Leaf type

Leaf type refers to how a leaf's blade is divided into discrete segments, and is judged by the number of segments the petiole bears. A simple leaf has a single, undivided lamina on its petiole - the lamina may be entire (Mango) or incised to some depth, but never right down to the midrib or petiole (Cucurbita). A compound leaf has its main rachis bearing more than one small lamina, called leaflets; only one axillary bud sits at the base of the whole compound leaf, never at the base of an individual leaflet, and compound leaves are thought to have evolved to increase total leaf surface area.

Pinnately compound leaves bear leaflets laterally along the rachis, alternately or oppositely, as in tamarind and Cassia. A leaf whose rachis stays simple and unbranched, bearing leaflets directly, is unipinnate - paripinnate if the leaflets are even in number (Tamarindus, Abrus, Sesbania, Saraca, Cassia) or imparipinnate if odd, with one unpaired terminal leaflet (Rosa, Azadirachta/neem, Murraya). If the primary rachis branches once into secondary rachii (pinnae) that then bear the leaflets, the leaf is bipinnate (Delonix, Mimosa, Acacia nilotica, Caesalpinia). If it branches again into tertiary rachii, it is tripinnate (Moringa, Oroxylum). If the rachis branches repeatedly and irregularly, the leaf is decompound (Daucus carota, Coriandrum sativum, Foeniculum vulgare).

Palmately compound leaves instead bear all their leaflets terminally at the tip of the petiole, radiating out like fingers from a palm: unifoliolate has one leaflet (Citrus, Desmodium gangeticum), bifoliolate has two (Balanites roxburghii, Hardwickia binata, Zornia diphylla), trifoliolate has three (Aegle marmelos/wood apple, Trifolium/clover, Lablab, Oxalis), quadrifoliolate has four (Paris quadrifolia, Marsilea), and multifoliolate/digitate has five or more (Ceiba pentandra, Cleome pentaphylla, Bombax ceiba). …