Ferns are among the few plants that reproduce via spores rather than seeds. The basic life cycle of a fern consists of alternating generations of sexual and nonsexual individuals. The gametophyte, the sexual stage of the fern life cycle that develops from spores, tends to be so small as to be inconspicuous. The larger, visible plants we know as ferns are actually the asexual stage of the fern’s life cycle, known as the sporophyte stage, which will produce the spores necessary to produce the next gametophyte generation. Nonflowering plants, like ferns, tend to thrive in moist environments. Sensitive fern is, in fact, strongly associated with wetland habitats, which makes perfect sense, since their complex gametophyte-sporophyte life cycle cannot be completed without water.
Most species of fern have distinctively shaped spore cases, appearing on a specific location of the sporophyte. For many, these spore cases appear as dots or lines on the underside of the leaves. For others, like the sensitive fern (Onoclea sensibilis), the spores develop in an entirely separate structure. This spore bearing structure is called the fertile frond. In sensitive ferns, these fertile fronds persist long after the leaves (i.e., sterile fronds) die back for the winter, allowing for identification of this species throughout much of the year. Despite these spore producing structures, sensitive ferns more often reproduce by budding new stalks from a creeping rhizome, than through their spores.
The name “sensitive” comes from the fact that the leaves of this species of fern are particularly vulnerable to cold. Its leaves rapidly die off after the first occurrence of autumn frost. Similarly, if a late spring frost occurs after sensitive ferns have already put forth leaves, the event can entirely kill the new growth, forcing the plants to start entirely new leaves.
Drawing the fertile fronds of sensitive fern in my perpetual nature journal.