Most of the ferns are small perennial herbs. Tree ferns with long pillarlike stem and a crown of leaves at the top are found only in tropical countries. The plant body is differentiated into stem, roots and leaves with well-developed vascular bundles. The stem is usually a creeping underground rhizome bearing many adventitious roots.

It remains covered by a large number of persistent leaf-bases. Pinnately compound leaves, often called fronds, spring up from the rhizome. The leaves, when young, remain coiled inwards thus, exhibiting circinate vernation.

General Characteristics of Ferns

The ferns are an extremely diverse group, and there is no single characteristic that defines them. The following features are present in most:

• Leaves, called fronds, are megaphylls. Most are compound with a rachis and numerous pinnae. Almost all have circinate vernation that are coiled (circinate) tightly in “shepherd’s crook” or crozier fashion over the growing tips. These unroll as they mature. Stems, for the most part, are rhizomes that grow at, or just under, the ground surface. They have only primary tissues. “Tree” ferns have erect, thick trunks, the bulk coming from roots clustered around the small true stem. Roots are simple, uncomplicated and arise adventitiously along the rhizomes near the base of the fronds.
• Sporangia are located, for the most part, on the undersides of ordinary leaves in clusters called sori (singular, sorus). In many ferns a small leaf outgrowth called an indusium covers each sorus. Ferns have the highest number of chromosomes known in vascular plants.
• Most modern ferns are homosporous.