Ferns and horsetails form a monophyletic group that is the sister group to the seed plants (Fig. 33.11). The majority of species in this group are ferns. You can recognize ferns by their distinctive leaves that uncoil during development from tightly wound “fiddleheads” (Fig. 33.12). This group also includes horsetails and whisk ferns, which traditionally were considered distinct groups because of their unique body organizations. Whisk ferns have photosynthetic stems without leaves and roots (see Fig. 33.11b). Thus, they look similar to fossils of early vascular plants. Molecular sequence comparisons, however, support the hypothesis that whisk ferns are the simplified descendants of plants that produced both leaves and roots.

The horsetails, represented by 15 living species of the genus Equisetum, produce tiny leaves arranged in whorls, giving the stem a jointed appearance (see Fig. 33.11c). Horsetail stems are hollow, and their cells accumulate high levels of silica, earning them the name “scouring rush.” Today, horsetails are small plants, most less than a meter tall. However, tree-sized versions grew side by side with the ancient lycophyte trees (see Fig. 33.9).

698

In contrast, many ferns produce large leaves—in some cases up to 5 m long, although typically the photosynthetic surfaces are divided into smaller units called pinnae. Yet, because ferns are not capable of secondary growth, their stems remain slender and frequently grow underground, with only the leaves emerging into the air. Many ferns exhibit little stem elongation and produce their leaves in tight clumps. However, some species, such as bracken fern, produce spreading stems with widely spaced leaves. Because bracken is toxic to cattle and other livestock, its tendency to invade pastures is a serious problem.

Despite the absence of secondary growth, some ferns can grow quite tall. Tree ferns, which can grow more than 10 m tall, produce thick roots near the base of each leaf that descend parallel to the stem to the ground. These roots increase the mechanical stability of the plant and also allow it to transport more water (Fig. 33.13a). Other ferns grow tall by producing leaves that twine around the stems of other plants, using the other plant for support. At the other extreme are tiny aquatic ferns such as Salvinia (see Fig. 33.11e) and Azolla (Fig. 33.13b) that float on the surface of the water.