Excerpts from Jim Conrad's
Naturalist Newsletter

from the March 7, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO; limestone bedrock, elevation ~39m (~128ft), ~N20.676°, ~W88.569°
FERN PROTHALLI
Our Roughhairy Maiden Ferns are so at home on our deeply shaded, mossy, water-saturated cistern walls that below the fronds it's easy to find prothalli, which is a little unusual. You can see a good one below:

fern prothallus (of Thelypteris hispidula)

In the picture, the prothallus is the roundish item at the picture's left, with a surface texture somewhat like that of a liver. Only one cell thick, the structure is some 5 mm across (1/5-inch).

Prothalli, which are capable of photosynthesizing their own food and are free living plants, are features of every fern's life history. To understand them, let's start with a green, wall-hanging frond like one of those shown above, whose multitudinous sporangia release innumerable spores into the air.

When a frond-produced spore lands somewhere, if the environment is just right, it does NOT, like a seed, sprout something that turns into a plant like the one that produced the spore. What issues from the spore turns into a prothallus. Most prothalli are more or less heart- or kidney-shaped like the one in our picture.

From prothalli arise tiny, spherical structures called "antheridia," which produce male sperm, and tiny, flask-shaped "archegonia," which produce female eggs. Often both antheridia and archegonia occur on the same prothallus.

When a sperm fertilizes the egg, then the egg does sprout rather like a germinating seed, in the sense that what the sprout turns into is the big thing we think of as the fern. In the above photo you can see a parsley-leaf-like blade emerging from inside the prothallus' sinus or indentation. That's the sprout we're talking about. Once the sprout is self sustainable, the prothallus disintegrates and the sprout grows into what we think of as a fern.

So, we have two very different forms of free-living, photosynthesizing plants alternating with one another during the life cycle of every fern. In biology class this phenomenon is referred to as "alternation of generations," and when it all sinks in, it's mind- boggling. It's almost like a dog and a snail alternating with one another in one life, with snails (like prothalli) having sex to produce dogs, but the dogs (like spore-producing fronds) walking around occasionally budding off spores, from which snails emerge.