The Lycophyta, Equisetophyta, and Psilophyta are collectively referred to as the fern allies because, like the ferns (Pterophyta), they reproduce by single-celled spores released from sporangia (spore sacs). They do not produce flowers or seeds and both ferns and fern allies contain well-developed conducting tissues to transport fluids within the plant. Fern allies, however, differ greatly in appearance from ferns because they generally bear small, simple leaves with an unbranched vein, whereas almost all ferns have larger, often lacy cut leaves called fronds that contain branching veins. The fern allies include some of the earliest known land plants, many of which are long extinct. Today, there are probably fewer species of fern allies than there were many millions of years ago.

The life cycles of the fern allies and ferns are similar. Alternating generations of sporophytes (diploid plants producing spores) live independently of gametophytes...