Petiole

petiole [ PET-ee-ohl ] noun: a leafstalk. adjective: petioled or petiolate

A petiole is the stalk that joins the blade (the flat part of the leaf, also called a lamina) to the node (the attachment point on a stem). The segment of the stem between two nodes is called an internode. The upper angle between a petiole/leaf (or branch) and the stem (or trunk) from which it arises is called an axil and new growth may occur within it from an axillary bud.

A petiole is a dynamic structure that “allows the leaf to swing freely in the wind in order to reduce the aerodynamic forces; it supports the weight of the leaf blade, as well as any moisture, rain, snow, and insects; it also enables the leaf blade to twist towards the sunlight and catch the sun’s rays.” (Pasini & Mirjalili 2006) Capturing light is crucial to photosynthesis where the leaf converts water (from the roots) and carbon dioxide to glucose (which feeds the rest of the tree) and oxygen.

Rhododendron leaf droop in January. — The undersides of rhododendron petioles develop folds or wrinkles as they bend downward in cold weather. Photo © Mary Free

However, petioles do not always tilt blades toward the sun. In the cold of winter for example, Rhododendron petioles can bend downward toward the stem at up to a 70 degree angle. This reduces the amount of leaf surface exposed to sunlight and helps prevent leaf damage during the time that Rhododendrons have lost the protective shade of the deciduous canopies of their natural habitat. This change in leaf position generated by the petiole is an example of thermotropism, where the temperature determines orientation.

Some leaf-climbers, like Clematis, have clasping petioles that act much like tendrils, grasping supporting structures so that the plants can climb. (Also see twine.) In On the Movements and Habits of Climbing Plants, Charles Darwin describes his observations of the different petiole movements of eight Clematis species.

Not all leaves have petioles though, and how a leaf attaches to a stem varies by species. (See pictures of plants illustrating different leaf attachments below.)

Petiolate or petioled leaves attach the blade to the stem.
Sub-sessile leaves have extremely short petioles.
Sessile leaf blades attach directly to the stem.
Clasping leaves are usually sessile leaves whose basal lobes clasp or wrap around the stem.
Perfoliate leaves occur when the base of a single blade or the bases of two leaves fuse together around the stem so that the stem looks like it has grown through.

*Lonicera sempervirens* petiolate and perfoliate leaves. Photo © Mary Free

A single plant can have more than one type of leaf attachment. For example, the opposite leaves of native Lonicera sempervirens (trumpet honeysuckle) are petiolate or sessile but fuse together and become perfoliate where the vine divides into two stems or when it terminates in an inflorescence. Some plants, like natives Erechtites hieraciifolius (fireweed), Mertensia virginica (Virginia bluebells), and Rudbeckia fulgida (orange coneflower) have petiolate lower leaves, which may become clasping (fireweed) or sessile as they ascend the stem.

Petiolate Leaves

Petioles of opposite leaves (eaten by monarch larvae) of native Asclepias syriaca (common milkweed). — Short perpendicular petioles of opposite leaves (eaten by monarch larvae) of native *Asclepias syriaca* (common milkweed). Photo © Mary Free

Stout one-inch petioles and axillary buds of Hydrangra macrophylla (mophead hydrangea). — Stout petioles (1″ long) and axillary buds of *Hydrangra*
*macrophylla* (bigleaf or mophead hydrangea).
Photo © Mary Free

Long petioles of star-shaped leaves of native Liquidambar styraciflua (sweetgum). — Star-shaped blades attached to long petioles (2.5-6″ long) of native *Liquidambar* *styraciflua* (sweetgum).
Photo © Elaine Mills

Short petioles (.25–.75"long) and axillary and terminal buds of native Quercus alba (white oak). — Short petioles (.25–.75″long) (with axillary and terminal buds) of native Quercus alba (white oak) restrict movement in the wind. Photo © Mary Free

Petioles of alternate leaved native Rhododendron maximum (great rhododendron). — Petiole orientation of native *Rhododendron maximum* (great rhododendron) during the warmer temperatures of June. Photo © Elaine Mills

Sub-sessile and Sessile Leaves

Sessile leaves of Diervilla sessilifolia (bush honeysuckle) — Sessile leaves of southern Appalachian native *Diervilla sessilifolia* (bush honeysuckle).
Photo © Elaine Mills

Sub-sessile (petioles less than 1/8") to sessile leaves of native Euonymus americanus (strawberry-bush). — Sub-sessile (petioles less than 1/8″) to sessile leaves of native *Euonymus americanus (strawberry-bush)*.
Photo © Elaine Mills

Some of the upper leaves of Mertensia virginica (Virginia bluebells) are sessile. — Some upper leaves of native ephemeral *Mertensia virginica* (Virginia bluebells) are sessile.
Photo © Mary Free

Upper stem leaves of nativar Rudbeckia fulgida var. sullivantii 'Goldsturm' (orange coneflower) are sessile. — Sub-sessile to sessile upper leaves of *Rudbeckia fulgida* var. *sullivantii* ‘Goldsturm’ (orange coneflower).
Photo © Mary Free

Sessile to sub-sessile leaves of nativar Solidago rugosa (rough-stemmed goldenrod) 'Fire Works.' — Sub-sessile to sessile leaves of nativar Solidago rugosa (rough-stemmed goldenrod) ‘Fire Works.’
Photo © Elaine Mills

Clasping Leaves

Erechtites hieraciifolius stem showing vertical lines and leaves alternate and sessile to clasping at the base. — Alternate upper leaves clasping at the base of native *Erechtites hieraciifolius* (fireweed).
Photo © Mary Free

Opposite clasping leaves of native Penstemon digitalis 'Husker Red' — Opposite clasping leaves of *Penstemon digitalis* ‘Husker Red’ (foxglove beard-tongue). Photo © Elaine Mills

Paired leaves clasping at the base of mid-south native Spigelia marilandica (Indian pink) — Paired leaves clasping at base of mid-southern native *Spigelia marilandica* (Indian pink).
Photo © Mary Free

Alternate, clasping leaves of Tricyrtis hirta — Alternate, clasping leaves of non-native *Tricyrtis hirta* (hairy toad lily).
Photo © Elaine Mills

Perfoliate Leaves

Native Silphium perfoliatum (cup plant) perfoliate leaves — Perfoliate leaves of native *Silphium perfoliatum* (cup plant).
Photo © Elaine Mills

Glabrous stems of Silphium perfoliatum (cupplant) — A perfoliate leaf of native *Silphium perfoliatum* (cup plant) creates a cup, which can hold rainwater.
Photo © Mary Free

Perfoliate leaves of native Uvularia perfoliata — Perfoliate leaves of native *Uvularia perfoliata* (perfoliate bellwort).
Photo © Elaine Mills

References

Chapter 2. Vegetative morphology of plants. UC Riverside. Department of Botany and Plant Sciences. (accessed October 18, 2021).

Harshberger JW. 1899. Thermotropic Movement of the Leaves of Rhododendron maximum L. Proceedings of the Academy of Natural Sciences of Philadelphia. 51(2): 219-224.

LeGrand H. Sorrie B. Howard T. 2021. Vascular Plants of North Carolina [Internet]. Raleigh (NC): North Carolina Biodiversity Project and North Carolina State Parks. (accessed October 19, 2021)

Pasini D. Mirjalili V. 2006. The optimized shape of a leaf petiole. Design and Nature III: Comparing Design in Nature with Science and Engineering. page 36. WIT Transactions on Ecology and the Environment, Vol 87. ISSN 1743-3541 (on-line). doi: 10.2495/DN060041.

Plants. North Carolina Cooperative Extension. (http://plants.ces.ncsu.ed accessed October 19, 2021).

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