BOTANY LABORATORY PRACTICUM (PLANT ANATOMY AND MORPHOLOGY) - O.A. SHEVCHUK - 2014

MORPHOLOGICAL AND ANATOMICAL STRUCTURE OF THE STEM

Background Information. The stem is an essential axial vegetative plant organ. Morphologically and functionally, it connects the underground absorption Organs (roots) and aerial absorption organs (leaves). It facilitates the upward transport of Water and mineral salts from the roots to the leaves, as well as the downward transport of synthesized Organic compounds from the leaves to consumption and storage sites. The stem exhibits apical growth, negative geotropism, radial Symmetry, and a complex internal Structure adapted to performing diverse physiological Functions. It Supports and positions leaves, branches, flowers, and fruits, stores nutrient reserves, initiates buds, regenerates, and serves as an organ of propagation.

A stem with its attached leaves and buds is called a SHOOT. The point of stem attachment where a leaf is borne is termed a node, while the region between two successive nodes is an internode. Depending on internodal development, Three types of shoots are distinguished: shortened (apple tree), normal (hazel), and elongated (ailanthus). The angle formed between the stem and the petiole of a departing leaf is called the leaf axil. Leaf arrangement on the stem can be alternate or spiral (apple tree), opposite (carnation, lilac)—where two leaves are positioned on opposite sides of the stem at a single node—or whorled (bedstraw), featuring three or more leaves arising from a single node.

Phyllotaxy is closely related to light exposure, often manifesting as a leaf mosaic. In many plants, internodes shorten toward the apex of the stem, while leaves become smaller and more densely arranged. Based on their position, three leaf formations are distinguished: upper (acrogenous), middle (mesogenous), and lower (cataphyllary). The stem terminates in an apex, which represents a shortened embryonic shoot. Axillary or lateral buds (solitary, serial, collateral) develop in the leaf axils. Buds that form on internodes, roots, or leaves are termed adventitious. In addition to these, there are floral or generative buds, which develop into flowers. Buds that remain dormant for extended periods and develop only under specific conditions (such as pruning or frost) are called resting or latent buds.

Stem growth occurs through The activity of the apical bud and is known as apical growth. This type of growth is also characteristic of first-, second-, and higher-order shoots developing from vegetative axillary buds. In grasses, horsetails, and certain other plants, rapid stem growth results from the activity of intercalary meristem located at the Base of the internodes. This type of growth is designated as intercalary.

Plant stems display considerable diversity. In cross-section, they can be cylindrical (grasses), multi-angled (umbellifers), triangular (sedges), quadrangular (lamiads), or flattened (prickly pear), among others. Based on spatial orientation, stems are classified as erect, growing vertically upward without bending under the weight of their branches, leaves, flowers, or fruits. Stems that trail along the ground and form adventitious roots at points of contact with moist soil, thereby rooting, are termed creeping. Stems with shortened internodes are called runners (cucumbers, pumpkins), while those with elongated internodes are stolons (strawberries). Prostrate stems that grow by clinging to other plants are known as climbing stems (pea, bedstraw, ivy). Twining stems (bindweed, hops) develop The ability to coil around the stems of other plants or supports.

Based on their life span and habit, plants are classified as trees, shrubs, subshrubs, and herbs. A tree is a plant in which the main stem is clearly differentiated from other shoots by vigorous growth in length and thickness, forming a crown. In a shrub, the main stem is not prominently differentiated, and numerous shoots develop intensely from its base near the soil surface. Subshrubs feature woody or corky lower shoot portions that remain perennial, while only the upper annual shoots die back. Herbs are plants whose aerial parts die off annually at the end of the growing season. Among them are annuals, which complete their entire Life Cycle of both underground and biennial organs within one growing season, and biennials, which form underground organs in the first year and develop aerial organs In the second year, completing their cycle with fruit and seed production (carrot, cabbage). Perennials retain underground organs and possess the ability to produce renewal buds annually over a long period (couch grass, sedges).

Stem development is accompanied by an increase in trunk and leaf biomass and the synthesis of large amounts of organic matter, which promotes branching. Several types of branching are recognized: monopodial, in which the main stem continues to grow via the apical bud for many years, while lateral shoots originate from the main axis and remain smaller than the main stem (pine, spruce); sympodial, in which the apical bud ceases growth after some time, and shoot elongation continues via a lateral bud, which subsequently stops growing, allowing the shoot to continue extending through a new axillary bud, and so forth (linden, plum); dichotomous branching, which occurs through the equal division of the apical meristem into two new growing points that retain this ability (clubmosses, Selaginella); and false-dichotomous (pseudodichotomous) branching, where the apical bud dies, and two oppositely placed sub-apical buds sprout to form two leading shoots, whose apical buds likewise die and are replaced by the sprouting of two lower opposite buds, and so forth (common lilac).

During their development, the stems of many plants undergo various Anatomical and morphological modifications, which can be subterranean (tubers, rhizomes, bulbs) or aerial (thorns, tendrils, cladodes).

In terms of Anatomical Structure, stems exhibit Primary and secondary structures. Their complex Organization comprises distinct blocks of typical tissue groups that determine their structural features. Primary Structure is associated with the activity and Differentiation of the Meristems of the stem apex (cone of growth). The outer meristematic layer—the tunica—gives rise to the epidermis and occasionally several layers of the primary cortex. The inner Cells of the growing apex—the corpus—give rise to all other Tissues. Consequently, the primary stem structure comprises the epidermis, primary cortex, and central cylinder (stele).

The epidermis typically consists of a single layer of living parenchymal cells with sinuous Cell walls, which enhances the mechanical cohesion of the protective tissues. This enables them to withstand the turgor pressure of cell enlargement and The formation of new tissues. Stomata are present within the epidermis, mostly on the abaxial (lower) surface, and various appendages develop on its surface. The primary cortex lies deeper. Its outer parenchymal layers frequently contain METABOLISM/14.html">Chloroplasts and perform an assimilatory function. In dicotyledonous plants, its cell walls thicken and differentiate into collenchyma. This provides mechanical resistance against wind, rain, and other environmental stresses. Many plants also contain strands of sclerenchyma alongside collenchyma.

The innermost layer of the primary cortex forms the endodermis, or starch sheath. Its cell walls sometimes become lignified or suberized. In the outer region of the central cylinder, one to two layers of parenchymal cells constitute the pericycle, from which medullary rays, adventitious buds, and lateral and adventitious roots originate. The multi-layered pericycle consists of prosenchymatous cells, which give rise to primary bast fibers (hemp).

The pith (medulla) occupies the major portion of the stem interior. Its cells are parenchymatous. Extending between the vascular bundles, it forms the pith rays (medullary rays). In the center of the stem, the pith often breaks down, leaving the stem hollow.

Vascular bundles originate from the procambium of the apical cone. The procambium forms conducting tissues—vessels and tracheids—along with storage xylem parenchyma, which together constitute the xylem, as well as sieve tubes, companion cells, and phloem parenchyma, which form the phloem.

In some cases, the procambium is entirely consumed in the formation of phloem and xylem, while in others, it persists and continues to produce new elements of the vascular bundles.

Regarding anatomical structure, several distinctions exist between monocotyledonous and dicotyledonous stems. In herbaceous monocots, the procambium is completely exhausted during the formation of xylem and phloem, resulting in closed collateral vascular bundles. These are arranged in a scattered, palm-like pattern rather than in a circle, as seen in dicots. The monocot stem lacks cambium and possesses no capacity for secondary thickening, which occurs solely through procambial activity and the enlargement of vascular bundle elements. The stem anatomy of grasses (Poaceae) comprises, from periphery to center: a uniseriate epidermis with or without stomata, followed by a continuous ring of sclerenchyma. Between the Ribs of the sclerenchyma lie chlorophyll-containing parenchyma cells enclosing an air cavity and stomata. Inside the sclerenchymatous sheath is the ground parenchyma, which fills the interior of the stem. Embedded within it are collateral closed vascular bundles, which are larger toward the center and smaller near the periphery.

In the stem anatomy of dicotyledonous plants, depending on whether the procambium originates as discrete strands or a continuous cylinder, a fascicular (bundle-type) or non-fascicular structure develops. Transverse sections of the fascicular type reveal the following tissue regions: epidermis, primary cortex, central cylinder, and pith. The primary cortex of most plants contains collenchyma, cortical parenchyma, and endodermis, whereas the central cylinder features a pericycle or sclerenchyma (forming a continuous ring or separate strands) and open collateral vascular bundles arranged in a circle. The central region is occupied by the pith, which extends outward as primary medullary rays separating the vascular bundles.

The non-fascicular type of stem structure is characterized by the absence of discrete vascular bundles, with xylem and phloem organized into continuous concentric cylinders.

The Introduction/11.html">Secondary structure of dicotyledonous stems results from the activity of the vascular cambium.

Certain plants exhibit a transitional type of stem structure: shifting from a fascicular to a non-fascicular type. In the sunflower, for instance, a fascicular stem structure forms during early developmental stages. Subsequently, interfascicular cambium develops in the gaps between adjacent fascicular cambia. The latter unites with the fascicular cambium to form a continuous cambial ring, which produces continuous layers of secondary xylem and phloem, thereby establishing a non-fascicular structural type.

In trees, shrubs, and perennial herbs, the epidermis is replaced by the periderm, and secondary medullary rays appear, among other modifications.

In woody dicotyledonous plants with a non-fascicular structure, the continuous cambial cylinder cuts off secondary xylem inward and secondary phloem outward. In the peripheral region, cork cambium (phellogen) arises, producing cork (phellem) externally and phelloderm internally. Consequently, the secondary stem structure of young woody branches displays the periderm, primary and secondary cortex, cambium, secondary xylem, and pith.

The primary cortex lies beneath the periderm. Its outer layers consist of collenchyma, while its bulk is composed of large parenchymatous cells. The innermost layer of the primary cortex is a uniseriate endodermis.

The secondary phloem comprises primary medullary ray regions and phloem wedges (trapezoids) consisting of hard bast (bast fibers) and soft bast (sieve tubes, companion cells, and phloem parenchyma).

The cambium is formed by living, thin-walled parenchymal cells arranged regularly one above the other. Beneath the cambium lies the most distinctly defined secondary wood. Due to the periodicity of cambial activity, annual rings are clearly distinguishable here, consisting of large-pored spring wood and small-pored, thick-walled autumn wood, xylem parenchyma, and portions of primary and secondary medullary rays.

The pith occupies the central part of the stem, containing both small, living brown cells and large, colorless dead cells.

TOPIC 2. MORPHOLOGICAL STRUCTURE OF THE SHOOT

General Remarks. In connection with their supportive and aerial Nutrition functions, plants have developed shoots of extraordinary morphological and structural diversity. When examining them, significant attention is devoted to their Morphology, particularly shoot typology based on growth habit, spatial arrangement, branching, and orientation. This will help students better master the practical aspects of the material and clarify The Role of shoots in plant developmental biology, as well as their potential economic use for propagation and crop cultivation.

Objective: to examine the morphological Features of the shoot, including its shape, dimensions, surface characteristics, direction of growth, diversity, and species Specificity (herbaceous and woody).

Materials and equipment: herbarium specimens of shoots from the following plants: dog rose (Rosa canina), stinging nettle (Urtica dioica), procumbent medic (Medicago lupulina), fat hen (Chenopodium album), white clover (Trifolium repens), common wheat (Triticum aestivum), littleleaf linden (Tilia cordata), catchweed bedstraw (Galium aparine), one-seed hawthorn (Crataegus monogyna), Pontic azalea (Rhododendron luteum), stag's-horn clubmoss (Lycopodium clavatum); living or fixed shoots of common lilac (Syringa vulgaris), horse chestnut (Aesculus hippocastanum), European larch (Larix decidua), common grape vine (Vitis vinifera), domestic apple (Malus domestica); indoor plants: ficus, monstera, ivy, cactus; potato tubers, Solomon's seal rhizomes, common onion bulbs; laboratory equipment, charts.

Tasks

1. Study the Morphology of the shoot.

2. Examine and study shoot types according to their growth habit.

3. Examine and study stems according to their shape and surface characteristics (smooth, pubescent, scaly).

4. Study shoot types according to their branching pattern.

5. Study shoot types according to internode development.

6. Examine and study subterranean and aerial modifications of shoots.

7. Analyze several shoots and provide a comparative morphological description of stem types based on their surface characteristics. Enter the data into Table 2.

Class="right">Table 2

Label the figures and draw Conclusions.

Conclusion

Self-Assessment Questions

1. Define THE CONCEPT OF a shoot. Name its constituent parts.

2. Describe the characteristics of stems according to their growth habit.

3. What attachment mechanisms are typical for stems?

4. Name the constituent parts of a shoot and their functions.

5. Classify the stem based on the shapes of its cross-section.

6. Name the types of shoots according to their branching pattern. Give Examples of herbaceous plants in which they occur.

7. What underground shoot modifications do you know, and in which plant species are they found?

8. What evidence can you provide that a potato tuber is a modified shoot?

9. Provide evidence that tendrils, thorns, and rhizomes are modified shoots.