyma is located between the protoxylem and metaxylem. The phloem within the bundle is well developed, represented by sieve tubes and tiny companion cells filled with cytoplasmic contents. Surrounding the phloem and xylem is a single layer of large, thin-walled bundle sheath cells, which in older leaves become fully or partially lignified at the top and bottom. Polygonal sclerenchyma cells adjoin these areas of the sheath ring, filling the entire space up to the upper and lower epidermis. In smaller bundles, the xylem is weakly developed and the phloem is more fully represented, surrounded by large parenchymal sheath cells filled with chloroplasts.

Methods for preparing a cross-section slide of a pine needle. Prepare the slide following the method used for preparing a lemon leaf cross-section.

Microscopic examination of the slide. Under low magnification of the Microscope, carefully examine and study the spatial and structural arrangement of individual tissue groups in the cross-section of the pine needle, and draw a schematic diagram of them.

Next, the internal structure should be studied exclusively under high magnification of the microscope. The slide clearly shows the cuticle covering a single-layered epidermis. The epidermal cells are nearly square with thickened tangential outer and inner walls. Stomata, consisting of sunken guard cells with thickened tangential walls, are present in certain areas. The stomata extend even into the underlying hypodermal tissue. The latter is formed by a single layer of horizontally elongated parenchymal cells. At the corners, the hypodermis consists of 2–3 cell layers. The Cell walls are slightly thickened yet lignified, protecting the internal Tissues of the needle. Beneath the hypodermis lies a multi-layered plicate (fold) parenchyma; its distinctive feature is The formation of inward wall projections resembling folds, hence the name plicate parenchyma. Chloroplasts are arranged along these folds, which is why it is also referred to as plicate chlorenchyma. These are large cells with thin walls. Schizogenous resin canals are embedded within the plicate parenchyma, located in the peripheral layer or separated from the hypodermis by 1–4 cell layers. The resin canals are rounded and consist of a resin duct canal in which droplets of resin can sometimes be seen. The canal is lined with living parenchymal epithelial cells whose function is to secrete resin. Surrounding the epithelium is a single layer of polygonal, tightly packed sclerenchyma cells with heavily thickened and lignified walls.

Moving further toward the center, a single layer of endodermal cells is visible. These cells are large, with somewhat thickened walls filled with starch grains. Below them lies a rather robust layer of transfusion tissue, formed by living parenchymal cells containing starch grains and hollow dead cells with bordered pits and slightly thickened walls (Fig. 72).

The needle slide reveals two vascular bundles whose xylems are formed by tracheids arranged in radial rows and oriented toward the outer flat upper epidermis, while the phloem, formed by sieve tubes, faces the convex lower part of the needle.

Bast fibers, or sclerenchyma fibers, are clearly noticeable between the vascular bundles and near the phloem. In cross-section, they appear as polygonal cells with heavily thickened walls all around their perimeter. It is precisely thanks to the sclerenchyma, endodermis, and hypodermis that the needle acquires its rigidity and firmness.

Conclusion. During The Study of the leaf, the features of internal differentiation and specialization of individual tissue groups were revealed. These features are characteristic of the LIFE FORMS OF various systematic plant groups. In dicotyledonous plants, the mesophyll is heterogeneous, represented by two types: palisade and spongy tissues; in monocotyledonous plants, it is homogeneous (in the form of spongy tissue); and in conifers, it is plicate. The identified differences are conditioned by the genetic Properties of the species and the action of environmental factors.

Class="center">Self-Assessment Questions:

1. Cells of which tissues and PARTS OF THE leaf possess thickened cell walls?

2. Name the primary tissues in the anatomical structure of the leaf.

3. Which tissue performs the function of photosynthesis, and which performs gas exchange and transpiration?

4. How does the lower epidermis of the leaf differ from the upper one?

5. What part of the leaf is called the mesophyll, and what is its structure in mono- and dicotyledonous plants?

6. Which tissue develops above and below the central vascular bundle in dicotyledonous plants?

7. What crystalline formations do you know of in leaf cells, and in which tissues are they contained?

8. What is The structure of the vascular bundle in the leaves of dicotyledonous plants?

9. Name the type of vascular bundle in a corn leaf and its constituent parts.

10. Which leaves are called isobilateral and how do they differ from dorsiventral ones?

11. In which leaves does plicate parenchyma develop, and how does it differ from palisade parenchyma?

12. What are bundle sheath cells, and in the leaves of which plants are they found?

13. What tissue is called transfusion tissue, and how is it differentiated?

14. Compare the structure of lemon and corn leaves, and highlight their differences.

15. In which of the studied leaves were storage reservoirs found, what is their origin, and what is their structure?