Simple Leaves and Compound Leaves: There are two types of leaves, e.g. (a) simple and (b) compound.
(A) SIMPLE LEAF — A leaf is said to be simple when it possesses one articulation and consists of a single leaf blade or lamina — this is due to the union of membranes of the midrib and its branches. The margin of the leaf blade may be entire or incised to any depth (therefore only lobed) but the incision never touches the base of the midrib or the petiole.
Simple leaves Examples: Black cherry, Maple, Oak, Banana, Mango, etc.
(B) COMPOUND LEAF — A leaf is said to be compound when it possesses two or more articulations and the leaf blade is broken up into a number of small segments, called leaflets, due to several incisions of the blade which touch the base of the non-membranous midrib or the petiole.
The leaflets are free from one another and arranged on the axis i.e. midrib known as rachis. The leaflets may be situated either laterally on the sides of the rachis or terminally i.e. at the apex of the rachis (petiole).
Compound leaves Examples: species of Sesbania, Saraca, Moringa olifera (Moringaceae), Citrus aurantium, Gynandropsis gynandra (Capparidaceae), etc.
Types of Compound leaf :
Compound leaves are of two types, viz. (i) Pinnate and (ii) Palmate.
1. PINNATELY COMPOUND LEAF :
In pinnate type of compound leaf, the leaflets are arranged either alternately or in an opposite manner on both sides of the rachis directly or on the branches of the rachis. This type of compound leaf looks like pinnae of a bird’s feather.
Pinnately compound leaves examples: Cassia, Acacia arabica, Moringa olifera (Moringaceae), Foeniculum vulgare, etc.
Types of pinnately compound leaf :
(a) Unipinnate – When the midrib i.e. rachis bears the leaflets directly, it is termed as unipinnate. Unipinnate type again may be of two kinds e.g.:—
(i) Paripinnate – When the leaflets are arranged in pairs i.e. rachis bears even number of leaflets, e.g. species of Sesbania, Saraca, Cassia, Tamarindus indica (Caesalpinaceae), etc.
(ii) Imparipinnate – When leaflets are arranged on the rachis in such a way that the terminal end of the rachis bears a single leaflet — So the rachis bears at the apex an unpaired odd leaflet, e.g. Clitoria ternata (Papilionaceae), Rosa centifolia (Rosaceae), Azadirachta indica (Meliaceae), etc.
(b) Bipinnate – In this case secondary branches are produced from the rachis which bear smaller leaflets called pinnules, i.e. in bipinnate the pinnately compound leaf is twice pinnate. Examples – Acacia arabica, Mimosa pudica (Mimosaceae), Caesalpinia pulcherrima (Caesalpinaceae).
(c) Tripinnate – In this type, the rachis produces secondary branches, which in turn again produce tertiary branches bearing leaflets. Hence in tripinnate type the leaf is thrice pinnate e.g. Moringa olifera (Moringaceae), Oroxylon sp. (Bignoniaceae).
(d) Decompound – When the leaf is more than thrice pinnate i.e. when the incision goes beyond the tripinnate condition, e.g. Daucus carota var. sativa, Foeniculum vulgare, Coriandrum sativum (Umbelliferae), species of Cosmos (Compositae), etc.
2. PALMATELY COMPOUND LEAF :
A palmately compound leaf is formed when the petiole bears terminally and articulated to it a number of leaflets which seem to be radiating from a common point like fingers of a palm. In palmately compound leaves, the rachis does not develop at all — hence leaflets are articulated to the petiole.
Palmately compound leaves examples: Citrus aurantium, Bignonia grandiflora (Bignoniaceae), Bombax ceiba (Bombacaceae), etc.
Types of palmately compound leaf :
(a) Unifoliate — When one leaflet is articulated to the apex of the winged petiole, e.g. Citrus aurantium, Citrus limon (Rutaceae) etc.
(b) Bifoliate — When two leaflets are articulated to the apex of the petiole, e.g. Balanites aegyptica (Zygophyllaceae), Bignonia grandiflora (Bignoniaceae), Hardwickia binata (Caesalpinaceae) etc.
(c) Trifoliate — When three leaflets are articulated to the apex of the petiole, e.g. Vitex negundo (Verbenaceae), Oxalis corniculata (Oxalidaceae), Aegle marmelos (Rutaceae) etc.
(d) Quadrifoliate — When four leaflets are articulated to the apex of the petiole, e.g. Marsilea quadrifolia (pteridophyte).
(e) Digitate or Multifoliate — When more than four leaflets are articulated to the apex of the petiole, e.g. Bombax ceiba (Bombacaceae), Gynandropsis gynandra (Capparidaceae) etc.