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

Topic 3. Structure of Vegetative Organs
Laboratory Work No. 15. Structure of the Leaf

Theoretical Background. A leaf is a lateral outgrowth of the axial Structure containing all of its constituent parts. The primary Functions of a leaf are Photosynthesis, gas exchange, and Transpiration. A leaf typically consists of a leaf blade (lamina), a petiole, a leaf sheath, and stipules. The main tissue of the leaf is the mesophyll, which in angiosperms can be of two types: palisade and spongy, and in gymnosperms, folded. It is the only plant organ that retains its Primary Structure throughout ontogeny.

Objective: to study the Structural Features of the leaf.

Materials and equipment: light microscopes, Glass slides and coverslips, dissecting needles, tweezers, glass rods, filter paper, distilled Water, phloroglucinol with Hydrochloric acid, aniline sulfate, glycerin, iodine in potassium iodide, plant material.

Slide. STRUCTURE OF THE philodendron leaf (Monstera deliciosa Lieb.)

A piece of a fresh green leaf containing a vein is taken, embedded in elderberry pith, and sectioned with a razor blade so that the vein is cut transversely. The sections are placed in a drop of water and then treated with various Reagents, such as phloroglucinol with hydrochloric acid or aniline sulfate, which cause lignified Cell walls to stain red or yellow.

The prepared sections are examined under a Microscope (Fig. 94) at low and high magnification. The leaf is covered externally by the epidermis, which consists of a single layer of living Cells containing Cytoplasm, nuclei, and vacuoles. The outer walls of the epidermis are thickened and covered with a cuticle, which is interrupted in places by Stomata.

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Fig. 94. Cytology/practical/72.html">Cross section of a philodendron leaf:

1 - epidermis, 2 - palisade parenchyma, 3 - spongy parenchyma, 4 - astrosclereid, 5 - druse, 6 - cell with fine raphides, 7 - xylem, 8 - phloem, 9 - sclerenchyma

Beneath the epidermis on the upper side of the leaf lies the palisade parenchyma, consisting of elongated cells tightly packed together. The Cells of the palisade parenchyma contain numerous METABOLISM/14.html">Chloroplasts distributed throughout The Cell lumen. These cells are elongated perpendicular to the leaf surface, resembling fence palings.

Beneath the palisade parenchyma lies the spongy parenchyma, extending down to the lower epidermis. It is formed of polygonal cells that are loosely connected, resulting in large intercellular spaces. A characteristic feature of this tissue is that its cells contain significantly fewer chloroplasts than those of the palisade parenchyma. The cell walls of the spongy parenchyma are cellulosic and unlignified. The primary function of this parenchyma is the evaporation of water transported up the stem from the roots to the leaves, as well as gas exchange. Atmospheric carbon dioxide enters the spongy parenchyma through the stomata and intercellular spaces, diffusing into the palisade parenchyma where it is utilized to synthesize Organic compounds. The organic substances enter the bundle-sheath (collecting) cells, which form a thin layer between the spongy and palisade parenchyma. These cells have endings that contact the palisade cells. Assimilates produced during photosynthesis flow through the collecting cells into other Tissues, specifically into the conducting elements of the phloem.

Branched sclereids occur within the mesophyll of the leaf. Their bases abut the epidermis, while their branches extend into the leaf mesophyll. Sclereids have heavily thickened walls and lack living contents at maturity, functioning as dead elements. Their primary role is mechanical, providing structural rigidity to the leaf blade.

In the Regions of the spongy parenchyma, cells containing calcium oxalate crystals and druses can be found. Additionally, there are cells containing numerous needle-like calcium oxalate raphides that dissolve upon Treatment with hydrochloric acid. In the center of the section, a collateral vascular bundle is visible, consisting of xylem and phloem. The xylem contains vessels, tracheids, and wood parenchyma composed of small parenchymatous cells with cytoplasmic contents. Their walls are lignified and stain a characteristic color with the aforementioned reagents. The xylem elements border the phloem elements, which comprise sieve tubes and companion cells. Their walls are glistening and composed of Cellulose. The vascular bundle is surrounded on all sides by sclerenchyma fibers that perform a mechanical function. This can be observed by comparing preparations treated with phloroglucinol and hydrochloric acid to untreated ones. In untreated slides, the walls of the sclerenchyma fibers are white and shiny, whereas in those treated with phloroglucinol and hydrochloric acid, they turn red, indicating that their walls are lignified.

Slide. Structure of the leaf of the yellow water-lily

(Nuphar luteum (L.) Smith.)

Plants developing under differing conditions exhibit varied anatomical structures, with aquatic plants showing particularly distinct adaptations. The leaf blade in such plants is thin, transparent, and often dissected into linear segments, as seen in water crowfoot. The Anatomical Structure of aquatic plant leaves is characterized by a range of adaptive features. Their assimilatory tissue is poorly developed due to insufficient illumination, and palisade parenchyma is either absent or weakly developed. In the spongy parenchyma, partitions arise between the upper and lower epidermis, enclosing large air cavities (lacunae). The dermal tissue is undeveloped, and the epidermal cell walls are not thickened. The cuticle is very thin or entirely absent. The epidermal cells are rich in chloroplasts and take over the function of assimilation. Stomata are absent on the leaf blades of submerged aquatic plants because the very thin blade can absorb gases and nutrients directly from the water across its entire surface. The epidermis of leaves and petioles in certain aquatic plants contains specialized cells or cell groups differing in shape, size, and greater wall permeability to water, known as hydropodes. Hydropode cells are rich in cytoplasm and contain small Plastids lacking starch grains.

In the leaf blades of submerged plants, the xylem is poorly developed, vascular bundles are sparse, and vessels are few. Very often, THE POSITION OF vessels in the xylem is occupied by a narrow intercellular canal. The phloem is more developed than the xylem, though weaker than in terrestrial plants, which is attributable to reduced assimilation rates. The supply of carbon dioxide and oxygen to the leaves is facilitated by an extensively developed system of air-filled intercellular spaces.

In plants with floating leaves, the leaf blade is very dense and coriaceous (leathery). Numerous stomata develop on the upper surface of such leaves. For instance, in the yellow water-lily, the upper epidermis of a floating leaf contains up to 500 stomata per 1 mm², whereas they are completely absent on the lower epidermis.

Fig. 95. Cross section of a water-lily leaf:

1 - epidermis, 2 - stomata, 3 - palisade parenchyma, 4 - idioblast, 5 - spongy parenchyma, 6 - air cavities, 7 - vascular bundle, 8 - sclerenchyma

A portion of the leaf blade with a vein from a water-lily or nymphaea is embedded in elderberry pith, and cross sections are prepared. These are treated with phloroglucinol and hydrochloric acid or aniline sulfate and then examined under a microscope (Fig. 95) at low and high magnifications. Externally, the leaf is covered by an epidermis with tightly juxtaposed cells. The outer walls of the epidermal cells are thickened and covered with a cuticle. The upper epidermis bears stomata, beneath each of which lies an air chamber. Below the upper epidermis is the palisade parenchyma, consisting of closely appressed cells arranged in multiple layers containing chloroplasts. Mechanical cells, or idioblasts, can also be observed. Situated beneath the palisade parenchyma is the spongy parenchyma, composed of loosely connected cells intersected by large air channels. A collateral closed vascular bundle runs through the spongy parenchyma, flanked on the lower side by sclerenchymatous fibers that impart mechanical strength to the leaf. Submerged leaves of water-lilies lack palisade parenchyma.

Slide. Structure of the leaf of the shade-tolerant plant European wild ginger (Asarum europaeum L.)

Fig. 96. Cross section of a wild ginger leaf:

1 - epidermis, 2 - palisade parenchyma, 3 - spongy parenchyma, 4 - air cavity

Thin sections are cut from a fresh or fixed leaf, placed in a drop of water or glycerin, and examined under a microscope at low and high magnifications. Externally, the leaf blade is covered by the epidermis (Fig. 96). The cells of the upper epidermis are larger than those of the lower. They feature thickened walls and are covered on the outside by a cuticle. Beneath the epidermis of the upper side of the blade lies the palisade parenchyma, which consists of a single layer of slightly elongated cells, though their shape differs little from the cells of the spongy parenchyma. This cell shape in the palisade parenchyma is typical of shade plants. The palisade tissue is characterized by the presence of large chloroplasts. The spongy parenchyma has a structure similar to that of the palisade tissue, but it contains numerous air cavities.

Slide. Structure of the Aloe Leaf (Aloe vera (L.) Burm.f.)

Xerophytic vegetation grows in arid regions. Under conditions of hot, dry air and insufficient soil moisture, xerophytic plants have developed a series of adaptations. Xerophytic plants are anatomically, physiologically, and morphologically adapted to severe moisture deficits in both soil and air. Among such plants, several distinct groups are recognized: succulents, thin-leaved xerophytes, and ephemerals.

Succulents possess leaves that resemble unique water reservoirs, where a large reserve of water accumulates. Such plants include aloe, agave, and among the plants of our local flora—orpine, houseleek, and others.

Fig. 97. Cross-section of an aloe leaf:

1 - cuticle, 2 - stomate, 3 - assimilation tissue, 4 - vascular bundles, 5 - water-storage parenchyma

Cross-sections are prepared from an aloe leaf and examined under a microscope (Fig. 97). Externally, the leaf is covered by an epidermis consisting of a single layer of flat cells. The outer walls of the epidermal cells are covered with a bumpy cuticle. On the lower, convex side of the leaf, the epidermis contains stomata.

Beneath the epidermis lies the palisade parenchyma, consisting of several layers of slightly elongated cells containing large chloroplasts. Adjoining the lower surface of the leaf is a tissue composed of rounded, tightly packed cells. Vascular bundles, structurally resembling those of corn or Dracaena, adjoin the palisade parenchyma. Spiral vessels are clearly visible within the bundles and can even be pulled out with a blade. A significant volume is occupied by the water-storage parenchyma, which is located in the central part of the leaf and consists of large, rounded cells. These cells are colorless and contain a vast amount of cell sap, which includes various organic compounds. The water-storage parenchyma forms a unique water reservoir that supplies the plant with water during periods without rain. The agave leaf is built on the exact same principle.

Slide. Structure of the Rye Leaf (Secale cereale L.)

Fig. 98. Cross-section of a rye leaf:

1 - epidermis, 2 - stomate, 3 - assimilation tissue, 4 - vascular-fibrous bundle, 5 - sclerenchyma

A rye leaf is taken to prepare cross-sections (Fig. 98), which are then studied under a microscope. It can be observed that the leaf is externally covered by an epidermis whose cells have thickened walls. The upper walls are covered with a cuticle. The epidermis contains stomata, beneath which lie air cavities. Parenchyma is present beneath the epidermis on both the upper and lower sides, but there is no structural difference between them because the chlorophyll-containing tissue of cereals is not differentiated into spongy and palisade layers. Numerous chloroplasts are visible within the chlorophyll-bearing parenchyma. Vascular-fibrous bundles are surrounded by sclerenchymatous fibers. Vessels and phloem elements are discernible within the bundles. Alongside each large vascular bundle lies a sclerenchymatous strand, and some cereals feature multiple separate strands. Sclerenchyma cells are located beneath the epidermis and along the margins of the leaf blade.

The leaf sheath, which clasps the stem, provides strength to the internodes; it also features a highly developed sclerenchyma.

Slide. Structure of the Feather Grass Leaf (Stipa sp.)

Cross-sections of the feather grass leaf are prepared. To do this, dry leaves are first soaked in hot water, then inserted into elder pith, and thin sections are cut with a sharp blade. These sections are placed on a glass slide in a drop of water and examined under a microscope at low and high magnifications. Preparing sections of feather grass is very difficult, so several must be made to select the best one.

the best one.

Fig. 99. Cross-section of a feather grass leaf:

A - in the rolled state;

B - in the unrolled state: 1 - lower surface of the leaf, 2 - upper surface of the leaf, 3 - stomata, 4 - vascular-fibrous bundles

Under the microscope (Fig. 99), it is noticeable that the lower surface of the leaf is smooth and even, lacking stomata. It is covered by an epidermis whose cells fit tightly together; their walls are thickened, especially the upper ones, and covered with a layer of cuticle. The upper side of the leaf is also covered by an epidermis, but it is uneven and irregular, featuring numerous large ridges and small outgrowths, and it contains stomata. The surface of the outgrowths is rounded, facing outward, and bears unicellular hairs. Stomata are located exclusively on the sides of the ridges and open into a narrow slit between them. With this arrangement, the stomata are protected from wind and direct sunlight. Thus, water transpiration is regulated, which is further facilitated by the leaves rolling into a tube during dry weather.

At the base of each ridge lie closed collateral vascular-fibrous bundles. The sieve tubes of the bundle face toward the lower, i.e., outer, side of the leaf. Narrow strips of mechanical tissue are visible in the large ridges, adjoining the bundle on both sides. Where the mechanical strip abuts the epidermis, it widens, and the epidermal cells here are significantly smaller. Thus, a rail-like beam appears to be formed between the two epidermal layers. This structure causes the leaf to roll up when water transpiration intensifies. In such cases, the delicate cells of the chlorophyll-bearing parenchyma decrease in volume, while the mechanical regions remain unchanged and shift. Unlike other plants, the stomata on the feather grass leaf blade are located on the upper surface rather than the lower. This arrangement ensures the Regulation of Water transpiration, especially during droughts, because the stomata end up enclosed inside the tube.

Slide. Structure of a Scots pine needle (Pinus sylvestris L.)

A pine needle is cut into small pieces, embedded in elderberry pith, and transverse sections are prepared. These sections are placed on a microscope slide in a drop of water and treated with various reagents: an iodine-potassium iodide solution, aniline sulfate, or phloroglucinol with hydrochloric acid. The cross-section is shaped like a nearly regular semicircle, with its convex sides facing outward and the flat sides facing inward toward each other. Under low magnification (Fig. 100), the key structural elements comprising the main Tissues of the needle can be identified. These include the epidermis, beneath which lies a mechanical tissue made up of sclerenchymatous fibers known as the hypodermis. The hypodermis is clearly visible on the section, particularly at the corners—the edges of the needle. It consists of cells with lignified walls. When treated with phloroglucinol and hydrochloric acid, the hypodermase cells stain red, while with aniline sulfate, they stain yellow. The hypodermis lies directly beneath the epidermis, thereby protecting the deeper-seated living tissues.

Fig. 100. Transverse section of a Scots pine needle:

1 - epidermis with a single-layered hypodermis, 2 - resin duct, 3 - stomata, 4 - plicate mesophyll, 5 - endodermis, 6 - epithelium, 7 - mechanical fibers, 8 - transfusion tissue with bordered pits

Beneath the hypodermis lies the chlorophyll-bearing or assimilation parenchyma, interspersed with resin canals surrounded by a ring of sclerenchymatous fibers.

The epidermal cells have heavily thickened walls; their cell lumens are small, and narrow channel slits extend from them toward the cell corners. The outer walls of the epidermis are covered by a cuticle that is interrupted by stomatal slits. Within the stomata, the living guard cells possess lignified walls. Consequently, the pine needle is rigid due to the presence of the hypodermis, and has low permeability because it is covered by a cuticle.

The assimilation, or chlorophyll-bearing parenchyma underlying the hypodermis consists of parenchymal cells with thin cellulosic walls. They form inward-projecting fold-like outgrowths, which is why this tissue is referred to as plicate (folded) parenchyma. In pine needles, the assimilation tissue is not differentiated into palisade and spongy layers, which is attributed to the specific arrangement of the needles on the branches and their uniform exposure to light.

The formation of these folds serves an adaptive function, as it increases the surface area available for chloroplast attachment. The central region of the needle lacks chlorophyll-bearing parenchyma. It is separated from the assimilation parenchyma by a layer of cells characterized by internal cytoplasmic contents, starch, and lignified cell walls—this is the endodermis. Inward from the endodermis lies parenchymal tissue composed of empty cells or those containing cytoplasmic remnants and starch. The walls of these cells are lignified and feature small bordered pits. This tissue is called transfusion tissue because it facilitates the Transport of Assimilates (plastic substances). The dead cells conduct water, functionally corresponding to tracheids, and are therefore termed tracheididal cells.

The central part of the needle contains two closed collateral vascular bundles connected by a strand of sclerenchyma. Phloem and xylem can be distinguished in each bundle; the phloem is located on the convex side of the needle, and the xylem on the flat side.

Slide. Longitudinal sections of a Scots pine needle (Pinus sylvestris L.)

Fig. 101. Longitudinal section of a pine needle:

1 - epidermis, 2 - stomata, 3 - air cavity, 4 - plicate mesophyll, 5 - endodermis, 6 - parenchyma with bordered pits,

7 - transfusion tissue, 8 - sclerenchyma, 9 - annular and spiral tracheids, 10 - tracheids with bordered pits, 11 - phloem

A pine needle is clamped by its middle section into elderberry pith, and the ends are cut off. Longitudinal sections are prepared from the clamped segment, placed in a drop of water on a microscope slide, and subsequently stained with an iodine-potassium iodide solution. The microscopic appearance depends on the plane of the section. If the section passes through the center of the needle, the microscope (Fig. 101) will reveal the epidermis on both sides, containing stomata—with only a single stoma visible on one side and numerous stomata on the other. The epidermal cells are

elongated, with thickened, pitted walls. A cuticle is discernible on the upper walls of the epidermal cells. As for the stomata, each is cut lengthwise, making only a single curved guard cell with unevenly thickened walls visible. Beneath the stomata lie large air cavities surrounded by parenchyma cells. The cells of the plicate assimilation parenchyma are slightly curved and elongated transversely across the needle, with intercellular spaces clearly visible between them. The plicate parenchyma cells are located on both sides of the needle, forming two layers. These are followed by the endodermis, whose cells are slightly elongated longitudinally.

The central region of the needle contains the transfusion tissue, which houses the closed collateral vascular bundles.

Review Questions

1. Characterize the differences in the anatomical structure of the stem in monocotyledonous and dicotyledonous plants.

2. Describe The structure of fasciculate (bundle-type) and non-fasciculate stems.

3. Compare the anatomical structure of stems in herbaceous and woody plants.

4. What causes the formation of annual wood rings?

5. What is the Functional Significance of stem pith rays?

6. Which histological elements are used to distinguish between the stems of gymnosperms and angiosperms?

7. Name the main Zones of the ROOT. What is the function of each?

8. Describe the structural features and Functions of the root cap.

9. Characterize the Introduction/19.html">Primary structure of the root.

10. What are the STRUCTURE AND FUNCTIONS of the root endodermis?

11. How do passage cells differ from the other cells of the root endodermis?

12. Compare the structure of epiblema and epidermis.

13. Characterize the central cylinder of a root with primary structure.

14. What type of vascular bundle is found in the root?

15. How does the transition from primary to secondary root structure take place?

16. State the Features of the secondary root structure.

17. Compare the structure of isolateral and dorsiventral leaves.

18. Describe the structural features of leaves in mesophytes, xerophytes, and hydrophytes.

19. Compare the structure of palisade and spongy mesophyll.

20. Which type of mesophyll is typical for angiosperms and which for gymnosperms?

21. In the leaves of which plants is aerenchyma present?

22. Characterize the structure of plicate mesophyll.

23. What are the structural features of a pine needle?

24. Name the Organs that contain the endodermis as a barrier tissue.