Word of the Week: Pubescent

pubescent [ pyoo-besuhnt ] adjective: covered with hairs (noun: pubescence)

 

Flora of Virginia describes Phlox divaricata (woodland phlox) calyxes as “densely glandular,” leaves as “sparsely villous or pubescent, some trichomes glandular,” and flower stalks as “glandular-hairy.”
Photo © Mary Free

Generally, the hairy surface (indumentum) of a stem, leaf, calyx, or corolla is described as pubescent. The individual hair (trichome) is an outgrowth of the epidermis. There are a number of different terms to describe hairiness, and these depend on the type (simple or glandular), shape (straight, sickle-shaped, hooked), length (minute, short, long), density (scarce, moderate, dense, heavy), and growing direction (relative to the surface they grow on) of individual hairs. Pubescent type may be used as a diagnostic tool in species identification, but can differ during the various stages of development from seedling to senile as well as among the seasonal forms of a particular species.

Why do plants have hair? Pubescence can protect them from insects and larger herbivores, reduce sun exposure and transpiration, and insulate. For example, small insects can stick to the hairs of Phlox divaricata (woodland phlox). Research on plants with viscous glandular hairs shows that the captured insects attract predators that scare off larger insect herbivores, thereby further protecting the plant, especially the buds. Both aphids and deer tend to avoid Stachys byzantina (lamb’s ear), a plant with woolly leaves. Interesting hairs make Cephalocereus senilis (old man cactus) a popular house plant, but in its native desert environment, its arachnoid pubescence helps the cactus retain moisture, reflect harmful sun rays, and manage temperature fluctuation.

Different Types of Pubescence (This list is not all encompassing and some types may be redundant.)

      • appressed: pressed close to a surface, as hairs flat against a stem
      • arachnoid: with long, cobwebby hairs
      • bristly: with stiff hairs
      • canescent: with dense, short, fine, grayish/white hairs
      • ciliate: with fringe-like hairs, as along a leaf margin
      • downy: with fine, soft hairs
      • glandular: with enlarged glands at hair tips
      • hirsute: with coarse hairs (less stiff than bristly, not sharply pointed)
      • hispid: with short, stiff, bristly, often pointed hairs
      • hoary: with close, fine, interwoven, grayish/white hairs
      • pilose: with distinct, long, soft, erect hairs
      • pubescent: with short, fine hairs; with hairs or trichomes of any type
      • scabrous: with short, stiff hairs
      • stellate: with star-shaped hairs (forked hairs on a single stalk or tufts of hair attached at a single point)
      • strigose: with sharp, rigid hairs pointing mostly in the same direction
      • tomentose: with densely matted, soft white hairs
      • velvety: with soft hairs
      • villous: with long, soft hairs, often curved, but not matted
      • woolly (or lanate): with long, soft, entangled hairs

Some Plants with Pubescent Parts*

Click on images  for larger views and captions.

*Although the plants above are provided as examples, various flora references may use different descriptions for the type of pubescence on the same species. The described pubescence may have been particular to the specimen observed, as plants of the same species may possess different pubescent characteristics.


Reference

Armstrong WP. 2001. Botany 115 Terminology. Leaf Terminolgy Part 2. Wayne’s World: 9 May 2001. [accessed 11 May 2021].

Barykina R, Alyonkin V. July 2016. Pubescence of vegetative organs and trichome micromorphology in some Boraginaceae at different ontogenetic stages. Wulfenia. 23:1–29.

Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas. page 801.

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