Tree ferns

I have made a tiny spool scroll of the life cycle of a tree fern (Dicksonia Antarctica). These are amazing plants with a remarkable natural history.

Ferns as a class of plant emerged on Earth a very long time ago (nearly 400 million years). Their life stages and structures pre-date the seed as an evolutionary technology. It is challenging to communicate the nuance of this process at work, where I usually just leave it at “Ferns are older than dinosaurs, they’re even older than seeds”. But the ferns we see around us today are… well… present. So the way that Ferns live and reproduce is very old, but this class of plant has continued to adapt and evolve – just like all living things – for the entire time they have lived on Earth. They just diverged from pines, firs and flowering plants many epochs ago in evolutionary time.

Nonetheless, stepping into a grove of tree ferns feels like spiralling through deep time. With rocks and soil covered in the greenest mosses and liverworts, with towering fronds covered in sori (spore sacs) overhead, the air is thick with water. I always start to imagine all sorts of bygone creatures exploring and stepping through these forests, that unite continents and extend beyond mass extinctions.

So I made a tiny spool scroll of their life cycle.

When the conditions are right, each leaflet of the tree fern Dicksonia antarctica have spore sacs known as sori. To the naked eye, they look like smooth, brown little nobs on the underside of a frond. But looking with my microscope revealed a tangle of spores, resembling sprouts with mossy roots, all clinging together until it’s time to release.

The spores need cool, wet soil to grow into their tiny reproductive forms known as gametophytes. These have both males and female parts, and need water to sexually reproduce. When the sperm has fertilised the egg, the gametophyte then sprouts a sporophyte. The sporeling is what may eventually grow into a huge tree fern.

Ferns are their own evolutionary branch, and their life history somewhat resembles both mosses and flowering plants and conifers. Reflecting on how a tree fern is made is a reflection of how all trees, and by extension all mammals, and animals we see today (including us) came to be on Earth.