Plant Anatomy: Practical Course - Panyuta O.O. 2019

Topic 3. Structure of Vegetative Organs
Laboratory Work No. 11. Structure of Monocot Stem

Theoretical Background. The stem is an axial, typically symmetrical plant organ characterized by prolonged apical growth. It facilitates the bidirectional Transport of substances between roots and leaves, Supports the plant crown, increases the overall assimilating surface area through branching, serves as a storage organ, and performs photosynthetic Functions in its youth.

Objective: to study the Structural Features of monocot stems.

Materials and equipment: light microscopes, Glass slides and coverslips, dissecting needles, forceps, glass rods, filter paper, distilled Water, phloroglucinol with Hydrochloric acid, and plant material.

Slide. Structure OF THE corn stem (Zea mays L.)

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Fig. 57. Cross-section of a corn stem:

1 - epidermis, 2 - sclerenchyma, 3 - ground parenchyma, 4 - vascular bundles

To examine The structure of a corn stem, select a stem section that lacks a central hollow cavity. Using a sharp razor, cut several ultra-thin sections, ensuring the cut extends from the epidermis to the center. Alternatively, one may limit the section to half the stem (yielding a thinner slice) or even take a smaller wedge. Place the prepared sections into a drop of water on a Microscope slide and examine them under low magnification. The slide (Fig. 57) reveals the epidermis with its cuticle and a layer of thick-walled mechanical Cells. These thick-walled cells represent lignified sclerenchyma fibers. Beneath them lie large, thin-walled Cells of the ground parenchyma, interspersed with scattered collateral vascular bundles. The ground parenchyma cells contain Cytoplasm, and their walls feature distinct pits, which become clearly visible if the slide is stained with chlor-zinc-iodine.

Certain structural variations can also be observed across different Zones of the section. Most notably, the diameter of the vessels decreases from the center toward the periphery. Bast fibers and a broad ring of small parenchymatous cells with thickened walls lie directly beneath the epidermis. In deeper layers of the parenchymatous tissue, The Cell walls become thinner. Vascular bundles are more numerous in the peripheral region and contain a higher proportion of bast fibers compared to those situated closer to the center. This peripheral architecture has an adaptive significance: it prevents the stem from breaking in the wind, as the windward side undergoes tension while the leeward side undergoes compression. In the former case, stretching occurs, whereas in the latter, contraction takes place. These mechanical stresses of stretching and compression are minimal in the center of the stem and maximal at the periphery.

Slide. Structure of the rye stem (Secale cereale L.)

Cross-sections of the rye stem are treated with phloroglucinol and hydrochloric acid (or aniline sulfate) to enhance the contrast of the lignified cell walls. The preparations are then examined under low and high microscope magnifications. Externally, the rye stem (Fig. 58) is covered by an epidermis composed of a single layer of cells with lignified walls. Underlying the epidermis is the sclerenchyma, consisting of thick-walled cells that form a wide peripheral ring. Pits are visible within the sclerenchyma cell walls, and middle lamellae are present between adjacent

cells.

Fig. 58. Cross-section of a rye stem:

1 - epidermis, 2 - sclerenchyma, 3 - assimilation tissue, 4 - vascular bundles, 5 - ground parenchyma

Sclerenchyma is characteristic of all grass stems, though its development can vary. Under conditions of dense crop sowing, excessive nitrogen or other mineral fertilizers, and high soil moisture, sclerenchymatous fibers fail to develop properly, leading to crop lodging. The rye stem features a chlorophyll-bearing parenchyma composed of parenchymatous cells containing METABOLISM/14.html">Chloroplasts. This tissue forms small patches within the sclerenchymal ring and consists of thin-walled cells with non-lignified walls. The chlorophyll-bearing parenchyma is clearly discernible only in cross-sections taken from fresh, green stems.

Towards the center of the stem—bordering the central cavity—lies a parenchyma composed of large cells. Once the stem has completed its growth, these cells lack cytoplasmic contents.

The arrangement of vascular bundles in rye is distinctive, as they form two rings: one peripheral and another located deeper within the large-celled parenchymatous layers. In both cases, the vascular bundles are surrounded by a strengthening sclerenchymal sheath. The vascular bundles are closed, meaning they lack a cambium. Peripheral vascular bundles are significantly smaller than those located closer to the center. Each peripheral bundle contains phloem towards the outside and xylem facing the center. The phloem consists of sieve tubes and companion cells containing living protoplasts. The xylem prominently features two large vessels separated by parenchymatous cells, with 2 or 3 spiral and annular vessels extending further inward toward the center. A central cavity is clearly visible in the middle of the stem.

Slide. Structure of the asparagus stem (Asparagus officinalis L.)

Fig. 59. Portion of a cross-section of an asparagus stem:

1 - epidermis, 2 - Stomata, 3 - primary cortex, 4 - endodermis, 5 - sclerenchyma,

6 - ground parenchyma, 7 - sieve tubes, 8 - companion cells,

9 - annular and spiral vessels, 10 - reticulate vessels

To prepare the slides, it is best to use young asparagus shoots that are just emerging from the soil and retain a whitish color. To gain An Overview of the stem anatomy of this plant, cross-sections are made across the entire SHOOT, mounted in a drop of iodine-potassium iodide solution, and examined under low magnification (Fig. 59).

A thin epidermal layer consisting of a single layer of cells is visible along the periphery. The outer walls of the epidermal cells are covered with a cuticle, while the lateral and inner walls lack thickenings. Stomata are present in some areas of the epidermis. Beneath the epidermis lies the cortex, composed of parenchymal cells containing chloroplasts and starch grains, which can be detected using iodine tests. The innermost layer of the cortex is the endodermis, or starch sheath, which separates the cortex from the central cylinder. The cells of the starch sheath are thin-walled and packed with starch grains. Underlying the endodermis are several layers of thick-walled sclerenchyma cells. The sclerenchyma cell walls are lignified, turning cherry-red when treated with phloroglucinol and hydrochloric acid, and yellow with aniline sulfate. The size of the sclerenchyma cells increases from the periphery toward the center. These cells gradually transition into the ground parenchyma, which consists of cells with slightly thickened walls. Vascular bundles are embedded within the ground parenchyma tissue, and their size increases noticeably from the periphery to the center. At the center of the stem lies the pith, where bundles are absent. As is typical for all monocotyledonous plants, the vascular bundles in the asparagus stem are closed, meaning they lack cambium and do not undergo secondary thickening. Each bundle contains phloem, located toward the periphery, and xylem. In the phloem, wide sieve tubes and smaller companion cells with a granular content are clearly visible. The xylem consists of spiral and annular tracheids, as well as spiral and reticulate vessels. Their cell walls are lignified, as confirmed by the corresponding color reactions.