Sporangia Special

What am I looking at?

This is a single sorus of a rock polypody fern (Polypodium virginianum). Sori can be found on the underside of this tiny fern’s fronds. They consist of an assembly of spore-producing sporangia (1) and paraphyses – supportive hairs shaped like a bowling pin, which play a protective function (2). Spores are visible through the walls of four more mature sporangia (3).

Click on the right arrow to see some additional views of this sorus.

Biology in the background

Ferns have a life cycle that involves two distinct, alternating generations. The spores released by the sporophyte – the asexually reproducing form that we commonly recognize as a fern – germinate into either a tiny, heart-shaped, hermaphroditic gametophyte (“hermaphroditic” means it has both male and female reproductive parts) or an even smaller, oval, purely male gametophyte.

The male organs, called antheridia, produce motile, ciliated sperm, and the female organs produce eggs. Fertilization requires the presence of water – which is why you don’t usually find ferns in dry places. The sperm find eggs following chemical signals, and upon contact the two gametes form a zygote that develops into an embryo within the archegonium, the female sex organ. The embryo eventually develops into a sporophyte, while the gametophyte usually atrophies and dies.

The sorus of a rock polypody fern can be up to 1.5 millimeters in diameter, or roughly 20 times the width of a human hair. A single spore is a little over 50 micrometers across, or a bit smaller than the width of a human hair.

Technique

These images were created using confocal microscopy.