pinnatisect

Pinnatisect describes a leaf morphology in which the blade is deeply dissected into numerous narrow segments arranged pinnately along a central axis. The incisions extend well toward the midrib or axis, causing the leaf to resemble a feather with many slender lobes or lobules.

In pinnatisect leaves, the dissections create segments that remain part of a single leaf blade rather than forming fully separated leaflets on a common rachis. The overall appearance is of a single, continuous blade that is deeply divided, rather than a compound leaf with discrete leaflets. The segments are typically lanceolate or linear and are arranged on both sides of the central axis.

The term is used in botanical descriptions across various plant groups, particularly among herbaceous species. A commonly cited example is Achillea millefolium (common yarrow), whose leaves are often described as pinnatisect due to their deeply divided, featherlike segments. Because terminology can vary among authors, pinnatisect is sometimes discussed alongside related terms such as pinnatifid or pinnately dissected, with slight differences in how deeply the blade is cut and whether the segments are considered separate leaflets.

Etymology derives from Latin pinnatus (feathered) and sectus (cut), reflecting the featherlike division of the leaf blade. In plant identification and morphology, pinnatisect is a descriptive term used to convey a specific pattern of leaf dissection that helps distinguish species or cultivars with similar leaf shapes.