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

LEAF MORPHOLOGY AND ANATOMY

General Information. The leaf is a lateral plagiotropic organ with limited intercalary growth. Only in a few plants (such as Welwitschia) is the leaf a permanent organ of indeterminate growth, whereas fern leaves grow by their apex. In the course of evolution, the leaf emerged in connection with the transition of plants to a terrestrial lifestyle. Its primary Functions are Photosynthesis, Respiration, and Transpiration. Typical leaves exhibit the following morphological parts: the blade (lamina), petiole, and stipules. At the Base of the leaf in many plants, stipules may fuse to form an ocrea (sheathing stipule). Grass leaves also possess a ligula (ligule) and auricles.

The blade is the expanded portion of the leaf, which transitions at its base into the petiole. If the petiole is absent, such leaves are termed sessile. In grasses, sedges, and umbellifers, the petiole expands to form a leaf sheath.

Leaf blades vary greatly in size, shape, and consistency. A distinction is made between simple leaves, which bear a single blade on the petiole (apple, buckwheat, nettle), and compound leaves, where the petiole bears several distinct leaflets that abscise independently during leaf fall (acacia, lupine).

Changes in the shape and Structure of leaves at different nodes of a SHOOT from bottom to top are referred to as heterophylly (Water crowfoot, arrowhead). If variations in leaf size and shape (less commonly) occur at the same node, but the leaves are oriented differently relative to the horizon and light, this phenomenon is termed anisophylly.

In the course of evolution, as an ADAPTATION TO ENVIRONMENTAL conditions, the leaves of certain plants have undergone modifications (metamorphoses). The main leaf metamorphoses include spines (barberry, cactus), tendrils (pea, vetch), and phyllodes, in which the petiole flattens and functionally replaces the blade. Insectivorous Plants exhibit fascinating leaf modifications: for instance, in the Venus flytrap, the blades of basal leaves have evolved into trapping mechanisms, whereas in the bladderwort, a part of the leaf is transformed into a bladder, and so forth.

Leaf Venation. All leaves contain Veins, or vascular bundles. The following types of venation are distinguished: parallel (grasses), arcuate (tulip, lily), pinnate-reticulate (bird cherry), palmate-reticulate (maple), and dichotomous (Ginkgo).

Anatomical Structure of the Leaf. In the vast majority of plants, leaves have a dorsiventral structure (the upper side of the leaf is dorsal, and the lower is ventral). The anatomical structure of a leaf is closely related to its function. In cross-section, the blade consists of the following Tissues: dermal (protective), assimilation (photosynthetic), vascular, and mechanical.

The dermal tissue of the leaf is represented by a single-layered epidermis enclosing the leaf on both the upper and lower sides. The outer Cell walls of the upper epidermis are covered with a cuticle, while the lower epidermis forms a thinner cuticular layer, often bearing hairs that reduce water evaporation. Stomata are located in the lower epidermis.

Assimilation Tissue. The region of the leaf between the two epidermal layers is called the mesophyll. In many leaves, the mesophyll is differentiated into palisade and spongy parenchyma or consists of uniform Cells. In pines and spruces, the leaf mesophyll is represented by folded parenchyma.

Vascular Tissues traverse the leaf mesophyll as vascular bundles. Most commonly, they are closed and consist of xylem, located on the upper side of the vein, and phloem. The xylem comprises vessels, tracheids, and ground parenchyma cells arranged as radial rays. The phloem contains sieve tubes and companion cells. In the finest ramifications of the bundles, phloem and vessels disappear, leaving only tracheids.

Mechanical Tissues are most frequently situated around or above the vascular bundles, thereby providing structural support for the leaf. The pine needle possesses a continuous layer of thickened hypodermal cells beneath the epidermis, which also performs a mechanical function. Mechanical tissue is predominantly represented by collenchyma and sclerenchyma, though sclereids may also be present.

Secretory Tissues are represented by glandular hairs and hydathodes.

TOPIC 3. LEAF MORPHOLOGY AND METAMORPHOSES

General Remarks. The Origin of the leaf in plants is associated with their transition from water to land. It first appeared as an external outgrowth in fossil lycophytes and subsequently transformed into a true plagiotropic lateral vegetative organ in representatives of later systematic plant groups. The leaf serves as an organ of photosynthesis, transpiration, and gas exchange. Owing to these diverse functions, it displays a high degree of lability and a kaleidoscopic variety of forms. Evidence of this is that Carl Linnaeus once utilized over 150 terms specifically for leaves. The leaf responds sensitively to environmental factors and undergoes specific modifications.

The structural parts of a leaf are stipules, the petiole, and the leaf blade.

Leaves lacking a petiole are termed sessile or epeptiolate. The leaves of wheat, oats, and rye are also petioleless, yet their lower portion clearly clasps the stem, forming an open sheath. At the margins, near the base of the grass leaf blade, appendages known as auricles develop. At the transition zone from the sheath to the leaf blade lies a ligule of varying size and shape. A careful examination of sedges reveals that at the lower base, the leaves completely encircle the stem, forming a closed sheath, whereas the ligule is absent at the junction of the sheath and the blade. Such leaves are termed exligulate. In the leaves of carrots, parsley, and angelica, the petioles are expanded at the base to form pouch-like swellings. In Representatives of the buckwheat family (such as sorrel), the stipules fuse at the base of the petiole to form an ocrea.

Leaves Classified by General Outline. Based on their general outline, leaves may be: acicular (needle-like) in pine; lanceolate in willow; linear in wheat and rye; ovate in hornbeam and lilac; obovate in lesser butterfly-orchid; rhombic in birch and black poplar; rounded in aspen and wintergreen; spathulate in bugle; sagittate in arrowhead; hastate in bindweed and sheep sorrel; reniform in wild ginger; cordate in linden and violet; and elongate in goat willow.

Leaves Classified by Apex Shape include: obtuse (wild ginger), acute (willow, thornapple), acuminate (clubmoss, black poplar), cuspidate (ivy, cornelian cherry), and emarginate (yellow loosestrife).

Leaves Classified by Leaf Blade Base Shape: hastate (bindweed), sagittate (arrowhead), cuneate (black poplar), cordate (violet), and rounded (aspen).

Leaves Classified by Margin Type. Leaf margins may be: entire (flax, water plantain), dentate (sage, birch), doubly dentate (hazel), serrate (nettle, mulberry), crenate (violet, ground ivy), and sinuate (saltbush).

Leaves Classified by Venation. Venation groups include: dichotomous (Ginkgo), reticulate (a - palmate-veined [maple], b - pinnate-veined [pear]), parallel (wheat), and arcuate (water plantain).

Leaves Classified by Blade Dissection may be: entire, incised, lobed, cleft, parted, lyrate, runcinate, reniform, and peltate.

Lobed leaves are characterized by incisions that do not exceed ¹/₄ of the leaf blade. These include trilobed (hops), palmatilobed (common grapevine), and pinnatilobed (oak) leaves.

Parted leaves are those in which the incisions of the leaf blade extend more than ¹/₄ or ²/₃ of the distance from the margin to the midrib. Examples include ternately parted, palmately parted (Norway maple), and pinnately parted (dandelion) leaves.

Divided leaves have incisions that reach the midrib or the base of the veins in palmately veined leaves. These include pinnately divided (tomato, greater celandine) and palmately divided (creeping buttercup, geranium species) leaves.

Entire leaves have a blade with no incisions at all (lungwort), whereas toothed or lobed leaves form only shallow marginal incisions.

Types of leaves. Leaves are classified as either simple or compound. Simple leaves consist of stipules, a petiole, and a single leaf blade.

Compound leaves are those whose common petiole bears several simple leaflets, each of which falls off independently and freely. These include the following types: trifoliate (clover, strawberry), palmate (lupine, horse chestnut), pinnately compound with an even number of leaflets (yellow acacia), and pinnately compound with an odd number of leaflets (white acacia, rowan, dog rose).

Leaf modifications, or metamorphoses, develop under METABOLISM/18.html">The Influence of various environmental factors. The most common ones in the temperate zone are spines, tendrils, and phyllodes.

Spines are characteristic of drought-resistant xerophytic plants as an adaptation to reduce water loss (transpiration). In barberry and cacti, leaves are modified into spines; in acacias, stipules are modified; and in milkvetch, the tip of the rachis becomes a spine.

A tendril is a leaf modification that serves to attach the plant to a support. These are typical of climbing plants (lianas). In the pea or vetch leaf, you can see how the terminal leaflet transforms into a tendril, helping the plant anchor to a support.

Phyllodes are modified leaves found in Australian acacias, in which the petiole expands to mimic a leaf blade and takes over the function of photosynthesis.

Leaf succulents are modifications featuring a highly developed water-storage parenchyma protected by a thick cuticle layer. Such adaptations are clearly visible in stonecrops, aloes, and other plants. A cross-section of such a succulent reveals a heavily developed ground parenchyma and poorly represented mechanical tissue.

Trapping Organs are a type of leaf modification in which the blade transforms into a device designed to capture insects. The leaf blade expands and develops numerous glandular digestive organs that secrete a sticky fluid containing digestive Enzymes. Such modifications are characteristic of sundews.

Objective: to study the Morphological diversity of leaf structure;

to become familiar with the parts of a leaf and how it attaches to the stem; to examine various shapes of simple leaves with entire blades, learn the principles of determining leaf shapes; and get acquainted with the Classification principles of compound leaves.

Materials and equipment: morphological herbarium of leaves; potted plants: aloe, asparagus, begonia, monstera, pelargonium, ivy, tradescantia, Ficus; leaves of apple, alpine clover, oats, wheat, barley, fescue, sow thistle, black sedge, parsley, carrot, angelica, sheep sorrel, knotweed, pine, willow, broadleaf plantain, aspen, wintergreen, lesser butterfly-orchid, birch, black poplar, bugle, wild ginger, arrowhead, bindweed, thorn apple, water plantain, hazel, sage, flax, nettle, mulberry, violet, ground ivy, orache, birthwort, lungwort, oak, grapevine, maple, dandelion, castor oil plant, hops, buttercup, radish, valerian, broad bean, walnut, acacia, rowan, cornelian cherry; wall charts: "MORPHOLOGICAL STRUCTURE OF the Leaf", "Leaf Modifications", "Generalized Diagram of Leaf Blade Shapes"; magnifying glasses, forceps, dissecting needles.

Tasks

1. Examine the constituent parts of a leaf.

2. Observe and study herbarium specimens representing leaf types based on the overall outline of the blade, its dissection, base, apex, and margin.

3. Examine and study the types of simple and compound leaves.

4. Study leaf metamorphoses.

5. Locate leaves with various venation types using herbarium specimens and potted plants. Record the results of your venation analysis in Table 3.

Class="right">Table 3. Leaf Venation

Label the drawings and draw Conclusions.

Fig. 13. Generalized diagram of leaf blade shapes

Fig. 14. Types of dissection in simple and compound leaves:

1 - lobed (dissected less than halfway to the midrib); 2 - cleft (dissected more than halfway to the midrib); 3 - parted (dissected almost to the base); 4 - compound leaflets with petioles; 1-3 - simple leaves; 4 - compound leaves

Conclusion

Self-Control Questions

1. What are simple leaves?

2. Name the structural parts of a leaf and their modifications.

3. What leaf types do you know based on the overall shape of the leaf blade?

4. Name the leaf types according to their apex and give examples.

5. Name the leaf types according to their base and give examples.

6. Name the leaf types according to margin shape.

7. What are compound leaves?

8. Give examples of palmatilobed, palmatipartite, and palmatisect leaves.

9. What types of leaf venation do you know in monocot and dicot plants?

10. What types of leaves are modified into trapping organs?

11. The leaves of which plants form spines?

12. Give examples of plants in which leaves have transformed into tendrils.

13. What criteria are used to distinguish entire, incised, lobed, cleft, and parted leaves?

14. What are leaf sheaths in grasses and sedges, and how do they differ?

15. What is the Biological Significance of the trapping apparatus?