“There’s something wrong with my fern! It has dots all over the leaves. It must be some sort of disease! What should I spray it with?”
I was sitting at our county extension’s Master Gardener Help Desk, answering questions when the call came in. The caller was quite agitated, afraid that her prize fern was on the verge of passing into the great garden in the sky. It took me some time to calm her down and explain that those black dots, neatly lined up in rows, were in fact a sign of health, and that her fern was not only thriving, but was on the brink of parenthood. Yes, those little bumps contained a zillion baby ferns, so to speak, in the form of spores.
I can see why my caller got confused. Most plants reproduce with flowers and seeds. Ferns, on the other hand, have a life cycle similar to that of mosses. It’s a complicated combination of gametes and spores that take turns in what biologists call an “alternation of generations
Just like mosses, ferns do best in areas of high humidity. (And just like my previous post on mosses, this one was also inspired by a recent trip to the Pacific Northwest, land of extremely high humidity. It rained pretty much every day of the two weeks I was there—normal for January. See photo, left, for why I was in Washington.)
This affinity for water is not an accident. Droplets of water are essential for fern reproduction. The ferns we all know and recognize produce spores, but before those spores grow into new ferns, a few other steps are involved.
As we would expect, the little packets containing the spores ripen and open, allowing the dust-like spores to drop to the ground. They are so fine, they also catch the breeze, allowing for a wide distribution of offspring. One the ground, they sprout and grow into tiny plants called prothallia. Prothallia are extremely small—only a quarter inch high—green, and shaped like a heart, and you’ve probably never noticed them.
The prothallia contain the gametes—eggs and sperm—which allow ferns to enjoy the benefits of sexual reproduction and its attendant reshuffling of chromosomes. The tricky issue for these terrestrial plants (as opposed to being aquatic) is that in order to find one another, the gametes need water in which to swim. You see the problem. Egg over here, sperm over there, and a bunch of dry dirt in between. That just won’t work. (Although, interestingly, sometimes a single prothallus produces both egg and sperm. This would minimize a need for travel. Still, it’s better to hook up with someone from another plant, as we wouldn’t want to marry a sibling, would we?)
And now you have an inkling of why ferns choose to live in places such as western Washington, where it drizzles incessantly all fall, winter, and spring. We also find ferns in tropical rainforests, cloud forests, or among the damp undergrowth pretty much anywhere. They need wet. Damp. Dank and dreary.
If you want to grow your own ferns from spores, and why not?, this is very important information. When your babies reach the prothallia stage you must mist them, providing little drops of water in which egg and sperm can find fulfillment, and the life cycle can continue.
Once one of those gametes has met with and joined with its soulmate, it grows into a fern, with fiddlenecks arising from the moist soil to unfurl into fronds which are eventually adorned with rows of black spots that we all know now are neither insect pests nor disease.
The fact that ferns also prefer low light, avoiding bright sunshine, and have thin, easily desiccated leaves, is a result of their need for wetness. If you have to live in a place with abundant drizzle and the resultant high humidity, you’d best adapt in every way.
And lest you now view ferns as a sort of leafy, ruffled moss, they are quite distinct. While ferns share a lot of characteristics of mosses—primarily their reproductive strategy—they differ by having the equivalent of blood vessels. They’re vascular plants, with phloem and xylem providing a way to move sap and nutrients from one part of the plant to another. In this way, ferns more closely resemble your garden daisy or oak tree. Pretty amazing, isn’t it?