glabrous [ GLEY-bruhs ] adjective Botany: smooth, having a surface lacking hairs, bristles, and glands
While the word comes from the Latin glaber (bald), and is so used in literature to describe, for example, the skin on an old man’s head, when used in the scientific sense, glabrous means skin that was never meant to have hair, like the palms of our hands, and in botany, plant parts that are without surface projections.
Various plant parts can be glabrous, including leaves, stems, calyxes, and even the sori of ferns, such as those of Dryopteris cristata (crested wood fern). Having glabrous (smooth or glossy) leaves is often a distinguishing characteristic in plant identification. Many grasses have glabrous stems, leaves, or leaf sheaths. Others, proving the adage that few things in botany are always true, are described as “smooth” or “nearly hairless,” or “barely pubescent.” Chasmanthium latifolium (river oats), for example, is described as having hairless stems by some and as “smooth or barely pubescent” by others.
Some plants include the epithet glabra in their Latin names, firmly describing a characteristic of the leaves or some other part of the plant. Ilex glabra (inkberry), a shrub in the holly family, has smooth, glossy or leathery leaves. Aesculus glabra (Ohio buckeye) has not only glossy, hairless leaves, but also the seed inside the prickly fruit is a smooth, glossy brown. Native tree Carya glabra (pignut hickory) adds a complicating wrinkle to the use of the epithet: its bark is smooth, with shallow cracks, when young, but becomes sharply ridged as it ages. Chelone glabra (white turtlehead) is a smooth-leaved perennial that attracts many pollinators and is a critical food support for the caterpillars of the Baltimore checkerspot butterfly
Left to right: Ilex glabra, Aesculus glabra ‘Nana,’ Chelone glabra.
Solidago caesia (blue-stemmed goldenrod) is a native species of goldenrod that has glabrous stems and smooth, hairless leaves and tolerates having less sun than some of the others. It has small blooms at the axils, though the largest flower clusters are at the stem terminals. The phyllaries, peduncles, and stems of the native perennial Silphium perfoliatum (cup plant) are also glabrous or mostly so, but the leaves that fuse around the stems to form the cups (which hold water for insects and small birds) feel like fine sandpaper. The deep blue flowers of native Baptisia australis (blue wild indigo) are held on glabrous stems with smooth, hairless leaves described as waxy or slippery, and are cupped in glabrous calyxes.
Left to right: Solidago caesia, Silphium perfoliatum (phyllaries/peduncle, stem/leaves), Baptisia australis.
Reference
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas. pages 265, 376, 380, 566, 782, 1225.
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