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.
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.
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.
Sub-sessile and Sessile Leaves
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