The Hidden Secrets of Ferns

Fern spores are produced in vast numbers on the understide of fern fronds during the summer months. For details of how they are catapaulted into the airstream, take a look at http://beyondthehumaneye.blogspot.com/2009/07/natures-siege-catapults.html

Each spore is less than a hundredth of a millimetre in diameter and can be carried vast distances on air currents. Ferns are often the first plants to colonise bare volcanic lava flows, carried there as spores on the wind.

All they need for germination is water and mineral salts. They swell, the brown spore case splits open and a hair-like rhizoid emerges, that anchors the spore to its substrate. Then a green photosynthetic cell emerges from the spore.

The photosynthetic cell divides longitudinally, forming the beginnings of a short chain of cells. The green blobs in the cells are chloroplasts.

The chain of cells continues to elongate until it reaches 6-7 cells long, dividing longitudinally and producing more rhizoids to anchor itself more firmly. At this stage the remains of the brown spore coat is still visible. During this stage of development the plant must be constantly wet - even a short period of drought will be fatal. I sowed these spores in September, so they've taken about four months to reach this stage, where they appear to the naked eye as a green film covering wet soil.

This is the next crucial stage in development and is about a millimetre long. The tip cell of the thread now begins to divide laterally and longitudinally, forming a flat plate of cells ....

... here you can see this two-dimensional tip division at higher magnification. The flat plate of cells that develops from this is known as the prothallus, and this is where fern sexual reproduction takes place..

These are two fully developed prothalli, each about 5mm. in diameter and only one cell thick. They are incredibly delicate and must remain permanently wet to survive. At this stage they are about six months old and male and female reproductive cells form on their surface.

These are the male antherozoids, enclosed in a structure called the antheridium. When this bursts the antherozoids are released in swarms and swim, propelled by lashing flagellae, like tiny spinning tops in the surface film of water, in search of a female egg cell inside a long-necked structure called an archegonium, which you can see here. After a successful fertilisation an embryo deveops which ultimately grows into a .......

.......new miniature fern plant. In the early stages, as seen here, it's still attached to the prothallus formed by the germinated spore but that soon withers away and the new fern grows by producing a series of ever-larger fronds. It usually takes about a year after sowing to reach this stage.

Provided you have the required patience, ferns are not difficult to grow from spores. For detailed instructions, visit http://website.lineone.net/~margaret_cole/SFG7/growing%20ferns.htm

   

For more on the fascinating world of ferns, visit http://www.nhm.ac.uk/hosted_sites/bps/

Polypody – the fern with the golden sporangia

Some polypody Polypodium vulgare ferns continue producing spores deep into winter and if you turn over a few fronds you’re eventually likely to find these golden cluster of sporangia. Unlike many ferns, the sporangia of this species are not covered by a membrane during their development and under the microscope they resemble nests of golden eggs, or maybe even party balloons if you're in a celebratory frame of mind.

Each sporangium is packed full of spores and when they’re ripe there’s a remarkable mechanism for catapulting spores out into the airstream, that you can read about at http://beyondthehumaneye.blogspot.com/2009/07/natures-siege-catapults.html

The gaping sporangium at the top of this picture (above) has burst open and has already catapulted out most of its spores. You can still see the spores packed into the surrounding unripe sporangia, through their transparent walls 

Polypody spreads vegetatively with creeping rhizomes, that either grow over the branches of trees or through old walls, and you can see it in its habitat over at http://cabinetofcuriosities-greenfingers.blogspot.com/2009/11/wall-ferns.html

Nature’s Siege Catapults

Turn over a fern frond at this time of year and you’ll find that the underside is covered in rows of what look like small blisters (bottom photo). These are scores of spore- producing structures called sporangia, clustered together under a membrane that keeps them moist while they’re developing. In the example illustrated here the membrane is kidney-shaped, which is characteristic of a buckler ferns in the genus Dryopteris. Once the spores in the sporangia are ripe the protective membrane withers and at this stage the sporangia – sometimes more than a hundred in each cluster – look like minute black eggs when you look at them with a hand lens (second and third pictures from bottom - double-click any image for an enlarged view). Each individual sporangium is a minute catapult that fires its spores out into the airstream. You need to look at a sporangium under a microscope to see its detailed structure and decipher how it works (top photo). Each sporangium, mounted in a stalk and stuffed full of spores, is egg-shaped and has a 'spinal column' of thick-walled cells (showing up in vivid colours in these polarised light micrographs) stretching about two thirds of the way around its vertical circumference. Once these cells are exposed to air they lose water through their thin outer wall that you can see in the second photograph from the top, drawing this outer wall inwards via the surface tension of the remaining water inside. This creates tension inside each cell and, repeated all the way along that ‘spinal column’ of cells, draws the 'spinal column' back like a bowstring, ripping open the sporangium and exposing the spores inside. Eventually the remaining water in each cell in the 'spinal colum', under immense surface tension, vapourises instantaneously and the natural springiness of the thick walls of the sporangium flicks the spine back to its original position, hurling out the spores like rocks from a Roman siege catapult. Once the spores - each around a hundredth of a millimetre in diameter - reach the airstream they can be carried vast distances. Ferns are often amongst the first plants to establish themselves on new volcanic islands and lava flows, thanks to their spores' incredible aerial mobility, which also allows them to colonise unlikely places in urban environments. Next time you're walking through any city, look up at the gutters and you'll see ferns growing that arrived as wind-blown spores. What happens next, after a spore lands and germinates, is an equally remarkable tale of frantic sexual reproduction .... but that’s another story.