Plant Anatomy: A Practical Guide - Paniuta O.O. 2019

Topic 3. Structure of Vegetative Organs
Laboratory Work No. 14. Anatomical Structure of the Root

Theoretical Background. The Internal Structure of a ROOT varies across its different zones. This is because these regions perform distinct physiological Functions. The uptake of Water and mineral nutrients takes place in the young, actively growing Regions of the root, which exhibit a Primary Structure.

In monocotyledonous plants, this primary structure persists throughout their life cycle, whereas in dicots, it is rapidly replaced by Secondary Growth.

In dicots, the bulk of the root possesses a Secondary structure. There is a fundamental difference between the Primary and secondary root structures.

The root tip is protected by a root cap. Shaped like a thimble, it shields the delicate, actively growing root zone from various types of mechanical damage. Directly behind the root cap and the 2–3 mm meristematic zone lies the root elongation zone. It consists of young Cells produced through Cell Division. Like the meristem, the elongation zone is quite compact, spanning just a few millimeters. It is easily distinguished by its light coloration, whereas the rest of the root is dark brown or nearly black. The cells at the root tip have delicate, thin walls. Capable of absorbing water and various solutes, they form the rhizodermis, or epiblema. The surface of the rhizodermis produces root hairs and is covered with a mucilaginous substance composed of pectin.

The root Hair zone is relatively small, reaching up to 2 cm in length. Root hairs are short-lived; they quickly wither and are replaced by new ones. In regions where root hairs degenerate, the rhizodermis also breaks down and is superseded by a temporary protective tissue known as the exodermis. It is composed of tightly packed cells and exhibits considerable structural Variability. For instance, in certain plants (such as hemp and peas), the exodermis remains undifferentiated. The exodermis functions as a protective layer until the periderm develops.

The Introduction/19.html">Primary structure of the absorptive root region can be divided into two main parts: the central cylinder, containing the vascular elements, and a wide outer layer known as the primary cortex.

The innermost layer of the primary cortex is the endodermis. It encircles the central cylinder and plays a crucial physiological role. The endodermis consists of a single layer of cells. Most endodermal cells feature thickened walls known as Casparian strips, while certain cells remain unthickened and unsuberized, ensuring communication between the central vascular cylinder and the peripheral Tissues. These are referred to as passage cells. Passage cells are typically positioned opposite the xylem bundles of the stele. The central core of the root is separated from the primary cortex by the pericycle, which consists of living cells with thin, delicate walls and dense Cytoplasm. In monocots, the walls of pericycle cells eventually thicken and lignify.

The pericycle can be uniseriate or multiseriate. A single-layered pericycle, consisting of a single row of parenchymatous cells, is the most common. A multiseriate pericycle is characteristic of gymnosperms.

The primary root structure displays a radial arrangement of xylem and phloem. Interspersed between the xylem strands is delicate phloem tissue, consisting of sieve tubes and phloem parenchyma.

Thus, the primary STRUCTURE OF THE root is characterized by a radial vascular bundle.

As previously noted, the primary root structure is typical of monocots, whereas in dicots, it quickly transitions into a secondary structure. These changes begin early in plant development, even within seedlings. They originate in the parenchymatous tissue as cells acquire the capacity for division and growth. Specifically, beneath the strands of primary phloem, a strip of meristem arises in the form of rows of short initial cells. It produces xylem cells inward and phloem cells outward.

Objective: to study the Anatomical Features of root structure.

Materials and Equipment: light microscopes, Glass slides and cover slips, dissecting needles, forceps, glass rods, filter paper, distilled water, phloroglucinol in Hydrochloric acid, aniline sulfate, hydrochloric acid, plant material.

Slide. Root Structure of Duckweed (Lemna minor L.)

Duckweed is commonly found On the surface of water in marshes, shallow rivers, and ponds. It is a small plant whose body consists of fronds and small roots that develop on the underside of the thallus. The thallus itself is green and packed with METABOLISM/14.html">Chloroplasts; however, it is not a true leaf, but rather a modified stem performing a foliar function.

Notably, duckweed is an exceptionally convenient model Organism for studying primary root structure. The plant can easily be kept alive in aquaria throughout the winter. Its roots drop off in late November but regrow by early February.

To examine the root structure of duckweed, gently pluck the root tips with forceps, place them on a glass slide in a drop of water, and observe under a Microscope at low magnification. The duckweed root is transparent, allowing its entire internal anatomy to be viewed microscopically. Externally, the root is covered by an epidermis composed of rectangular cells. These cells absorb water and dissolved nutrients essential for the normal growth of the plant. The root tip terminates in a protective cap. It resembles a single-layered pocket that serves a protective function. Chloroplasts are visible within the cap cells. Scattered among them are occasionally secretory cells containing bundles of calcium oxalate crystals in the form ofraphides.

Situated beneath the rhizodermis is the primary cortex, which consists of parenchymatous cells. The central cylinder lies inward from the cortex.

After examining the slide, make a biological drawing of it.

Slide. Root Structure of Wheat (Triticum sp.)

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Fig. 83. Root structure of wheat:

1 - root cap, 2 - apical meristem, 3 - elongation zone, 4 - absorption

zone

To study the root structure of wheat, grains should be set out to germinate three to four days prior to the lab session. The small rootlets developing on the seedlings can then be used to examine the primary root structure.

To prepare the sample, cut off 3-4 tips up to 2 cm long with scissors, place them in a drop of water on a microscope slide, and examine them under a microscope at low magnification.

Under the microscope (Fig. 83), it can be seen that the root tip is covered by the root cap. The root cap has the shape of a sharp cone that covers the rounded root apical meristem (growing point). The cap consists of cells containing living cytoplasmic contents, nuclei, and thin cellulosic walls. The size of the cap cells is not uniform throughout; they are smallest where they come into contact with the apical meristem. This exact area houses the meristematic tissue of the cap, which borders the meristematic tissue of the growing point. This structural feature is characteristic of monocotyledonous plants. In gymnosperms and dicotyledonous plants, the growing point and the root cap share a common meristematic tissue.

The cap cells are loosely connected to one another and gradually slough off in the outer layer. This phenomenon can be observed directly on the slide. As already noted, the cap cells vary in size: they are large at the periphery and small toward the center. This is because they originate at different times and increase in volume as they grow. A characteristic feature of the root cap is that it continuously grows at the expense of the meristem adjacent to the apical meristem. The growing part of the root moves through the soil layers, pushing soil particles aside and encountering significant resistance along its path. Consequently, the root cap protects the embryonic Tissues of the root from damage.

The root apical meristem is located immediately behind the cap. It consists of meristematic cells with cellulosic walls, living cytoplasmic contents, and a nucleus. To examine the growing point more clearly, one should gently press the preparation with a finger or a dissecting needle through a piece of filter paper placed over the coverslip. In such cases, the elements comprising the growing point become more distinct. Along with young embryonic cells, older cells that are beginning to differentiate are also visible. These cells gradually transform into the elements of the tissues lying above them. A characteristic feature of the cells in the root apical meristem is the absence of vacuoles. The cavities of these cells are filled with cytoplasm and contain large nuclei that are visible even without staining. Inside the root, a dark strip is visible, sharply differentiated from the other parts. This is the central cylinder, which consists of Vascular Tissues.

Beyond the growing point lies the zone of elongation, followed by the maturation zone (zone of absorption) bearing root hairs. Starting from the root tip, the length of the hairs increases gradually. On the slide, all stages of root hair development can be traced, from a barely perceptible protrusion arising in the rhzdermis cells to quite long hairs whose cells have a tubular shape.

Root hairs are not separated by partitions from the cells from which they originate. Thus, root hairs are unicellular outgrowths of rhizodermal cells. The Hair cells have thin cellulosic walls, and no cuticle is formed on them. The cytoplasm forms a thin layer adjacent to the walls. A nucleus is visible within it, and the entire cell cavity is occupied by cell sap. The size of root hairs varies among different plant species. In woody plants, root hairs are shorter than in herbaceous plants. The function of root hairs is to absorb water and dissolved mineral nutrients from the soil, which are essential for plant nourishment. In the center of the root within the absorption zone, spiral and pitted vessels can be observed.

Slide. Structure of Solomon's seal root (Polygonatum officinale All.)

To study the primary structure of the root, the root of Solomon's seal can be used. When making sections, the root of Solomon's seal is clamped inside an elderberry pith because the roots are soft and cut poorly. For a better examination of the root tissues, the sections are treated with phloroglucinol and hydrochloric acid or with aniline sulfate.

Fig. 84. Cytology/practical/72.html">Cross section of Solomon's seal root:

1 - rhizodermis, 2 - cork layer,

3 - exodermis, 4 - mesodermis,

5 - endodermis, 6 - passage cells, 7 - pericycle, 8 - xylem, 9 - phloem, 10 - parenchyma

At low magnification (Fig. 84), it can be established that the root is externally covered by the rhizodermis with root hairs. The rhizodermal cells are slightly elongated, and the hairs are long and curved at the edges. Beneath the rhizodermis lies the exodermis. Passage cells are scattered among the cells of this layer. The exodermal cells are radially elongated, empty, and their walls are lignified. Below the exodermal layer lies the mesodermis, or the parenchyma of the primary cortex, which is subdivided into outer and inner regions. The outer region is composed of parenchymatous cells with thin cellulosic walls that do not stain with the applied Reagents. In some mesodermal cells, calcium oxalate crystals in the form of raphides are visible. In addition, cytoplasmic contents and a nucleus can be seen within them.

The inner layer of the mesodermis, adjacent to the central cylinder, consists of parenchymatous, loosely connected cells with intercellular spaces. The Cells of the inner layer are smaller than those of the outer layer, and their size increases from the center toward the periphery. As already noted, they form a loose tissue, whereas the cells of the outer layer fit tightly against one another, even though their walls are thinner than those of the inner layer cells.

In the cross section, the difference in volume between the central cylinder and the peripheral part is striking. Here, the primary cortex occupies a significantly larger volume than the central cylinder, which does not correspond to the ratio typical of the stem.

The innermost layer of the primary cortex is called the endodermis. It demarcates the central cylinder from the peripheral region. The endodermal cells are slightly elongated; under The Influence of reagents, they stain red or yellow, indicating that they are slightly lignified. Passage cells, whose walls remain thin, can also be observed here in the endodermis. Beneath the endodermis lies a layer of thin-walled living cells—the pericycle—which borders the endodermis on one side and the central cylinder on the other. In the central part of the root, there is a radial vascular bundle where phloem and xylem alternate and are arranged in radial groups. The xylem cells are stained red or yellow, depending on the reagent used to treat the slide. The size of the xylem cells is non-uniform: they are larger toward the center and smaller toward the periphery. The walls of the xylem cells are lignified. Phloem, consisting of unstained thin-walled cells, is noticeable between the xylem groups. A certain regularity is observed in the number of xylem and phloem strands, namely: the number of xylem strands corresponds to the number of phloem strands.

Longitudinal sections can be prepared from the root of Solomon's seal. To do this, a piece of the root is cut, placed horizontally into a split elderberry pith, and several longitudinal sections are made. The prepared sections are placed on a microscope slide in a drop of water, treated with phloroglucinol and hydrochloric acid or with aniline sulfate, and examined under a microscope (Fig. 85).

Fig. 85. Longitudinal section of Solomon's seal root:

1 - scalariform vessels, 2 - parenchyma,

3 - tracheids, 4 - sieve tubes,

5 - pericycle, 6 - endodermis, 7 - mesodermis, 8 - exodermis, 9 - cork layer, 10 - rhizodermis

Moving from the central part to the periphery, scalariform vessels are clearly visible in the center. Next, a strip of elongated cells containing cytoplasmic contents and thin walls is noticeable. These are pith cells, followed by short cells—tracheids. Depending on the plane of the section, the tracheids will either adjoin the scalariform vessels or be separated from them by pith cells. Further out are the phloem elements (sieve tubes), followed by elongated cells with very thin walls—the pericycle. The endodermis lies beyond the pericycle, and the mesodermis lies beyond that.

The endodermal cells are longer than the pericycle cells.

Two layers can be distinguished within the mesoderm: an inner and an outer layer. The cells of the inner layer are elongated and have intercellular spaces, while those of the outer layer are shorter.

A longitudinal section shows the same pattern observed in the transverse sections, namely, that the length of the mesoderm cells increases from the periphery toward the center. In the inner part, a distinct layer of cells is visible, containing accumulated crystals of calcium oxalate in the form of raphides. The exodermis layer lies beyond the outer zone.

Externally, the root is covered by a thin layer of rhizodermis.

If Solomon's seal is unavailable, longitudinal and transverse sections can be prepared from the root of a common onion instead.

Fig. 86. Transverse section of an onion root:

1 - rhizodermis, 2 - exodermis, 3 - outer cortex, 4 - mesoderm (inner cortex), 5 - endodermis, 6 - pericycle, 7 - phloem, 8 - xylem, 9 - cavity, 10 - vessels

To prepare the slides, onions are pre-planted in soil in small pots or boxes. After 10–15 days, white roots develop, from which sections can be made. Sections of the onion root (Fig. 86) reveal that the endodermis has a different structure here compared to that of Solomon's seal. The difference is that it consists of small cells with thin, unthickened walls, yet marked by distinct dark lines resembling thickenings. These form on the radial walls and are called Casparian strips. The pericycle cells are also small, with

thin walls. There is no pith in the onion root. Instead, a large central cavity is surrounded by vessels that form xylem bundles alternating with phloem bundles.

Slide. Structure of the root of the German iris (Iris germanica L.)

Fig. 87. Transverse section of an iris root:

1 - rhizodermis, 2 - exodermis, 3-4 - mesoderm, 5 - endodermis, 6 - passage cells, 7 - pericycle, 8 - xylem, 9 - phloem

Elderberry pith is split in half, and a young iris root fragment is inserted into it. Sections are cut with a sharp razor, placed on a microscope slide, treated with aniline sulfate and hydrochloric acid, and examined under a microscope at low and high magnification.

The root structure of the iris is very similar to that of the common onion and Solomon's seal. A transverse section (Fig. 87) shows that the root is externally covered by the rhizodermis, consisting of slightly elongated parenchymal cells, a significant portion of which bear root hairs. Beneath the rhizodermis lies the primary cortex, divided into three layers: the outermost layer is the exodermis, followed by the mesoderm, whose cells are loosely arranged with prominent intercellular spaces, and the innermost layer is the endodermis, consisting of dissimilar cells. These cells are prismatic in shape; the vast majority have thickened walls, while only a small fraction have thin cell walls—these are the passage cells. The endodermis performs a mechanical function. In young rootlets, the endodermal cells have a smooth, thin wall with dark spots on the radial walls where suberin is deposited. As the root grows, the lateral and inner walls thicken and become lignified (except for the passage cells). The function of the passage cells is to allow the transit of water and dissolved mineral compounds. A layer of cells called the pericycle lies beneath the endodermis. It consists of uniformly shaped cells with thin walls and living cytoplasmic contents. The pericycle is also known as the root-forming layer because lateral roots originate from it. This can be observed by making a transverse section of a young root such that it passes through the region where a lateral root emerges.

Beneath the pericycle in the central cylinder lies a radial vascular bundle. There is no pith.

Slide. Structure of the root of the pumpkin (Cucurbita pepo L.)

The primary structure of the pumpkin root is as follows: in the center of the root lies a vessel, from which vessels of smaller diameter extend in four rays. This is the primary xylem. Primary phloem lies between the rays of the primary xylem, and parenchyma is found between the primary xylem and primary phloem. Thus, the vascular bundle is closed and radial.

All of this is surrounded by a ring of the pericycle.

The primary cortex lies exterior to the pericycle.

Secondary changes begin when cambium is formed between the primary xylem and phloem from parenchymal cells through tangential division. It is arranged in four arcs whose ends approach the pericycle.

The cambium deposits secondary xylem elements inward and secondary phloem elements outward. The pericycle forms four medullary rays that originate from the ends of the primary xylem and intersect the secondary xylem and phloem.

Fig. 88. Transverse section of a pumpkin root:

1 - primary xylem, 2 - secondary xylem, 3 - medullary ray, 4 - cambium, 5 - secondary phloem, 6 - primary phloem, 7 - periderm

To prepare sections, take a section of the pumpkin root that lacks root hairs. The material is prepared and preserved in advance, or pumpkin seeds are sown in soil-filled boxes 15–20 days before the work begins to allow them to germinate. Once the seeds have sprouted and the young seedlings are well-developed, select young plants and take the part of the root devoid of root hairs—that is, the portion of the taproot that has already begun to age. Cut cross-sections from it, place them in a drop of water on a microscope slide, and examine them under low and high magnification.

In the sections (Fig. 88), four radii consisting of tiny vessels can be seen in the center of the root. This is the primary xylem. Secondary xylem lies between the radii of the primary xylem. Here, large vessels immediately catch the eye. Located on the outside is the cambium, through The Cell division of which secondary changes and root thickening have taken place.

Closer to the periphery lies the secondary phloem, which merges with the primary phloem so that they can no longer be distinguished. Further out is the primary cortex, which may terminate externally in a periderm (cork).

Attention should be paid to the four medullary rays, which begin as triangles at the ends of the primary xylem radii and pass between the regions of secondary xylem and phloem.

Slide. Structure of the root of the broad bean (Vicia faba L.)

Broad bean seeds are sown in pots or in glass dishes containing sand 15–20 days before THE START OF the exercise. When the root reaches a length of 15–20 cm, secondary structure develops in it, making it suitable for studying the Anatomical Structure. A piece of such a root lacking root hairs is taken, inserted into a freshly split piece of elderberry pith, and sectioned with a razor blade. The prepared sections are first placed on a microscope slide in a drop of water and then treated with stains—phloroglucinol with hydrochloric acid or aniline sulfate. The slides are then examined under low and high magnification and sketched with the appropriate structural elements labeled.

The sections (Fig. 89) show that the root is externally covered by the primary cortex, which terminates in the endodermis. Dark dots are noticeable on the radial walls of the endodermal cells. These dark dots are folds of the radial walls of the endodermis, which stain red with safranin, yellow with aniline sulfate, and cherry-red with phloroglucinol and hydrochloric acid.

Fig. 89. Cross-section of a broad bean root:

1 - mesoderm, 2 - endodermis, 3 - pericycle, 4 - xylem, 5 - hard bast (sclerenchyma), 6 - soft bast (phloem), 7 - cambium

The endodermis consists of a single layer of cells, followed by the pericycle. The central cylinder appears as a four-rayed structure consisting of xylem, with phloem located between these four rays. All xylem rays converge in the center, where a large vessel is visible. The size of the xylem vessels decreases from the center to the periphery. The xylem is formed of parenchyma cells with heavily pitted walls, pitted and reticulate vessels, as well as tracheids. The largest vessels are pitted, whereas tracheids have spiral thickenings of their cell walls. The phloem elements lie closer to the periphery, positioned between the xylem rays, and appear glossy under the microscope. The phloem contains Hard and Soft bast. The hard bast is located toward the periphery and consists of thick-walled fibers, performing a mechanical function. Its cells are initially living and later die, with their walls becoming lignified. This is evidenced by the fact that they stain red with phloroglucinol and hydrochloric acid, and yellow with aniline sulfate. The soft bast performs a conducting function, transporting assimilates (organic nutrients) from the leaves to the root. The soft bast is located adjacent to the xylem and separated from it by a narrow strip of cambium.

The cambium consists of four arcs whose ends face the periphery. The concave sides of these arcs are located between the xylem rays. The ends of the cambium arcs are joined where the sharp strands of the xylem terminate—that is, closer to the periphery, beneath the pericycle. In these areas, the cambial cells connecting the sections of the cambial arcs are formed from pericycle cells. Thus, in the Secondary structure of the bean root, a continuous layer of cambium separating the xylem from the phloem can be observed. It consists of small, elongated cells that divide by tangential and radial walls. During cell division by tangential walls, newly formed cells are deposited toward the center and the outside. Those cells deposited toward the center transform into xylem, while those deposited externally become phloem. When cells divide by radial walls, the cambium layer expands without rupturing. In sections of an older root, the central part consists of xylem, which occupies a large volume compared to other elements and has a quadrangular shape. As for the cambium, it forms a continuous ring that separates the vascular bundles; phloem elements are located toward the periphery of this ring, and xylem elements toward the center.

Slide. Structure of the root of the sugar beet (Beta vulgaris L.)

The roots of certain plants are characterized by an enhanced ability to store nutrients, which are deposited in a highly developed thin-walled parenchyma. Sugars or insoluble CARBOHYDRATES predominate among these substances. Such plants include radish, beet, parsley, carrot, parsnip, etc.

Take a small sugar beet root 2–3 cm in diameter, and make cross-sections from its lower part, where it is no more than 6–10 mm thick. Making a thin section

through the entire root is difficult, so it is better to capture a significant portion of the root with the razor blade, making up to 75% of its diameter. Care must be taken to ensure that the section extends to the center and from the center to the periphery. The prepared section is first placed in a drop of water on a microscope slide, then treated with phloroglucinol and hydrochloric acid or aniline sulfate, and examined under low magnification (Fig. 90).

Fig. 90. Cross-section of a beet root:

1 - periderm (cork), 2 - cortical parenchyma,

3 - phloem, 4 - cambium, 5 - xylem, 6 - primary xylem

One's attention is drawn to the presence of concentric rings in the beet root—wide rings alternating with narrow ones. In the wide rings, the cells are loosely arranged (parenchymatous tissue), whereas in the narrow ones, they are denser (conducting bundles).

In the center of the root is a core consisting of primary xylem in the center and secondary xylem arranged as two wide regions between two rays of primary xylem. Externally, the secondary xylem is bounded by the cambium, followed by the phloem elements.

The beet root possesses multiple cambia that arise successively as continuous rings. It has been established that in beet seedlings, the first cambium appears on the tenth day. Wood and bast are formed from this cambium, but subsequently, several more separate

cambia arise, resulting in The formation of new conducting tissue elements and a large amount of storage parenchyma.

The wide concentric rings consisting of parenchymatous cells originate from The activity of the pericycle. They lie external to the core and alternate with the rings of conducting bundles. The formation of new rings occurs sequentially from the center to the periphery.

Additional bundles in the beet root possess a typical collateral structure. They are radially elongated and surrounded by small parenchymal cells.

A distinct cambial strip is visible within the bundle, while a weakly developed interfascular cambium lies between the bundles in the parenchymal tissue. As a result of interfascular cambium cell division, parenchymal cells are laid down both outward and inward toward the center, forming the rays of the interfascicular parenchyma.

In the beet root, xylem parenchyma is formed in significantly smaller amounts than phloem parenchyma. The number of cambial rings can reach 10–12. Consequently, the thick, fleshy root crop of the beet develops primarily through the activity of additional cambial rings.

The xylem elements in the beet root have thickened walls and consist of pitted and spiral vessels. In the phloem, alongside soft bast, small strands of hard bast are discernible, which perform a mechanical function. Exterior to the phloem elements, directly toward the periphery, lies the cortical parenchyma. It consists of living cells with non-lignified walls. The outer layer of the root is covered with cells possessing suberized walls.

Slide. Structure of the Radish Root (Raphanus sativus L.)

Take a thin radish root and make transverse sections at a distance of 2–3 cm from the tip. Examining them under a magnifying glass, one can notice a small pith in the center of the root, from which medullary rays extend, intersecting the xylem and phloem.

Between the medullary rays located within the xylem, vessels and parenchyma cells containing reserve nutrients are clearly visible.

Fig. 91. Transverse section of a radish root:

1 - primary xylem, 2 - secondary xylem, 3 - Vessels of the secondary wood, 4 - wood parenchyma, 5 - secondary medullary rays, 6 - cambium, 7 - phloem and primary cortex

The detailed structure of the radish root can be studied using thin sections. Using a razor, prepare sections so as to capture at least 70% of the entire diameter, ensuring that all parts from the periphery to the center are present in the sections. Treat the prepared sections with aniline sulfate or phloroglucinol with hydrochloric acid and examine them under a microscope at low and high magnification.

In the center of the root (Fig. 91) lie the elements of the primary xylem, immediately followed by the secondary xylem, which occupies a large volume and essentially forms the central cylinder of the root. Large vessels are noticeable within the secondary xylem, and a significant portion is occupied by wood parenchyma as the primary storage tissue of the central cylinder. A large quantity of plastic (reserve) substances accumulates in its cells.

The secondary xylem is penetrated by medullary rays, the cells of which also store plastic substances. The secondary xylem is separated from the peripheral tissues by the cambium, which consists of elongated cells with delicate walls. Further out lie the elements of the phloem and primary cortex.

The detailed structure of the radish root can be examined on microscope slides under high magnification. One can observe that in the center of the root, a narrow strip of primary xylem appears red or yellow, depending on the reagent used to treat the slide. The primary xylem consists of two strands of xylem elements that develop centripetally and connect in the center. Primary medullary rays extend from each primary xylem strand toward the periphery. They are wider toward the periphery and narrower toward the center. The medullary rays consist of large parenchymal cells with thin, non-lignified walls. Secondary medullary rays are clearly expressed in the secondary xylem and are significantly wider than the primary ones. The vessels have a large diameter, and their walls are stained red or yellow. Mostly, the vessels form small groups amidst the main parenchymal tissue. The cells of the primary and secondary medullary rays, as well as the xylem parenchyma, are mostly living, containing cytoplasmic contents and reserve substances. The cambium lies beyond the secondary xylem, followed by the elements of the phloem and the primary cortex. The phloem consists of sieve tubes and bast parenchyma. Typically, the phloem forms small areas opposite the secondary xylem regions. Secondary medullary rays, consisting of large parenchymal cells with living contents, pass between the areas of secondary phloem. The primary phloem is located exterior to the secondary;

it is pushed toward the periphery as a result of the activity of the cambium, from which the elements of the secondary xylem and phloem arise. External to the phloem lies the cortical parenchyma, whose large cells contain living contents and cellulosic walls. Externally, the root is covered by a periderm.

Slide. Structure of the Parsley Root (Petroselinum hortense Hoffrn.)

Fig. 92. Transverse section of a parsley root:

1 - primary xylem, 2 - secondary xylem, 3 - vessel, 4 - wood parenchyma, 5 - secondary medullary rays, 6 - secondary phloem, 7 - sieve tubes, 8 - bast parenchyma, 9 - cortical parenchyma, 10 - periderm, 11 - essential oil canal, 12 - cambium

The Anatomical structure of the parsley root has its own distinctive features. Specifically, while the central cylinder of the root develops similarly to that of the radish, the cortex here is more developed and occupies a volume almost equal to that of the central cylinder.

Transverse sections are made from thin parsley roots. They are treated with phloroglucinol and hydrochloric acid or with aniline sulfate and examined under a microscope.

The structure of the parsley root is clearly discernible even at low magnification (Fig. 92). There is no pith in the central cylinder. Instead, it contains the primary xylem, which includes vessels reaching almost to the center. Bordering it is the secondary xylem, consisting of vessels, wood parenchyma, and secondary medullary rays. When treated with phloroglucinol and hydrochloric acid or aniline sulfate, the walls of the vessels stain in a characteristic color, whereas the cells of the wood parenchyma and medullary rays (both primary and secondary) remain unstained because their walls are non-lignified and composed of Cellulose.

The secondary phloem is highly developed in the parsley root. It consists of sieve tubes and bast parenchyma. Secondary medullary rays run between the strands of secondary phloem. The cortical parenchyma is located beyond the phloem. Externally, the parsley root is covered by the secondary protective tissue—the periderm. In the cortex and bast on transverse sections, essential oil canals surrounded by epithelial cells are noticeable. They secrete aromatic substances—Essential Oils. In the parsley root, reserve substances accumulate not in the central cylinder, but in the cortex. This explains why the cortex is so highly developed. The carrot root has a very similar structure. In the central cylinder, between the phloem and xylem, lies a distinct, well-defined cambium.

Slide. Structure of the Littleleaf Linden Root (Tilia cordata Mill.)

Take a thin linden root and prepare transverse sections, treating them with phloroglucinol and hydrochloric acid or aniline sulfate. Under the microscope (Fig. 93), one can determine that externally the root is covered by a relatively thin layer of yellowish-brown cork, which is part of the periderm. The periderm consists of many layers of cork featuring narrow, thick-walled cells whose cavities are filled with brown contents. Directly beneath the cork, a layer of elongated cells with thin walls containing living cytoplasmic contents and nuclei is clearly visible. This is the phellogen, or cork cambium, from which the cork is formed. Phelloderm is noticeable beneath the phellogen. Next comes the primary cortex (Fig. 93, II), which includes collenchyma composed of cells with walls bearing shiny white or yellowish thickenings.

Fig. 93. Cross-section of a linden root:

I - Periderm: 1 - cork, 2 - phellogen, 3 - phelloderm.

II - Primary cortex: 4-5 - parenchymal cells.

III - Secondary cortex: 6 - hard bast, 7 - soft bast, 8 - sieve tubes.

IV - Wood (Xylem): 9 - medullary rays, 10 - cambium, 11 - pitted vessels, 12 - tracheids, 13 - wood fibers, 14 - wood parenchyma

Collenchyma cells contain starch and cytoplasmic contents; in addition, some collenchyma cells contain calcium oxalate crystals. Besides collenchyma, the primary cortex contains a small number of polygonal parenchymal cells with thickened walls. Fine-grained contents, crystals, or druses can be observed within the cavities of these cells, alongside many empty cells.

Compared to the primary cortex, the secondary cortex (Fig. 93, III) is highly developed and features a complex structure. It clearly displays groups of hard bast, soft bast, and bast parenchyma. Hard bast consists of thick-walled elements—bast fibers—which have shiny lignified walls, with thickening occupying almost the entire internal cavity. These fibers provide mechanical strength to the secondary cortex.

Thin-walled soft bast, containing distinct sieve tubes and companion cells, is located between the bands of hard bast. Bast parenchyma consists of two groups of cells. Cells of the first group are flattened and retain living contents; these cells lie adjacent to the hard bast. Cells of the second group are polygonal and empty, with some containing calcium oxalate crystals.

Medullary rays are situated between the bast bundles and consist of large parenchymal cells tightly adjoining one another. The cells of the medullary rays in the phloem are living, containing cytoplasm and starch grains.

These are followed by a narrow strip of cambium, which consists of elongated cells containing living granular contents and nuclei.

The secondary wood also has a very complex structure and occupies a significant volume.

Here, the structure of the root closely resembles that of the stem.

In the secondary wood, a prominent place is occupied by wide, polygonal, and hollow pitted vessels bearing pits on their lateral walls. The second group consists of radially flattened tracheids with pitted walls, forming continuous layers. Libriform, or wood fibers, consists of small, polygonal, hollow cells with smooth walls. A substantial portion is comprised of wood parenchyma, which consists of small polygonal cells containing cytoplasmic contents and starch. Thus, the wood parenchyma of the root performs a storage function.

The arrangement of secondary wood elements in the root is identical to that in the stem. Annual rings can be observed here, in which the distribution of vessels mirrors the stem: large vessels are concentrated at the inner margin of the annual wood ring because they form in the spring. As for the smaller vessels, they are located in the outer regions of the annual ring, having developed from the cambium in the autumn. This difference in size between spring and autumn vessels makes it possible to delineate annual layers, just as is characteristic of the stem.

The proportions of individual xylem elements making up the secondary wood are unequal. Libriform fibers account for the largest volume, forming the bulk of the secondary wood. In contrast, The amount of fundamental parenchymal tissue and various vessels is considerably smaller. It should be noted that when comparing the anatomical structure of an older root with a stem, the differences noticeable in their primary structure gradually disappear, and the root becomes almost indistinguishable from the stem.