Fern prothallus

[Lisa Millbank]

We grow a few herbs and vegetables inside over the winter, but today I noticed a weird little thing growing in the cilantro pot.  I thought it was a tiny weed sprout at first, but it appears to be a growth stage of a fern called a prothallus, about 2mm across.  

We've never managed to find one of these things before, and you'd think that with the jillions of ferns everywhere, they'd be easier to find.  I guess it's mostly just their small size that makes them hard to locate.  We always thought they were interesting because a prothallus (gametophyte) grows from a spore and only has one set of chromosomes.  The prothallus has male and female sex organs on it.  Ideally, the sperm can swim to another prothallus nearby to mix and match their genes, but most ferns can also fertilize themselves.  The union of the sperm and egg creates a diploid cell with two sets of chromosomes, which begins to grow out of the prothallus and becomes an adult fern plant (sporophyte), which will eventually produce its own spores.

It would be interesting to know if the spore is from an exotic fern that came in the potting soil I used, or if it was a local spore that happened to land on the soil recently.  I'll try to keep the prothallus alive and see if it begins to grow into a recognizable fern plant.  Or maybe I'll find out I was wrong about it being a fern, as there are a few other groups of spore-bearing plants that have prothalli.

Lisa Millbank