Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Class Topic 8.21. Anatomical Structure of the Stem of Fiber Crops

General Remarks. Fiber crops are characterized by a non-bundle type of stem Structure. The procambium is laid down as a continuous ring even within the SHOOT apex. It develops into a cambium, which cuts off a continuous cylinder of phloem towards the periphery and xylem towards the center. A distinctive feature of these crops is the presence of bast fibers. These are represented by either one type (in flax) or two types (in hemp, ramie) of bast fibers. Primary bast fibers are long and elastic with weakly lignified walls, whereas secondary fibers are short and heavily lignified, which limits their use in the textile industry. Structurally, primary bast fibers originate from the pericycle or parenchyma, while secondary fibers are of cambial origin.

Objects:

1. Stem of fiber flax (Linum usitatissimum L.)

2. Stem of cultivated hemp (Cannabis sativa L.)

Tasks:

1. Study the Anatomical Features of the non-bundle stem structure in herbaceous plants.

2. Examine The structure of the flax stem and its bast fibers.

3. Using the hemp stem as an example, identify the structural differences compared to the flax stem.

4. Draw a sector of the cross-section for each stem and label their constituent parts.

Equipment and Materials: MBR-1 microscopes, magnifying glasses, razors, scalpels, forceps, charts, prepared slides, Reagents, etc.

Procedure for preparing a cross-section of a flax stem. To prepare the slide, take a pre-cut piece of flax stem and insert it into a slit in a piece of elderberry pith to a depth of up to 2 cm. Using a scalpel, trim The surface of both the stem and the elderberry pith so that it is perpendicular to the stem axis. Using a razor or blade, make a series of thin sections, preferably from different PARTS OF THE flax stem (upper, middle, and lower). Using a magnifying Glass, select the best sections: complete, thin, and containing all tissue groups. Place the sections on a Microscope slide and treat them with phloroglucinol or an iodine-potassium iodide solution. Cover the sections with a coverslip. Secure the finished slide on the microscope stage using clips.

Microscopic examination of the slide is best carried out in two stages. First, examine the self-prepared or permanent slide under low magnification. A careful examination makes it easy to identify and distinguish the familiar tissue groups: epidermis, primary cortex, phloem, cambium, secondary xylem, and pith. Draw a diagram of the cross-section sector and label these tissue groups in your lab manual. In the second stage, switch to high magnification. Observe and illustrate the fine details and Structural Features of the aforementioned tissue groups in general and each one in particular. The epidermis is distinct and single-layered. The Cells are parenchymatous, thin-walled, rectangular, and contain living cytoplasmic content. The outer Cell walls are covered with a cuticle, which protects the stem from wetting and the penetration of larvae and spores.

Stomata are clearly visible in the epidermis. They consist of two guard cells with thickened tangential walls. Between the cells is a stomatal pore that opens and connects to the intercellular air space of the stem (Fig. 64).

Primary cortex. Beneath the epidermis lies a rather thick, multi-layered parenchyma. Its distinctive feature is the large size of the cells filled with METABOLISM/14.html">Chloroplasts, which is why it is often referred to as chlorenchyma. This is typical of young stems. The innermost layer is formed by the endodermis. These cells are large, oval, or angular in shape, and contain starch grains, hence the name starch sheath.

Bast fibers. Just inside the endodermis, the bast fibers are easily noticeable. The slide clearly shows distinct groups of large cells with heavily thickened walls all around the perimeter. As a result, The Cell lumens remain very small. These sclerenchyma cells develop from the pericycle or primary cortex parenchyma, meaning they are primary bast fibers.

Phloem. Moving inward on the prepared slide, the phloem becomes visible. Examine its sieve tubes and companion cells. The former are larger, hollow cells, whereas the latter are filled with dense Cytoplasm. Interspersed among and around these elements is the phloem parenchyma, formed by medium to large-sized cells.

The cambium is clearly visible. It is well-defined and grid-like in appearance, with cells arranged in radial rows, one above the other. The cells contain cytoplasm and are capable of division.

The secondary xylem occupies the largest part of the slide: its composition includes vessels, libriform fibers, and tracheids. Interspersed among them are living xylem parenchyma cells with dense cytoplasm, as well as medullary ray parenchyma. The inner part of the xylem, appearing as distinct sectors between adjacent medullary rays, represents the primary xylem. It consists of similar elements: tracheids and xylem parenchyma.

In the center of the stem is the pith, which partially breaks down in the lower part, resulting in a hollow cavity. Radiating outward from the pith are the medullary rays, formed of living parenchymatous cells.

Procedure for preparing a cross-section of a hemp stem. The hemp stem slide is prepared in the same way as the flax stem slide. It is advisable to take sections from the apex, middle, and lower parts of the stem. Under the microscope, the stem appears solid and hollow, with lignified basal regions.

Microscopic examination of the slide. It is best to begin the study under low magnification. At the same time, the same tissue groups as in the flax stem can be observed. Draw a schematic representation showing their proportions and spatial distribution. A detailed Study of the self-prepared slide is best performed under high magnification. Let us examine the individual tissue regions.

The epidermis forms the outer layer of the hemp stem. It is clearly visible and consists of medium-sized, living parenchymatous cells. Its outer walls are impregnated with cutin, forming a continuous lipid-like film called the cuticle. Here and there, the continuous epidermis is interrupted by stomata, which consist of two guard cells. In addition, unicellular, curved rough hairs (trichomes) lying parallel to the stem surface can be observed in places. Beneath the epidermis lies the primary cortex, composed of collenchyma and parenchyma.

Collenchyma is a characteristic feature of the hemp stem. It is located directly beneath the epidermis or as a layer within the cortical parenchyma. It is a lamellar collenchyma formed by elongated parenchymatous cells with thickened tangential walls.

A significant portion of the peripheral stem area is occupied by the cortical parenchyma. Calcium oxalate druses can be observed within its living, rounded cells. In a young section, the innermost layer consists of a single layer of starch sheath cells. The cells of this layer often merge with the cortical parenchyma and are poorly distinguishable on slides.

Bast fibers are easy to identify in the prepared slide. There are two types of bast fibers: Primary and secondary. Primary bast fibers are located directly beneath the cortical parenchyma as a continuous band or separate isolated patches. They are formed by prosenchymatous cells. In cross-section, these cells are polygonal, tightly packed, with uniformly thickened walls around their entire perimeter. Structurally, primary bast fibers are of pericyclic origin or derived from the primary cortex parenchyma.

Deeper within the secondary phloem lie the secondary bast fibers. These are composed of smaller cells that are similarly polygonal, tightly packed, and possess uniformly thickened walls. Cambial in origin, they are more lignified and shorter than primary fibers. Their commercial quality is inferior to that of primary bast fibers, which restricts their use to the manufacture of burlap and twine. Sieve tubes are conspicuous as relatively large, hollow cells, accompanied by much smaller companion cells with dense cytoplasmic contents. Parenchyma cells are scattered individually or in groups throughout various areas.

The cambium exhibits a familiar structure, consisting of living, thin-walled, rectangular parenchymatous cells. These cells are filled with cytoplasm and retain the capacity for division.

Wood, or secondary xylem, occupies the greater part of the cross-section and is easily recognized by the presence of large, pitted vessels. These vessels show no regular arrangement and are scattered throughout the secondary xylem. The bulk of the wood is represented by medium-pitted vessels with exceptionally thickened walls, mostly arranged one above another in regular radial rows, separated by xylem parenchyma and medullary ray parenchyma. Xylem parenchyma is better developed in the hemp stem than in the flax stem. Within the secondary xylem matrix, distinct groups of cells with heavily thickened cell walls can be observed, forming the libriform fibers. The libriform is particularly distinct in cross-sections taken from the lower part of the stem, which exhibits a higher degree of lignification compared to the mid-stem region.

Primary xylem is found in the lower inner region adjacent to the pith. It contains the same structural elements: pitted vessels, fine-pitted vessels, xylem parenchyma, and primary medullary ray parenchyma (Fig. 65).

Primary medullary rays extend outward from the pith parenchyma toward the periphery, while those that do not reach the pith form secondary rays. Both types of medullary rays are formed from living, thin-walled parenchymatous cells with dense cytoplasmic contents.

Conclusion. The stems of fiber crops feature a non-bundle (stele-like) type of structure, which originates in the shoot apical meristem as a continuous procambial ring. The division of its cells gives rise to a secondary meristematic tissue—the cambium—which produces a continuous cylinder of phloem toward the periphery and a continuous cylinder of xylem toward the center. The anatomical structure comprises the following tissue systems: epidermis, primary cortex, secondary cortex, cambium, secondary xylem, and pith.

In the flax stem, bast fibers are of pericyclic origin. In male hemp plants (staminate plants), they are exclusively pericyclic, whereas in female hemp plants (pistillate plants), they are of both pericyclic and cambial origin. Unlike the flax stem, the hemp stem develops collenchyma just beneath the epidermis.

Review Questions:

1. Name the tissue characteristic of the stems of fiber crops.

2. What is THE ORIGIN OF bast fibers in flax and hemp?

3. Characterize the constituent parts of the secondary phloem.

4. Which tissue is located between the phloem and the xylem? How does it function?

5. In the anatomical structure of which plant are both primary and secondary bast fibers distinguished?

6. Which fiber crop features collenchyma in its stem structure, and where is it located?

7. How do primary medullary rays differ from secondary ones?