A typical leaf consists of three parts: Leaf base or hypo podium, petiole or mesopodium and leaf lamina or blade or epipodium. The entire leaf is said to be phyllopodium.
A. The leaf base (Hypo podium):
The part of the leaf attached to the stem is known as leaf base. In many monocot plants the leaf base expands into a sheath.
When it partially clasps the stem it is called semiamplexicaul (e.g., banana, If it clasps the stem wholly, it is called amplexicaul (e.g., grasses, Aethusa, in plants like Caesalpinia, Clitoris and Manqifera, the leaf base is swollen and called pulvinus.
In a few cases, where the leaf bases are winged, they extend down the stem up to the lower node. It is known as a decurrently leaf base, e.g., Symphytum.
B. The Petiole (Mesopodium):
It is the stalk of the leaf starting from leaf-base up to the origin of lamina. Sometimes it may not develop at all. A leaf with a petiole is called petiolate and one devoid of it is sessile.
The petiole is commonly a cylindrical structure more or less circular in cross section, or grooved having a longitudinal furrow. Different types of structural outgrowths and modifications are marked on the petiole.
In Citrus, it becomes winged and resembles the leaf lamina. In Eichhornea, it swells into a spongy bulb with numerous air chambers that help the plant to float.
In Clematis, the petioles are tendrillar. In Quisqualis, the leaf blades sometimes fall off and the petiole becomes hard spines. In Accaica moniliformis, it is modified into a sickle-shape lamina, called phyllode.
In many monocot plants of the grass family, Gramineae (Poaceae), the sessile leaves develop some peculiar outgrowths at the junction of the sheathing leaf base and the lamina.
It looks like a tongue-shaped membranous structure on the inner surface of base and is called the ligule. It is further strengthened by two out growth from the two sides of the junction between the sheath and lamina, known as auricles.
C. The lamina or leaf blade (Epipodium):
The lamina – flattened structure is the most important part of the leaf. It manufactures food and it is the place where all the functions of leaf, like gaseous exchange, transpiration, photosynthesis, etc. are performed.
Leaves vary widely, mainly due to different structural aspects of lamina. The structural aspects of lamina can be studied under the following headings:
It is the description of the form which is of various types as given below:
Acicular: needle-shaped as in pine;
Linear: Longer and slightly broad as in many grasses, Tuberose, etc.;
Lanccolate: Lens-shaped as in Nerium and Polyalthia;
Oblong: More or less rectangular as in banana;
Subulate or awl shaped: Wide at the base and narrow towards the apex as in Isoetes;
Ovate or egg-shaped: as in China rose and banyan; Cordate or heart-shaped: as in Piper betel;
Sagittate: arrow head-shaped as in Sagittaria.
Hastate: sagittate-like, but the two basal lobes directed outwards as in some sp. of Ipomoea;
Reniform or kidney-shaped: rounded above with a deep notch at the base as in Centella asiatica;
Lunate: Like a half-moon with two pointed basal lobes as in Adiantum;
Obovate: wider at the apex and reverse shape of ovate as in jackfruit.
Obcordate: reverse of cordate as in Bauhinia;
Spatulate: spatula-like as in Euphorbia;
Cuneate: Wedge shaped as in Pistia;
Elliptical: Like an ellipse as in Vinca and Rotund or orbicular as in Lotus,
(ii) Base of lamina:
The base of the lamina is generally studied in sessile leaf. When the leaf base surrounds the stem like two flaps as in Calotropis, it is called auricultate. When the articulate base completely clasps the stem it is called amplexicaul. When the basal lobes fuse together after completely clasping the stem, it looks as if the stem has perforated the leaf. This is called per foliate as seen in Aloe perfoliata.
In Swerita chirayita, the bases of two opposite leaves fuse together to appear like one leaf, through the centre of which stem has passed. It is called connate leaf base.
(iii) Margin of lamina:
The margin of the lamina may be as follows.
(a) Entire: Margin is smooth, e.g., Banyan.
(b) Repand: Margin is wavy, e.g., Polyalthia.
(c) Serrate: Margin is toothed with teeth pointed upwards, e.g., China-rose.
(d) Bi-scrrate: Margin is toothed, but each tooth again serrated, e.g., elm tree.
(e) Retroserrate: Margin is incised like a saw but teeth pointed downwards.
(f) Dentate: Margin is toothed, the teeth are pointed outwards, e.g., water lily.
(g) Bi-dentate: Margin is toothed and each tooth again dentate.
(h) Crenate: Margin is toothed and the teeth rounded, e.g., Centella.
(i) Bi-Crenate: Margin is toothed and the teeth crenate.
(j) Spiny: Margin is pointed to form spines, e.g., pineapple.
(k) Lobed or incised: Margin is incised into various depth and forming small lobes,, e.g., Mustard and Raphamus sativus.
(iv) Apex of lamina:
The apex of lamina is of following types
(a) Acute: Apex is narrow and pointed forming an acute angle, e.g., mango.
(b) Obtuse: Apex is broad angled and blunt, e.g., banyan.
(c) Acuminate: Apex is drawn out into a long tapering tail, e.g., Ficus religiosa.
(d) Mucronate: Apex is broad but the tip forms sharp point, e.g., Vinca.
(e) Cuspidate or spiny: Apex forms a spinous structure, e.g., Phoenix, Pandanus, palm, etc.
(f) Tendrillar: Apex is provided with a thin, wiry, coiling structure known as tendril for climbing, e.g., Gloriosa.
(g) Cirrhose: The mucronate apex ends in a small, fine thread-like structure, e.g., banana.
(h) Truncate: Apex is cut abruptly, e.g., Paris polyphylla.
(i) Rctuse: The obtuse apex is slightly notched, e.g., Pistia Clitoria.
(j) Emarginate: The obtuse apex is deeply notched, e.g., Bauhinia.
(v) Surface of lamina:
(a) Glabrous: The surface of lamina may be smooth without any hair, e.g., mango.
(b) Glaucous: The surface is covered by waxy coating, e.g., lotus, Calotropis.
(c) Scabrous: The surface is rough, e.g.
(d) Rugose: The surface is somewhat wrinkled is, e.g., Rubus rugosus.
(e) Viscose: The surface is sticky because of some excretion, e.g., Cleome viscosa.
(f) Gland-dotted: The surface is with gland dots, e.g., lemon.
(g) Hairy: The surface is covered with hairs. Based on the different types of hairs, it may be:
(i) Pubescent: hairs are soft and woolly as in tomato,
(ii) Pilose: hairs are long, distinct and scattered as in Grewia flavescens,
(iii) Villose: hairs are long, soft and closely arranged as in Leucas aspera,
(iv) Tomentose: haris are short, dense and cottony as in Terminalia and Calotropis, etc.
(v) Hirsute: hairs are stiff, fine and scattered as in Eclipta Alba.
(vi) Hispid: haris are long and rigid as in Cucurbita.
(h) Spinose: The surface is covered by small prickles as in brinjal.
(vi) Texture of lamina:
Lamina of the leaves has different textures.
(a) Herbaceous : The leaf lamina is thin and membranous as in China-rose and rose.
(b) Coriaceous: The leaf lamina is firm and leathery as in mango.
(c) Succulent: The leaf lamina is soft and juicy and more or less brittle as in Kalanchoe.