Sex and the Secret Life of Ferns

Rock polypody – Polypodium virginana – grows on acidic rock faces.

I have been wondering if a story about sex might not ease our worries for a bit. Please forgive my anthropomorphism.

Interrupted fern – Osmunda claytoniana

One summer’s day when the sunshine and raindrops felt right, the capsule in which they had grown gently pulled open exposing their tiny dusty selves to the air. Then, suddenly, the walls sprang back hurling them out into the breeze. Drifting along, the spores flew far then fell to the ground. Some of them, finding themselves on good damp dirt either soon, or many years later, sprouted from a single cell into a tiny green heart-shaped gamete. Search for years on hands and knees on disturbed soil with a magnifying loupe on your eye, and you will probably never see the secret sex life of ferns that follows.

Spore capsules – sori on the back of marginal wood ferns – Dryopteris marginalis

Even though these heart-shaped plants could have lived alone making sugars from what sunlight filtered down to the soil, they longed for more. For the tiny haploid fern prothalli only had one set of chromosomes. The little gametes wanted to be like people and trees and flowering herbs (we will not get into mosses, bananas, seedless watermelons, nor hybrid ferns) and find their other half. Searching to be diploid, with two pairs of chromosomes, the gametes  sprout a male organ on top and a female organ on the bottom of its little heart-shaped self. The male organ (antheridium) feels a twinge of hormones coming from a neighboring gamete and ejects its many spermatozoa to swim off through the water in search of an egg. The female organ (archegonium) had grown a single egg nestled down in a swollen chamber below a tube. When the sperm approaches she opens her tube and lets him swim in. There, a new life begins, a single celled zygote.

It first grows rhizomes to connect with the dirt and then pushes up a leaf that unfurls into what we would recognize as a fern frond. The little heart-shaped gamete that started it all shrivels and dies. The fern—the sporophyte will grow through the summer photosynthesizing. Eventually single-celled haploid spores will appear in safe capsules called sori, often hidden on the undersides of leaves. One day when the sunshine and raindrops are right, the cycle will continue.

In spring, as the snow melts and we are walking again into the forest looking for sprouts of green, we are greeted with tiny fiddleheads and wonder what type of ferns these are. All ferns unfurl from a curled-up leaf—an important diagnostic. Many of the common ferns we find here in New England are also found in temperate forests of Eastern Asia, particularly Japan. Sensitive ferns, ostrich ferns, maidenhair ferns, interrupted fern, and silvery spleenwort are some of the ferns once found all around the globe during the warm Tertiary Period. A period that began with an asteroid impact and the sudden extinction of dinosaurs, it came to an end as the planet cooled, and the glaciers grew.

Maidenhair ferns – Adiantum pedatum
fiddleheads of Adiantum pedatum

The Tertiary was a great time to be a fern, as forests thrived around the globe, even into sub-alpine zones. Land bridges connected one continuous circumboreal region. But as glaciers scoured the northern lands, and as mountain ranges like the Rockies and the Himalayas rose creating dry rain shadows, fern habitat was reduced to a few refugia. As the ice melted, areas like New England that had a free corridor for plants to move northward once again became home for forests and ferns.  Spring will be here soon. Remember when you walk in the forest, there is more going on underneath your feet than you know.

Unfurling fiddle heads of interrupted fern – Osmunda claytoniana
fertile spores on the “interrupted” fronds of Osmunda claytonia

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