Let us learn about Diversity in the Leaf. After reading this article you will learn about: 1. Definition of a Leaf 2. Parts of a Leaf 3. Types.
Definition of a Leaf:
The leaf is a flattened, lateral outgrowth of the stem in the branch, developing from a node and having a bud in its axil. It is normally green in colour and manufactures food for the whole plant. The leaves take up water and carbon dioxide and convert them into carbohydrates in the presence of sunlight and chlorophyll. Leaves always follow an acropetal development and are exogenous in origin.
Parts of a Leaf:
A typical leaf of Ficus religiosa (pipal) has a broad thin, flat structure called the lamina. The thin stalk below the lamina is the petiole. The lamina possesses a network of veins. The veins have both xylem and phloem elements which are continuous with similar tissues of the stem through those of the petiole.
A strong vein, known as the midrib, runs centrally through the leaf- blade from its base to the apex; this produces thinner lateral veins which in their turn give rise to still thinner veins or veinlets. The lamina is the most important part of the leaf since this is the seat of food manufacture for the whole plant.
Types of Leaves:
Proceeding from or near the root, e.g., onion, radish, etc.
Pertaining to the stem, e.g., palms.
Cauline and Ramal:
Pertaining to the main stem as well as its branches, e.g., mango.
Phyllotaxy of Leaves:
A single leaf arising at each node, e.g., Hibiscus rosa-sinensis.
On different sides of the axis with the bases at the same level.
In pairs at right angles to one another, e.g., Calotropis.
A pair of leaves that stands directly over the lower pair in the same plane, e.g., guava.
More than two leaves arranged in a circle round an axis, e.g., Spergula, Alstonia.
The leaf blade is situated on the petiole, e.g., Hibiscus, Ficus, etc.
Without a petiole or stalk, e.g., Ixora.
Having a short stalk, e.g., Polygonum.
The leaf with stipules, e.g., rose, Ixora,
The leaves having no stipules, e.g., Ipomoea.
The stipules may be of several types. They are as follows:
Normally two stipules are developed at the base of a leaf petiole; they may be foliaceous, e.g., in Lathyrus-, free lateral, e.g., in China rose; adnate, e.g., in rose; interpetiolar, e.g., in Ixora, Spergula- spiny, e.g., in Acacia, Euphorbia splendens; tendrillar, e.g., in Smilax.
Leaf Base of Sessile Leaves:
Two sessile opposite leaves meeting each other across the stem and fusing together, e.g., Lomicera flava.
Clasping or surrounding the stem, as base of leaf, e.g., Sonchus.
Leaf with expanded bases surrounding stem, e.g., Calotropis.
Having leaf base prolonged down stem as a winged expansion or rib, e.g., Laggera pterodonta.
A leaf with basal lobes so united as to appear as if stem ran through it, e.g., Aloe perfoliata.
Simple and Compound Leaves:
A leaf which may be entire or incised to any depth, but not down to the midrib or petiole.
A leaf made up of two or more leaflets, e.g., pea, and several other members of Leguminosae. The compound leaves may be of several types.
They are as follows:
A. Palmately compound Leaf:
Having one leaflet only, e.g., Citrus.
Palmate compound leaf with two leaflets, e.g., Prinsepia, Balanites.
Such palmate compound leaf having three leaflets growing from same point, e.g., Oxalis, Vigna, Trifolium, Melilotus, etc.
Compound palmate leaf with four leaflets arising at a common point, e.g., Marsilea (a pteridophyte).
Compound palmate leaf with five or more leaflets arising at a common point, e.g., Gynandropsis pentaphylla, Bombax ceiba.
B. Pinnately Compound Leaf:
A compound leaf having leaflets on each side on an axis or midrib.
Having leaflets on each side of an axis, e.g., Cassia.
The central axis produces secondary axis which bears the leaflets, e.g., Acacia.
The secondary axes produce the tertiary axis which bear the leaflets, e.g., Moringa.
More than thrice pinnate, e.g., old leaves of coriander.
Pinnately compound without a terminal leaflet, e.g., Cassia.
Pinnately compound leaf with an odd terminal leaflet, e.g., pea.
Margin of Lamina:
With continuous margin, e.g., Psidium, mango, madar.
With large saw like teeth on the margin, e.g., Nympluiea, watermelon.
With serrate edges themselves toothed, e.g., China rose, nim.
The margin is wavy, e.g., Polyalthia.
With obtusely toothed margin, e.g., Bryophyllum, Centella.
Having margin or apex deeply cut into irregular lobes, e.g., many members of Ranunculaceae.
Irregularly incise, fringed.
Minutely incised or fringed.
Bearing fine hairs on the margin, e.g., Cleome viscosa.
Curled or extremely undulate margin.
Bearing many spines, e.g., Argemone.
Comb like margin.
Leaf margin divided into many lobes, e.g., Ranunculus.
Ending in a sharp point forming an acute angle, e.g., mango.
Drawn out into long point; tapering; pointed, e.g., Ficiis religiosa.
With blunt or rounded end, e.g., Banyan.
Having a notch at apex, e.g., Bauhinia.
Terminating abruptly, as if tapering end were cut off, e.g., Caryota mens,
Abruptly terminated by a sharp spine, e.g., apex of leaflet of Cassia obtusifolia.
Terminating in a point.
Provided with awns or with a well developed bristle.
Obtuse with a broad shallow notch in middle, e.g., Oxalis.
Leaf with prolongation or mid-rib forming a tendril, e.g., Gloriosa.
Forming abruptly to a small tip, e.g., Dalbergia.
System or disposition of veins in the leaves.
They are of several types:
Like net work, e.g., in most of dicots.
Parallel veined, e.g., most of monocots.
(a) Unicostate reticulate:
Having only one principal vein, e.g., mango, banyan, etc.
(b) Multicostate reticulate:
Having many principal veins, e.g., castor, cucumber, etc.
Shape of Leaf Lamina:
Long and narrow leaf, e.g., many grasses.
Lance-shaped leaf, e.g., bamboo, Nerium, etc.
Round or orbicular:
Leaf with a circular leaf blade, e.g., lotus, garden nasturtium, etc.
An ellipse-shaped leaf, e.g., guava, jack, etc.
Leaf with an egg-shaped leaf lamina, i.e., slightly broader at the base than at the apex, e.g., banyan, China rose, etc.
Spatula-shaped leaf, i.e., broad and round at the top and narrower towards the base, e.g., Calendula and Drosera.
Leaf with two unequal halves, e.g., Begonia.
Leaf with wide and long leaf lamina. Here the two margins run more or less straight up, e.g., banana.
Kidney-shaped leaf, e.g., Indian pennywort.
Leaf with heart shaped leaf lamina, e.g., betel.
Inversely heart-shaped leaf blade, e.g., wood-sorrel.
Leaf with an arrow shaped leaf blade, e.g., arrow-head and some aroids.
Sagittate leaf with its two lobes directed outside, e.g., water bindweed and Typhonium.
Lyre-shaped leaf lamina, i.e., with a large terminal lobe and some smaller lateral lobes, e.g., radish, mustard, etc.
Long, narrow and cylindrical leaf, i.e., needle-shaped, e.g., pine (a gymnosperm).
Wedge shaped leaf, e.g., water lettuce.