Ferns and many other so-called lower vascular plants lead an interesting double life. The life form we see is the large, spore-producing one, the sporophyte. But there is a hidden form that is hard to find and seldom seen. This very tiny plant that produces both eggs and sperm is known as the gametophyte. When a fern sperm swims to and fertilizes an egg, a new sporophyte is born.
A Rock and a Wet Place
by Gray Merriam
How did the fern below begin to grow on top of that rock? Ferns do not have seeds, so it was not simply a seed landing here.
The ferns that we see — the “fronds” — are called sporophytes, literally spore plants. The little rusty spots on the underside of the frond contain multitudes of spores waiting to launch onto the wind. Some of those spores landed on this boulder.
But each spore has only a half set of chromosomes. The “adult” fern has a double set — a full set of genetic instructions. That full set was completed by a seldom-seen, tiny but critical stage in the fern’s life history — the gametophyte, literally the
gamete plant. A little heart-shaped flap of green tissue, it grew from a spore that landed on the rock. So it, too, had only a half set of chromosomes and genes. But on its underside, two types of “organs” developed. One produced fern eggs, or female gametes, and the other produced male gametes, or sperm.
The eggs and the sperm of any one gametophyte usually do not ripen at the same time. So the sperm had to swim to another gametophyte to fertilize an egg. Yes, fern sperm swim between gametophytes to complete the fertilization that reproduces the double set of chromosomes and genes.
Without a carpet of moss, the boulder would never have held a film of water to let that gametophyte survive or to let the sperm cell swim to the egg that it fertilized.
Many things happen in the woods after dark!
