Explanation of Terms
A frond is said to be pinnate (Latin, pinna, a feather), when its primary divisions extend to the rachis, as in the Christmas fern (Fig. 1). A frond is bipinnate (Latin, bis, twice) when the lobes of the pinnæ extend to the midvein as in the royal fern (Fig. 2). These divisions of the pinnæ are called pinnules. When a frond is tripínnate the last complete divisions are called ultimate pinnules or segments. A frond is pinnátifid when its lobes extend halfway or more to the rachis or midvein as in the middle lobes of the pinnátifid spleenwort (Fig. 3). The pinnæ of a frond are often pinnátifid when the frond itself is pinnate; and a frond may be pinnate in its lower part and become pinnátifid higher up as in the pinnátifid spleenwort just mentioned (Fig. 3).
The divisions of a pinnátifid leaf are called segments; of a bipinnátifid or tripinnátifid leaf, ultimate segments.