Wednesday, October 11, 2023

Flinging Spores and Fern ID

Sori on the back of a lady fern frond. Dozens of brown sporangia emerge from beneath the edge of a nearly translucent indusium.

The back of this lady fern frond (Athyrium filix-femina) is covered with sori, clusters of spore-forming bodies called sporangia. Each sorus holds dozens of them, all covered by a protective flap of tissue called an indusium. When a sporangium matures and dries, a line of cells over the top of the sporangium contracts, causing it to fling open and catapult its spores. You can watch it happen here in a sorus of an unidentified fern.

A spore print made by a lady fern frond. The spores are so small they look like dust. 

Unlike seeds, spores don’t contain embryonic plants. They’re little more than tiny packages of DNA that give rise to the next generation of ferns. They do this by first growing a small, heart-shaped prothallus, a body that produces egg and sperm cells. The flagellated sperm cells swim through a film of water to fertilize the egg cells, which then grow into the ferns we recognize. Because water is needed for this kind of reproduction, many ferns rely on damp or humid habitats.

A closeup of the back of a lady fern frond with a labeled sorus, sporangia and indusium.
Lady fern is identified in part by the shape of its sori. They are usually curved or
horseshoe-shaped. They appear in late summer and early fall.

A cluster of brown fertile fronds of ostrich fern. Their shapes resemble ostrich feathers.
Fertile fronds of ostrich fern.
Not all ferns have sori that look like those of lady fern. Some are located along the edges of the frond, some have indusia of different shapes or sizes, and some have no indusia at all. Other ferns, such as ostrich fern (Matteuccia struthiopteris) and sensitive fern (Onoclea sensibilis), produce spores on fronds specialized for that purpose – in other words, the whole frond is devoted to forming spores. For all these variations, timing is important. Sori and reproductive fronds (aka fertile fronds) may appear only at certain times of the year, and the appearance of sporangia and sori can vary depending on how old they are. 

Both reproductive and vegetative characteristics are helpful to identify a fern. Here are some resources to learn more about fern anatomy, biology and identification.

  • Ferns. U.S. Forest Service. This site may take a few tries to load.
  • Dichotomous Key to Ferns of Wisconsin, by Tim Gerber, UW-La Crosse.
  • Key to Fern Traits, by Areca Treon. This is a key to ferns in Cedar Creek Ecosystem Science Reserve near Bethel, MN.
  • Ferns of Minnesota, by Rolla Tryon. Illustrated by Wilma Monserud. University of Minnesota Press, 1980. ISBN 0-8166-0932-2.
  • Ferns and Lycophytes of Minnesota: The Complete Guide to Species Identification, by Welby R. Smith (author) and Richard Haug (photographer). University of Minnesota Press, 2023. ISBN 1517914663.

 For more information about lady fern, try these sites:

Plants for Bee Specialists

Jerusalem artichoke, Helianthus tuberosus , is one of several sunflower species favored by the sunflower mining bee, a specialist pollinator...