Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
Laboratory Class. Topic 9.22. Leaf Morphology and Metamorphoses

General Remarks. The Emergence of leaves in plants is associated with their transition from Water to land. A leaf is an organ of Photosynthesis, Transpiration, and gas exchange. Due to these Functions, it exhibits high lability and a kaleidoscopic variety of forms. Evidence of this is the fact that Carl Linnaeus used over 150 terms to describe the leaf alone. The leaf responds sensitively to environmental factors and undergoes various modifications.

Objects: leaves of apple, alpine clover, oats, wheat, barley, meadow fescue, thistle, black sedge, parsley, carrot, angelica, sheep's sorrel, knotweed, pine, willow, greater plantain, aspen, wintergreen, lesser butterfly-orchid, birch, black poplar, bugle, wild ginger, arrowhead, bindweed, jimsonweed, water plantain, hazel, sage, flax, nettle, mulberry, violet, ground ivy, orache, birthwort, lungwort, oak, grapevine, maple, dandelion, castor bean, hops, buttercup, radish, valerian, broad bean, walnut, acacia, rowan, cornelian cherry.

Tasks:

1. Study the structural PARTS OF THE leaf.

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

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

4. Study leaf metamorphoses.

5. Sketch Different types of leaves and label their parts accordingly.

Equipment and Materials: magnifying glasses, forceps, dissecting needles, live distributed material, herbarium, charts.

METHODOLOGICAL GUIDELINES FOR studying leaf Morphology. We recommend collecting various leaf types and mounting them in a morphological herbarium. At the same time, systematize the leaves according to their Structure or composition, the outline of the leaf blade, the CHARACTERISTICS OF THE apex, base, margin, and dissection. Separate simple and compound leaves, and demonstrate their structural features.

Among the distributed material as well as during field trips, identify various types of hereditary leaf metamorphoses and traumatic modifications caused by biotic and abiotic environmental factors.

Macroscopic study of leaf structure. By visual observation or using a magnifying Glass, study the structural parts of the leaf. If you find and collect a young leaf of an apple tree or alpine clover, you will easily identify the following structural parts: stipules, petiole, and leaf blade. Upon careful examination of a thistle or corn cockle leaf, you will not find a petiole. Such leaves are called sessile. The leaves of wheat, oats, and rye also lack a petiole, but it is evident that their lower part embraces the stem and forms an open sheath. Appendages known as auricles emerge at the margins of the Base of the grass leaf blade (Fig. 66).

At the junction where the sheath meets the leaf blade, There is a ligule of varying size and shape. A careful study of sedges reveals that at the lower base, the leaves completely clasp the stem to form a closed sheath, while the ligule is absent at the transition zone from the sheath to the leaf. In the leaves of carrots, parsley, and angelica, the petioles are broadened at the base and form sac-like swellings. In members of the buckwheat family (Polygonaceae, such as sorrel), the stipules fuse at the base of the petiole to form an ocrea.

Leaves classified by overall outline. From the provided leaves, select and sketch the following leaf types in your sketchbook based on their overall outline (Fig. 67): acicular (needle-like) — in pine; lanceolate — in willow; linear — in wheat, rye; ovate — in hornbeam, lilac; obovate — in lesser butterfly-orchid; rhombic — in birch, black poplar; orbicular (round) — in aspen, wintergreen; spathulate (spatulate) — in bugle; sagittate (arrow-shaped) — in arrowhead; hastate (spear-shaped) — in bindweed, sheep's sorrel; reniform (Kidney-shaped) — in wild ginger; cordate (Heart-shaped) — in linden, violet; elongate — in goat willow.

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Figure 67. Types of leaves According to the overall outline of the leaf blade

Leaves by apex shape: obtuse (wild ginger), acute (willow, jimsonweed), acuminate (clubmoss, black poplar), cuspidate (ivy, cornelian cherry), emarginate (groundsel).

Leaves by leaf blade base shape. From the presented live and herbarium material, select leaves based on the shape of the leaf blade base and arrange them in the following order: hastate (bindweed), sagittate (arrowhead), cuneate (black poplar), cordate (violet), orbicular (aspen).

Leaves by margin shape. Select leaves with an entire margin (flax, water plantain), dentate (sage, birch), doubly serrate (hazel), serrate (nettle, mulberry), crenate (violet, ground ivy), sinuate (orache).

Leaves by venation. Examining the underside of the leaves reveals a multitude of Veins, or venation patterns. Study The Nature of venation and sketch the following venation groups (Fig. 68): dichotomous (Ginkgo), reticulate: a.) palmate-nerved (maple), b.) pinnate-nerved (pear), parallel (wheat), arcuate (water plantain).

Figure 58. Types of leaves according to venation pattern

Leaves by leaf blade dissection. Using herbarium specimens and live material, examine and study entire, cleft, lobed, parted, and divided, lyrate, runcinate, reniform, and peltate leaf types.

Lobed leaves are characterized by incisions that do not exceed ¼ of the width of the leaf blade. Among them, three-lobed (hops), palmatilobed (common grapevine), and pinnatilobed (oak) types are distinguished.

Parted leaves are those in which the incisions of the leaf blade extend more than ¼ or ²/₃ of the distance from the margin to the midrib. Examples include trifid-parted, palmatiparted (Norway maple), and pinnatiparted (dandelion) (Fig. 69).

Dissected leaves have incisions that reach the midrib or the base of the veins in palmate-veined leaves. These include pinnatisected (tomato, greater celandine) and palmatisected (meadow buttercup, certain geranium species).

In entire leaves, the leaf blade has no incisions (lungwort), whereas in lobed leaves, the blade forms only shallow incisions along the margins.

Types of leaves. Leaves are categorized into simple and compound. Simple leaves consist of stipules, a petiole, and a single leaf blade. As you have observed, most of the leaves discussed earlier belong to this category.

Compound leaves are those in which multiple simple leaflets are borne on a common rachis, with each leaflet falling off freely and independently. These include the following types: trifoliate (clover, strawberry), palmate (lupine, horse chestnut), paripinnate (yellow acacia), and imparipinnate (black locust, rowan, dog rose).

Leaf modifications or metamorphoses develop in response to various ecological factors. The most common in the temperate zone are thorns, tendrils, and phyllodes.

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

Tendrils are modified leaves that serve to attach the plant to a support, typical of climbing plants (lianas). In the leaves of the pea or vetch, you can see how the terminal leaflet is transformed into a tendril used for anchorage.

Phyllodes — these are leaf modifications found in Australian acacias, where the petiole expands to mimic a leaf blade and takes over the function of photosynthesis.

Leaf succulents — these modifications feature a highly developed water-storing parenchyma protected by a thick cuticular layer. Such adaptations are visible in stonecrop, aloe, and others. A cross-section of such a modification reveals a massively developed fundamental parenchyma and poorly represented mechanical tissue.

Carnivorous traps — these are modifications where the leaf blade is transformed into a specialized trap for catching insects. The leaf blade expands and develops numerous digestive glands that secrete a sticky, enzyme-rich fluid. Such modifications are characteristic of sundew.

Conclusion. The morphological analysis of various leaf types demonstrates that the leaf is one of the most variable plant Organs. Leaves are divided into two main groups: simple and compound. Simple leaves are further classified by the shape of the leaf blade, its base, apex, margin, and degree of dissection. Compound leaves are characterized by the branching pattern of the rachis and the arrangement of the simple leaflets upon it.

Self-Assessment Questions

1. What are simple leaves?

2. Name the component parts of a leaf.

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

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

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

6. Name the types of leaves based on their margin shape.

7. What are compound leaves?

8. Provide examples of palmatilobed, palmatiparted, and palmatisected leaves.

9. What types of leaf venation do you know in monocotyledonous and dicotyledonous plants?

10. The leaves of which plants form thorns?

11. Give examples of plants in which leaves are modified into tendrils.

12. What are the criteria for distinguishing entire, lobed, parted, and divided leaves?

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