PLANT ANATOMY - Yu.I. Korniievskyi - 2017

Lecture Notes

Lecture No. 5. ANATOMICAL STRUCTURE OF THE LEAF

Lecture Outline

1. Leaf Morphology

1.1. Phyllotaxis on the stem

1.2. Simple entire leaves of various shapes

1.3. CHARACTERISTICS OF THE leaf blade

1.4. Leaves with various venation types

1.5. Shape of the leaf blade

1.6. Leaf formations

1.7. Varieties of leaves with a dissected leaf blade

1.8. Classification of leaves by stomatal distribution

2. Metamorphosis of the leaf and its parts

3. Water/140.html">Anatomical Structure of the leaf

Leaf (folium)

A lateral, temporary, flattened, monosymmetric, bifacial SHOOT element with limited growth in seed plants, or nearly unlimited growth in ferns.

Leaf Functions: Photosynthesis; Transpiration (evaporation); gas exchange.

Functions of modified leaves:

✵ spines — protective, moisture-conserving;

✵ succulent leaves — water-storage;

✵ tendrils — supporting;

✵ carnivorous traps — digestive;

✵ scale-like leaves — protective.

1. Leaf morphology

Depending on their orientation relative to the shoot axis, Two Sides of a leaf are distinguished:

✵ the morphological upper or inner (ventral, adaxial) side, facing toward the stem;

✵ the morphologically lower or outer (dorsal, abaxial) side, facing away from the stem.

Phyllotaxy refers to the pattern of leaf arrangement on the stem:

✵ alternate or spiral (one leaf per node);

✵ distichous-opposite and decussate-opposite (two leaves per node);

✵ whorled or verticillate (more than two leaves per node);

✵ basal rosette - a significant crowding of nodes on a shortened shoot;

✵ false-whorled (verticillastrate) - resulting from the enlargement or splitting of stipules.

1.1. Phyllotaxy on the stem

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1 - alternate; 2 - opposite; 3 - whorled; 4 — rosette.

The pattern of leaf arrangement on the stem is called phyllotaxy and occurs in several types: alternate or spiral (one leaf per node); distichous-opposite and decussate-opposite (two leaves per node); whorled or verticillate (more than two leaves per node). False-whorled phyllotaxy (in The Madder family, Rubiaceae) is formed due to the growth or splitting of stipules. When nodes are significantly crowded on a shortened shoot, a basal rosette of leaves is formed.

Regardless of their arrangement type on the stem, leaves are oriented so that sufficient light falls on their surface. Mutual adjustment in leaf positioning, or leaf mosaic, is achieved through varying sizes, Asymmetry of the leaf blades, uneven growth, bending, twisting of petioles, etc.

1.2. Simple entire leaves of various shapes

1 - triangular-ovate, 2 - broad-elliptic, asymmetrical, 3 - rhombic, 4 - flabellate (fan-shaped), 5 - linear-lanceolate, 6 - reniform (Kidney-shaped), 7 - orbicular-peltate, 8 - cylindrical (fistulose), 9 - acicular (needle-like), 10 - hastate, 11 - sagittate, 12 - scale-like, 13 - spathulate, 14 — obovate.

During leaf fall, simple leaves separate entirely from the node and possess a single continuous blade that is either unlobed or more-or-less lobed.

If the incision of the margin does not exceed 1/3 of the half-blade, the simple leaf is considered entire; if it exceeds this, it is considered cut or dissected. For entire leaves, all morphological features are described: blade shape, base, apex, margin, and venation type.

Leaves classified by their attachment to the stem

a - sessile, b, c - amplexicaul (stem-clasping), d - decurrent and perfoliate, e - leaf with a petiole expanded into a sheath.

When a petiole is absent, the leaf is termed sessile. Regarding their attachment to the stem, sessile leaves can be decurrent, perfoliate, amplexicaul, or semi-amplexicaul. A base forms from the lower part of the primordium, connecting the leaf to the stem and ensuring its growth. This base varies greatly: indistinct, expanded, with auricle-like structures or stipules (Polygonaceae), in the form of a leaf pulvinus (a small thickening at the Base of the petiole or leaf blade), or an expanded sheath that more or less clasps the stem (Asphodelaceae, Poaceae).

Petiole - the stem-like structure between the leaf blade and the leaf base that connects the leaf to the stem. Petioles can be short or long. In some plants, the petiole features a callus - a pronounced thickening at the upper or lower end (poplar); in others, it expands into a membranous sheath (Apiaceae, Orchidaceae) that clasps the stem.

1.3. Characteristics of the leaf blade

✵ thickness and structure (thin, thick, coriaceous, membranous, succulent, firm, brittle, loose);

✵ shape of the blade, determined by the length-to-width ratio and its resemblance to geometric figures or common objects;

✵ shape of the blade's structural parts: the apex, the base, and the margin;

✵ type of venation;

✵ surface texture (smooth, glossy, matte, hairy, pubescent, velvety, bristly).

When describing leaf margins, In addition to the primary descriptor (dentate, serrate, crenate, sinuate, wavy), qualifiers such as finely, coarsely, doubly, or triply are frequently required.

1.4. Leaves with Different Venation Patterns

Leaf venation refers to the arrangement and branching pattern of Veins (vascular bundles) within the mesophyll of the leaf blade. Venation can be open, where veins do not anastomose and extend all the way to the margin, or closed, where veins repeatedly anastomose to form a network.

Members of the class Dicotyledones predominantly exhibit closed pinnate and palmate venation, whereas Monocotyledones typically show open parallel and arcuate venation. Ginkgo biloba leaves feature dichotomous venation.

1 - camptodromous-reticulate, 2 - craspedodromous, 3 - campylodromous, 4 - reticulate-pinnate, 5 - palmate, 6 - palmate-craspedodromous, 7 - palmate-reticulate, 8 - palmate-campylodromous, 9 - arcuate, 10 - parallel, 11 - dichotomous.

1.5. Shape of the Leaf Blade

Leaf shape is one of the Characteristic Features of a plant species. Based on the outline of undivided blades, leaves can be: orbicular, with a circular outline; oval, having rounded base and apex of approximately equal width, with the length being twice the width; elliptic, featuring acute base and apex of roughly equal width, with the length twice the width; ovate, where the blade base is broader than the apex and the length is twice the width; lanceolate, with a length 3–4 times exceeding the width; linear, with a length 5–10 times exceeding the width.

Compound leaves consist of multiple leaflets, either petiolate or sessile, attached to a common axis (rachis) or petiole (as in trifoliolate and palmate leaves, or pinnate leaves where the rachis is an extension of the common petiole). During leaf fall, each leaflet of a compound leaf detaches independently, leaving leaf scars where they joined the rachis. In contrast, in dissected leaves, which externally resemble compound leaves, the entire leaf separates from the stem rather than its individual segments. Depending on the number and arrangement of leaflets, leaves are classified as ternate, palmate, or pinnate; if the rachis branches, the leaves become bipinnate or tripinnate.

1.6. Leaf Formations

Within an annual shoot, different leaf formations are distinguished: basal, cauline, low-growing (cataphylls), middle (foliage), and upper (hypsophylls). Most plants exhibit heterophilly, meaning that middle leaves located at different positions on the shoot vary in the degree of development of their component parts, size, shape, and dissection.

Low-growing leaves include underdeveloped protective scale leaves, bud scales of aerial and subterranean buds, and the earliest, smallest, undifferentiated brownish leaves on a shoot (e.g., in lily of the valley), as well as scales on rhizomes (couch grass), imbricate bulbs (lily), runners (strawberry), and tubers (potato).

Scale leaves of the lower formation are called cataphylls (from Greek kate – down, and phyllon – leaf).

Upper leaves or floral leaves are known as hypsophylls (from Greek hypso – high/upper, and phyllon – leaf), including bracts, involucre bracts, spathes, bracteoles, involucels, and bracts proper.

Bracts are reduced to membranous, transparent scales that quickly wither or drop off after the inflorescences unfold (lily of the valley, lilac, bird cherry).

Middle shoot leaves are typical for each species, green, highly differentiated, and perform the primary physiological functions.

1.7. Types of Leaves with a Dissected Blade

In dissected leaves, the outlines of the apex and base are not always clearly defined, and margin characteristics apply not to the blade as a whole, but to its free segments—lobes, clefts, or divisions. Dissected leaves are categorized based on blade shape, as well as the arrangement and number of free segments, into pinnate (free segments arranged on both sides of the midrib), ternate, and palmate (free segments radiating from a common point). According to the degree of dissection and relative size of the free segments, simple dissected leaves are classified as:

✵ lobed, where the dissection extends deeper than 1/3 but less than 1/2 of the half-blade, with the free parts referred to as lobes;

✵ divided, or partite - cut into lobes whose length exceeds 1/2 of the half-blade;

✵ dissected - cut into segments all the way to the base of the blade (in ternate and palmate leaves) or to the midrib (in pinnate leaves).

1.8. Classification of leaves based on stomatal presence

2. Metamorphosis of the leaf and its parts

1 - stipules of a compound leaf modified into spines, 2-4 - stages of Transformation of a simple barberry leaf into a ternate spine with an axillary shortened vegetative shoot, 5 - leaf of a cotton thistle with spine-tipped veins, 6 - colored floral bracts, 7 - climbing leaf petioles of nasturtium, 8 - tendrils of a compound vetch leaf, 9 - trapping apparatus, 10, 11 - bilobed leaves with Teeth; a - phyllode, b - tendril-like part of the petiole, c - petiolar part of the petiole with nectar, d - leaf blade in the form of a colored pitcher lid.

3. Anatomical STRUCTURE OF THE leaf

The leaf possesses the same primary tissue systems as the stem, but differs in their relative distribution, which is associated with the limited growth of the leaf and its primary functions. The anatomical structure of the leaf blade ensures photosynthesis, Respiration, and transpiration. Covered by the epidermis, it contains assimilation parenchyma, or chlorenchyma, which forms the green part of the leaf - the mesophyll. Within the mesophyll are supporting Cells, strands of sclerenchymatous fibers, crystal-bearing idioblasts, and secretory structures. The mesophyll is penetrated by a network of veins consisting of one or more closely associated collateral vascular bundles, in which the phloem faces the lower side of the leaf and the xylem faces the upper side. The cambium functions only in the midrib during the period of leaf blade growth.

3.1. Structure of the dorsiventral leaf type

Dorsiventral, or bifacial, leaf blades are characteristic of leaves with a more illuminated upper (ventral, adaxial) side and a shaded lower (dorsal, abaxial) side. Depending on light intensity, one or more layers of palisade parenchyma adjoin the upper epidermis, while a multi-layered spongy parenchyma adjoins the lower. Stomata are present either exclusively in the lower epidermis (hypostomatic leaf), exclusively in the upper epidermis (epistomatic leaf), or occasionally in both, though typically more abundant per unit area in the lower epidermis (amphistomatic leaf).

A - leaf of Japanese camellia, B - petiole of green alder, C - leaf blade of yellow water-lily.

1 - upper epidermis, 2 - lower epidermis with stomata, 3 - palisade parenchyma, 4 - spongy parenchyma with druses, 5 - collenchyma, 6 - sclerenchyma, 7 - vein xylem, 8 - vein phloem, 9 - aerenchyma, 10 - astrosclereids.

3.2. Structure of the isolateral leaf type

Isolateral, or unifacial, blades are characteristic of linear, lanceolate, ensiform, cylindrical, and similarly shaped leaves that occupy an approximately vertical position in space and receive nearly equal illumination on both sides. Their mesophyll may be homogeneous or heterogeneous. The density of stomata on the morphologically upper and morphologically lower epidermal surfaces is approximately equal (amphistomatic leaves).

A - homogeneous mesophyll, amphistomatic leaf; B, C - mesophyll differentiated into palisade and spongy layers.

1 - single-layered epidermis with sunken stomata, 2 - multi-layered epidermis, 3 - stomatal crypts, 4 - spongy mesophyll, 5 - palisade mesophyll, 6 - vein.

3.3. Structure of the radial leaf type

The radial, or centric, type is typical of needle-like leaves of conifers and similar xeromorphic angiosperm leaves with a low surface-area-to-volume ratio. The needles of evergreen plants feature adaptations for reducing transpiration, tolerating low temperatures, and withstanding mechanical stress. In pine needles, the epidermal cells are thick-walled and covered by a multi-layered cuticle. Stomata are numerous, arranged in longitudinal rows, and clearly visible from the surface as light stripes against a dark Background because the outer stomatal vestibule is filled with a whitish granular substance. Beneath the epidermis lies the hypodermis - a supportive, protective, and water-storage tissue that is interrupted beneath the stomata. The mesophyll is represented by plicate (infolded), and more rarely palisade and spongy, parenchyma. Below the hypodermis, numerous schizogenous resin ducts encircled by sclerenchyma extend in a circle along the axis of the leaf. In the center of the leaf are one to three vascular bundles enclosed by a sclerenchymatous sheath, a thick-walled starch-bearing endodermis, and central transfusion tissue. The bundles are open, with the xylem facing the adaxial side and the phloem facing the abaxial side.

Scale-like leaves of conifers (cypress, juniper, thuja) resemble needle-like leaves in their anatomical structure. They possess a thick-walled epidermis with a cuticle and a hypodermis. Stomata in the epidermis are sparse, the mesophyll is homogeneous but not always plicate, the endodermis is often inconspicuous, and there are typically one or two vascular bundles.

Structure of a pine needle:

1 - epidermis, 2 - stomata, 3 - hypodermis, 4 - plicate chlorenchyma, 5, 6 - resin ducts with sclerenchyma, 7 - endodermis, 8 - vascular bundles, 9 - sclerenchyma, 10 — transfusion tissue.

Leaf and leaf part metamorphism is associated with protective functions, moisture retention (bud scales, spines), expansion of the photosynthetic surface, and reduction of transpiration (flattened petioles or phyllodes). Simple leaves (barberry), leaflets of compound leaves (vetchling), stipules (black locust), vein tips, or the rachis can transform into spines, awl-shaped structures, or bristles. Leaves modified into brightly colored bracts and perianth segments attract insects, birds, and other pollinators, while the trapping Organs of carnivorous plants provide nitrogen Nutrition. Tendrils in lentils and peas are also Examples of leaf metamorphosis.