Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Class Topic 8.17. Anatomical Structure of Monocotyledonous Stems

General remarks. Monocotyledonous grass plants have specific Anatomical Features in their stem Structure: beneath the epidermis lies sclerenchyma rather than the primary cortex; vascular bundles of the closed collateral type are arranged chaotically; cambium and The ability to undergo secondary thickening are absent, and the stem takes the form of a culm (straw), etc.

Objects:

1. Corn (maize) stem (Zea mays L.)

2. Rye culm (Secale cereale L.)

Tasks:

1. Prepare temporary cross-section slides of the corn stem on your own.

2. Study the Water/140.html">Anatomical Structure of the corn stem and sketch a sector of the cross-section.

3. Study and sketch a closed vascular bundle of the corn stem.

4. Study the anatomical STRUCTURE OF THE rye culm using a prepared slide.

Equipment and Materials: microscopes, hand lenses, razor blades, scalpels, forceps, handout materials, permanent slides, charts, Reagents, etc.

Method for preparing a cross-section slide of a corn stem. Place a drop of iodine-potassium iodide solution onto a Glass Microscope slide. Take a piece of sliced corn stem in your left hand so that it extends 5—8 mm above your fingers. Level the top surface of the stem using a scalpel. Take a razor blade in your RIGHT HAND AND cut a series of cross-sections. The section must be strictly perpendicular to the stem axis. Furthermore, a mandatory requirement is to take the slice from the periphery, though it is not necessary to cut all the way across the entire stem. Ideally, the section should be as thin and transparent as possible. Select the 2—3 best sections using a hand lens, place them into the drop of solution, and cover with a coverslip. Secure the slide.

Microscopic examination of the slide. First, examine the slide under low microscope magnification. At this power, you can distinguish the following tissue blocks: the epidermis, the sclerenchymatous ring, the ground parenchyma, and the vascular bundles. Observe them and make a pencil diagram of their arrangement, then fill in the structural details.

Using high magnification, thoroughly and systematically examine all Tissues and sketch their features onto your diagram. The slide clearly shows a single-layered epidermis formed by parenchymatous Cells elongated in the horizontal direction. Their Cell walls are somewhat thickened and lignified. The outer wall is covered by a continuous cuticular film. Stomata, each consisting of two guard cells, are occasionally visible in the epidermis.

Beneath the epidermis in many monocots, a primary cortex is visible; however, in the corn stem, it is noticeable only at a young stage as a thin layer of chlorophyll-bearing parenchymatous Cells of the primary cortex. As the plant ages, this changes, and a pericyclic sclerenchymatous ring comes to lie directly adjacent to the epidermis. This ring is formed by polygonal, prosenchymatous, lignified cells with hollow lumina uniformly thickened around their entire perimeter. On a finished slide, they exhibit a reddish-magenta color. The cells are tightly packed. The tissue is characterized by high elasticity, giving the stem mechanical strength, especially against bending.

Deeper lies the robust region of the ground parenchyma (Fig. 57). Its cells are parenchymatous, thin-walled, and rounded, with numerous intercellular spaces; their size increases from the periphery toward the center. They are filled with living protoplasts, which is why young stems are used as green fodder or for silage.

Embedded within the ground parenchyma are closed collateral vascular bundles. In the peripheral region, they are smaller in size and more densely packed, whereas toward the center, they occur less frequently and increase in size. You should select one of the best-defined bundles among them for detailed study and close-up sketching under high microscope magnification.

Pay attention to the fact that the bundle is surrounded by sclerenchymatous tissue, which is horseshoe-shaped and multi-layered at the top and bottom, and single-layered on the sides. The phloem, oriented toward the periphery, and the xylem, oriented toward the center, are clearly visible within the bundle. The phloem is readily discernible as a delicate meshwork where hollow, thin-walled, polygonal cells form sieve tubes, while small, square cells with dense cytoplasmic contents form companion cells (Fig. 58). Living phloem parenchyma cells are found among and around them.

The xylem is easily distinguished on the slide. Flanking it are large, pitted, rounded vessels with thickened cell walls. Thick-walled parenchyma, composed of polygonal, elongated parenchymatous cells with uniformly thickened walls, interconnects these vessels. In the middle, extending from this tissue toward the center, are two smaller, rounded vessels — an annular-spiral and an annular vessel. They border a large lacuna (rupture cavity) that formed in place of vessels destroyed early in the stem's development. Living, thin-walled xylem parenchyma containing nutrient reserves adjoins this cavity on both sides.

Method for preparing a cross-section slide of a cultivated rye culm. Take a piece of elderberry pith and make a longitudinal slit down the middle to a depth of 1.5—2 cm. Insert the rye culm, taken from the internode region, into the slit. Level the surface using a scalpel or razor blade. Cut a series of thin cross-sections and place them on a glass slide. Apply a drop of phloroglucinol to the section, followed 5—10 min later by a drop of Hydrochloric acid, then rinse with water using a pipette. Cover the specimen with a coverslip and examine the finished slide under a microscope.

Microscopic examination of the slide. Using low microscope magnification, examine the slide and schematically sketch the individual tissue blocks in your lab notebook: the epidermis, sclerenchyma, ground parenchyma, and vascular bundle. Under high magnification, carefully examine each of the named parts, their proportions, and structural features (Fig. 59). Externally, the stem is covered by an epidermis consisting of a single layer of parenchymatous cells slightly elongated horizontally. The outer walls are slightly thickened and covered by a continuous, thin cuticular film. The epidermis is not continuous and is interrupted in places by stomata. Each stoma consists of two guard cells. The latter are Kidney-shaped with tangentially thickened walls and thin-walled radial walls, which allows the stomata to open and close. Beneath the stomata lies a substomatal cavity, extending deep into the regions of chlorophyll-bearing parenchyma. Its cells are rounded, thin-walled, and filled with METABOLISM/14.html">Chloroplasts. In certain areas, the epidermis connects directly with the sclerenchyma, which assumes the appearance of a gear wheel, with the indentations of its outer part filled with chlorophyll-bearing parenchyma.

The sclerenchyma cells are polygonal with cell walls uniformly thickened around the entire perimeter. The cells are tightly packed, providing the stem with resistance to bending stress.

Further inward, the ground parenchyma occupies the greater part of the area. Its cells are large, rounded, and feature intercellular spaces. The innermost layers are formed by smaller cells.

Embedded within the mass of ground parenchyma are closed collateral vascular bundles. They are capped above and below by sclerenchymatous tissue, which is poorly developed or absent on the sides. The outer sclerenchyma covers the phloem portion of the vascular bundle. The greater part consists of sieve tubes, which appear hollow on the slide. Small, square companion cells adjoin them. Below lies the phloem parenchyma. The xylem adjoins the phloem and is easily identified by two large, pitted vessels connected by polygonal cells of thick-walled parenchyma. Just below and centrally located are two smaller vessels — an annular and an annular-spiral vessel — which are rounded with thickened pink cell walls. Flanking them is the xylem parenchyma, formed of polygonal parenchymatous cells. Below this, a prominent rupture cavity (lacuna) is visible, bounded underneath by sclerenchyma.

Two Types of vascular bundles can be distinguished in the rye stem: large ones in the ground parenchyma (described above), and smaller, incompletely developed bundles located within the sclerenchyma.

Conclusions. The anatomical structure of the monocot stem is characterized by several distinctive features: 1. Vascular bundles are of the closed collateral type. 2. Cambium is absent. 3. There is no capacity for secondary thickening. 4. Vascular bundles are scattered irregularly, following the so-called palm type, while their partial alignment is due to The formation of the stem cavity. 5. Sclerenchyma located beneath the epidermis strengthens the culm against bending.

Self-Assessment Questions

1. Name the mechanical tissue, its Location, and origin in the corn stem.

2. What accounts for the bundle-type structure of the stem?

3. What type of vascular bundles is typical for the stems of monocotyledonous plants?

4. Name the histological elements of the phloem.

5. List the histological elements of the xylem.

6. Which tissue occupies the largest portion of the monocot stem?

7. Why are the vascular bundles in monocot stems referred to as fibro-vascular bundles?

8. What types of vessels are present in the xylem of the rye stem?