Leaf Stalk

Fern leaves vary in the relationship of the petiole, or leaf stalk (often referred to as stipe in ferns), to the blade (the expanded part of the leaf). Many strap-shaped leaves essentially have no petiole and are described as sessile; broad, ovate, or triangular leaves commonly have a pronounced leaf stalk, called a stipe, and are termed petiolate or stipitate. Narrowly elongated leaves in ferns are usually erect, spreading, or, in certain epiphytes, pendent. Leaves that are broadly ovate or triangular tend to be borne at right angles to the incident light. Broad-leaved ferns thus become more or less bent at the blade base, with an arch at the top of the petiole.

Anatomically, the petioles or stipes of fern leaves show nearly as much diversity in cross-sectional pattern as do the stems. The simplest vascular strands of fern petioles are commonly crescent-shaped single bundles. In more and more elaborate petiolar patterns, the crescent takes on the form of the Greek letter omega (Ω), opening adaxially (i.e., upward or toward the central axis of the plant). The latter shape, with many variations, occurs widely among ferns, especially those considered on other grounds to be primitive. Double-stranded ferns (the omega now divided into two parts and unconnected below) are usually associated with more specialized genera (e.g., Athyrium and Thelypteris). Any of the generalized patterns may exist as broken-up strands; the separation is commonly associated with size, small leaves having only three to nine strands, large leaves of tree ferns having many. Petiolar vascular bundle shapes have been found to be so definitive as to have a certain value in separating fern genera and families.