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
Look at the palm of your hand and then at a leaf. If the veins branch from the place where the petiole joins the leaf blade, like fingers from a hand, then the leaf has palmate venation. Besides being palmately veined, some leaves are palmately lobed or incised. A leaf can be simple, where the blade is undivided, or compound, divided into leaflets arranged along a rachis.
Among those trees with simple leaves with palmate venation are the redbud, maple, sweetgum, and sycamore pictured below. 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 palmately veined 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.
Left to right: Geranium maculatum, Pelargonium tomentosum, annual geranium, Tiarella cordifolia var. collina, Allaria petiolata, Vitis spp. and palmately veined and palmately lobed Ampelopsis brevipendiculata.
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 pictured below, 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.
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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.