palmate [ PAL-meyt, -mit, PAHL-, PAA-mayt ] adjective: of a leaf, lobed, veined, or divided from a common point with the veins forming a branching pattern that radiates from the place where the petiole joins the leaf blade, like fingers from a hand
Two common forms of venation that are the starting point for many plant identification systems are palmate and pinnate. A third venation pattern is fan-shaped, as in gingko trees (though in some identification systems Gingko biloba is treated as a variation of two-lobed venation, as the Latin name hints). A fourth form, arcuate, has a strong midrib, but also curved secondary veins in a more heart-shaped arrangement. The parallel venation of a fifth form appears in most monocot plants. (Refer to this Broadleaf Forms and Arrangements chart for illustrations of phyllotaxy, types, shapes, margins, and venation.)
A leaf can be simply or compoundly palmate in form. Besides being palmately veined, some of them are palmately lobed or incised. Among the simple palmate leaves are trees like redbud, maple, sycamore, and sweetgum. Because sycamore and sweetgum leaves can be mistaken for maple, other diagnostic tools like leaf arrangement, bark, and fruit may be necessary for identification.
Some shrubs, vines, and herbaceous plants also have palmate leaves. These include currants and gooseberries, grapes, and the Geraniaceae (geranium family, including native wild cranesbill or wild geranium and the often beautifully incised leaves of the Pelargoniums or scented geraniums). Aggressively invasive weeds like Allaria petiolata (garlic mustard) and the beautiful but so harmful Ampelopsis brevipendulata (porcelain berry) have palmate leaves as well.
Palmately compound leaves illustrate how individually pinnate leaves join to form a palmate complex. Native plants with compoundly palmate leaves include Parthenocissus quinquefolia (Virginia creeper) and the small tree Aesculus pavia (red buckeye). Invasive Vitex agnus-castus (chaste tree) and Schefflera (umbrella tree, in this area usually seen as a tropical houseplant) also have compoundly palmate leaves. The palmately trifoliate arrangement of the leaves of non-native Trifolium repens (white clover) exemplifies another variation. (The Latin quinquefolia and trifolium translate to five-leaves and three-leaf respectively.) In all these diverse structural forms, the leaflets radiate from the same petiole attachment point.
Interestingly, pinnate venation seems more common than palmate, at least in temperate climate zones. A study published in 2008 by academics working in California, Massachusetts, and Spain found palmate venation in up to 30% of the woody angiosperms in their local flora. This study examined the relationship between venation and the effective delivery of water to the leaves and found that woody plants with palmate venation suffered less hydraulic interruption from mechanical or insect damage to leaf veins. In pinnate leaves, if the midrib was damaged, the leaf suffered severe declines; palmate leaves with multiple main veins instead of a single midrib were more successful at surviving insect or mechanical damage.
The researchers “inferred” that the advantage in hydraulic efficiency granted by palmate leaves in certain conditions may account for the form’s repeated appearance in a wide range of geographic and biological areas over the 130 million years of evolution of flowering plants. The sustained flow of liquids apparently outweighs what the botanists call the “relatively high construction and maintenance cost” of the palmate leaves, though they also cited other factors, such as vein density and capacity.
Reference
Sack L, Dietrich EM, Streeter CM, Sanchez-Gomez D, Holbrook NM. 2007. Leaf palmate venation and vascular redundancy confer tolerance of hydraulic disruption. Department of Ecology and Evolutionary Biology, Univ. of California, LA, Harvard Forest, Harvard University.
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