Tuesday, July 26, 2011

The sex life of sword ferns

Oooh, baby, get a load of those sori!
Photo by kqedquest 

Good ol' sword ferns are abundant in virtually any moist forest west of the Cascades. Compared to plants that flaunt gaudy flowers as a reproductive come-on, sword ferns lead quiet, strait-laced sex lives. They reproduce with decorum, forgoing both flowers and seeds.

Ferns make more ferns in a two-step process. A mature plant produces small rusty circles, called sori, on the underside of its fronds. These circles, which neatly line up along the margins of each leaflet, contain microscopic spores, and each plant produces millions.

The spores eventually fall to the ground or are carried by the wind. If moisture and light conditions are right, the spores will grow into tiny heart-shaped plants called prothallia. These reproduce sexually to become young fern plants.

Sperm and egg cells grow on the undersides of the prothallia leaves and require a drop of moisture to unite. Then the sperm cells, drawn by a chemical attractant, swim through the water to fertilize the eggs. A fertilized egg divides and redivides and finally creates new leaves unlike those of the heart-shaped prothallia. Additional leaves become more fern-like, eventually taking on the characteristic broad-sword shape the plant was named for.

Thoughout the spring and summer I like to note the progression in the cycle by occasionally tipping up the underside of a frond to see how the sori are developing.

Go ahead and take a peek--they won't mind.


  1. DaveOnFidalgo ...July 27, 2011 at 6:38 AM

    Wow, I didn't know about the second part.  Thanks for this.

  2. How would someone ever make the protothalia connection to sword ferns?  Some magic spells require a thimblefull of fern seed.  That's a lot of sori to harvest!

  3. I've always been fascinated by those little brown spores (sori) on the under parts of the sword fern.  But I never knew exactly how the reproduction cycle went.  Thanks for shining the spot light on these "puritans" of the plant world.