I have always thought of ferns as rather improbable. For one who has grown up with the concept of seeds, the lengths a fern has to go to in order to produce any offspring seems extraordinary. But what I always forget is that when ferns rocked up in the Devonian Period, seeds had not been invented. Neither had flowering plants. Or trees. Green things had only just managed landfall and ferns had made leaps and bounds since they had left their ancestors in the water. They had, for example, evolved a waterproof outer skin (cuticle) to stop them drying out and they had developed a system of tubes which meant water and sugars could be moved round the structure that these tubes also supported. Now equipped for life on the land, they needed to create more of themselves, and they did this (still do this) in the most incredible way.
The ferns I see when I stroll around the woods are not the whole story; they are in fact half the story as ferns have a complicated life cycle which involves two distinct forms. The leafy fern plant is the dominant part of the cycle – it contains two full sets of chromosomes, one from each parent. At this stage, I am used to plants producing pollen (only one set of chromosomes – haploid) which blows around or is carried by insects to the ovary (also haploid) at the base of a flower. When the one set of chromosomes from the pollen join with the one set of chromosomes in the ovary, a seed is produced with two sets of chromosomes and off we go again with the diploid plant growing much like the parents. No such sophisticated easy life for the fern. Instead, it produces spores.
Spores are haploid and are produced in structures called sori, normally on the underside of the fern leaves; you can see the sori as they ripen in the autumn as little brown dots. Ripe spores are ejected, often forcibly, from the leaf surface. Spores are tiny – if they are caught by the wind they can travel for hundreds, possibly thousands of miles. And where a spore settles, it grows into tiny heart shaped structure called a prothallus. This prothallus in an independent individual in its own right. Though only haploid, it is free living and photosynthetic (it can create its own energy from the sunlight). The prothallus is often short lived, but it takes part in something amazing. It contains both the female and male reproductive structures, so produces sperm and eggs and when these are mature there is only one more element necessary for this epic part of the ferns lifecycle to begin.
The element required is water; just a film of water is enough; a throwback to the watery origins of the plants. With this transport infrastructure in place, the sperm can move from the male part of the prothallus and swim to the female part in order to fertilize the egg, resulting a new diploid plant – the fern plant we started off with. Although tricky, it sounds doable until you realise that it is not a good thing, from a genetic point of view, for a sperm to fertilize an egg on the same prothallus as it originated from. In fact, many ferns actively ensure it cannot happen. That’s when you realise the enormity of the task ahead of that tiny sperm which, released into a hostile, unpredictable environment, has now to swim off and find another prothallus somewhere with an egg that needs to be fertilized. The fact that ferns have survived over 360 million years is testament to the efficacy of this system, but what are the odds? It seems impossible, and yet, nature has made a success of this method of reproduction. Indeed, once it was at the forefront of evolving processes, the revolutionary solution to dispersal and colonisation.
Ferns were prominent species in the Devonian and Carboniferous Periods, and have survived in great numbers since these Palaeozoic times. But what I still struggle to comprehend is the scale of the thing. How many cumulative miles have been swum by those tiny sperm to allow ferns to spread round the planet? All I can say, it’s a phenomenal achievement.