Page:Parsons How to Know the Ferns 7th ed.djvu/57

 They take various shapes in the different species. They may be round or linear or oblong or kidney-shaped or curved. At times they are naked, but more frequently they are covered by a minute outgrowth of the frond or by its reflexed margin.

This covering is called the indusium. In systematic botanies the indusia play an important part in determining genera. But as often they are so minute as to be almost invisible to the naked eye, and, as frequently they wither away early in the season, I place little dependence upon them as a means of popular identification.

A fertile frond is one which bears spores.

A sterile frond is one without spores.